A Question for the Injection "Masters"
 

Assuming the following:
- rebuilt, calibrated original set of injectors
- rebuilt head, including prechambers, valve guides, seals
- non-contaminated fuel

With no apparent fuel leaks, can worn delivery vales/sealing washers & rings be the cause of nailing? If so, how?

I say no but I will wait for others to weigh in also.

Nailing is caused by a bad spray pattern most fruequently.

I believe the Delivery Valve Crush Washers on your IP sit between the Delivery Valve body and the Delivery Valve holder (that the Hard Lines Attache to); so unless some one reused them I would not think that they would have a wear issue and should seal.
I believe I have read of some nailing caused when the O-rings leak but I am not sure why this would be the case.

I would be more inclined to think the Nailing was caused by IP timing not being correct.
By "rebuilt, calibrated original set of injectors" did this mean brand new Nozzles and Opening/Pop Pressure set on them? And, who did the rebuild?
Does the Nailing ever go away?
Any other issues like smoke?
Do you know if anyone was inside of the IP; for instance to change the Delivery Valve Holder O-rings?

1. Indie mechanic removed #1 delivery valve to check timing. (took it to indie thinking that nailing was indication that timing was off) Timing was less than 1/2 degree off so he left it alone. He replaced DV holder o-ring. Miraculously (or coincidentally??) after reconnecting the line the nailing went away. Slowly, about 500 miles later the nailing reappeared, only milder.

2. Injectors rebuilt by Diesel Injector Service in Portland. Certified Bosch technicians - very busy shop. New nozzles and opening/pop set confirmed.

No real smoke - maybe a little at start-up - especially when its in the "nailing mode."

My pump utilizes both copper crush and o-rings. See here

The only other thing I can think of is perhaps a small amout of air is getting in.

Also while I have not read it at the below site in the manual there is some sort of modification to the Overflow Valve; in the manual. I have not read it as I do not have that Engine or IP so I do not know the details or what problem it is supposed to solve.

http://www.ps2cho.net/downloads/MB%20CD/W124/Main.html

Random thoughts:
Is it possible that the EGR/Smog system could be causing the Nailing? Too much exhaust gas mixed with the fresh air might be causing late burning of the Fuel?

Is the Engine operating temp normal?

I also remember folks complaining of the inside of their Intake Manifolds getting really cruddy to the point they are difficult to clean.

Also it seems that after 1985 model year or so there is more complaints of nailing. Rebuilt Injectors and Timing set properly usually cure a 617 that has good compression and no air leaks.

Thanks Diesel911,
I think I have the problem solved. A faithful friend brought a stethoscope to locate the responsible cylinders/injectors and it turned out to be only one. #5 had a pronounced knock at the RPM that was most prone to elicit the nailing while all the other cylinders appeared to be the same. Once I determined that there was only one misfiring cylinder, we jumped in the car and drove to the injector shop where I originally had them rebuilt. Took the #5 injector out and gave it to the shop owner. He invited us to come back and watch him pop-test and verify opening pressure. It consistently showed 121 bar. He remarked that it was off, but that "it shouldn't be a miss." He offered to disassemble and reset the pressure. Readjusted to 140 bar, I re-installed the injector/line and the nailing went away. Just goes to show that you have to make sure of injector adjustments, and that if you are having someone else rebuild them for you, a trust relationship is valuable.

Car runs like it should now.

Excellent job trouble shooting!

I wounder what happend when they did the origional Injector Rebuild; since rebuilding the Injectors is many times simpler than rebuilding a whole IP.

Not sure what happened, when the injectors were pulled for the initial rebuild, two were completely blown (held no pressure) two were off spec (85, 90 bar) and one was about 129. Not completely familiar with the shim/pressure relationship, but the #5 injector may have had a shim settle more than the others?

Of course now I am keeping a close ear for the other four and if any of them start to nail I know where to start first. :)

Another sad story of a professional shop doing an improper job.

My concern now is the other four. If the shop cannot set one of them properly, why should we believe the other four are on spec? The lack of nailing is not the definitive answer to a proper job.

How smooth is the idle? If it's not like a gasser, I'd pull the other four and take them back as well...........let him prove that they are all at 140.

Thanks Brian,
I am seriously mulling that very question now. The idle is perfect presently. Injector noise in general is a bit loud, but even - unlike the sound before he reset #5. Stethoscope does not reveal any inconsistencies between all five, so I am taking a drive today out to Bonneville Dam for a test run and will report back any incorrect combustion.

- geoff

Note to self... build an injector pop tester.

http://www.peachparts.com/Wikka/DieselInjectorTester

Agreed - but, I gave this guy $200 to rebuild my injectors to spec. He knows I am a regular on this forum, and he is a good guy, so he has been real nice to me. I have decided to take out the other four and go to the shop and watch him pop test them. If they are all at 140 I will be another satisfied customer, consider the #5 event a fluke, and be on my way. But I doubt it will go that way.

This got me to wondering if a defective nozzle could change pop pressures if/when the plunger rotates.

Good question - I think I will take all five out and have them tested. If the nozzle is defective then he should replace that as well.

Number five injector was of course opening too early causing the early ignition knock I imagine. You have kind of proved it does not take very much to upset the individual cylinder timing. More reason than ever to at least check the injectors every 100k as recomended by mercedes.

I feel that old milli volt test would have picked up the hotter cylinder quite easily as well . Or perhaps just reading the glow plug temperatures with a laser thermometer with the engine warmed up and running.

At this point I am not aware of anyone trying that. Or if it would have any merit.

Since quite a few of us own one of those temperature readers I may look into it somewhat. Or someone else could and post if the plugs sit at different temperatures. It might be informative. I assume the sensitivity of the whole glow plug to internal individual pre combustion chamber temperatures is going to be the question.

What does it really cost to scan the operating surface temperature of the exposed glow plug end? A major temperature difference on any given plug might indicate something is unusual. Could be as simple as excess carbon buildup I imagine.

Needless to say the glow plugs should be wiped clean when cool as a different surface color would effect the laser temperature reading I think. I also would not want to see anyone burn themselves.

Who knows one day we may yet stumble on something quite useful that is cheap and fast to establish certain faults. In my limited knowledge on a well balanced warmed up engine all the external portions of the glow plugs should be at simular temperatures.

As long as all the glow plugs are the same brand regardless of internal element resistance . I think they should transmit heat equally well from the combustion pre chamber. Just a thought. Worth looking into?

I have this crazy compulsion to stir the pot periodically. :D The benifit from my perspective is at least it gets people thinking . Their contributions then tend to increase general knowledge in my humble opinion.

Sometimes for example we will just reach halfway or much less to a possible usable method. Yet even with that shortfall we come to understand applications of different troubleshooting approach possibilities better. Even more important is the possibility of one thing or a combination of them leading to a real usable senario at some point. That personally is my ultimate hope.

Me and a buddy were talking about a "combustion efficiency" display in our ideal diesel car of the future. Kind of like the way some of the new Detroit gassers display fuel efficiency - only not a hokey gimmick, but a real indicator of what goes on in each cylinder. I think your glow plug as a sensor idea is along this same line.

On airplanes often airplanes will have an Exhaust gas temprature for each cylinder so that folks building their own cooling plenums (air) can verify that all cylinders are running smoothly and equally. I beleive they just port the exhaust manifold at a uniform distance from the head with a high temp thermocouple. Sounds like a lot of work, but it would be interesting.

Sas....this worked out to be an excellent thread. Good on you and I am so happy to learn that you solved it. There are some great contributors on this forum and Diesel911 is one of them. Again the lesson is never assume anything.

Your OP was and I quote:

"Assuming the following:
- rebuilt, calibrated original set of injectors
- rebuilt head, including prechambers, valve guides, seals
- non-contaminated fuel

With no apparent fuel leaks, can worn delivery vales/sealing washers & rings be the cause of nailing? If so, how? "

So my answer held true because you did NOT have properly calibrated injectors.

Properly calibrated injectors isolate the spray formation and pattern from pump issues (except for timing...of course) due to the pop pressure being the final arbiter in the sequence.

This is not about me being right or wrong (no ego tied up here! ) but I would like you to privately acknowledge that I did not mislead you :confused:

It could be that dirt or something got trapped under or between the Shim pack and the Pop pressure was OK when tested. Later it got smashed or worked it self out.
Also it is one of the least skilled jobs in a Fuel Injection shop; it may be that they were training someone or just in a hurry.

The thing about shimming the Injectors is that each Spring from each Injector will not have exactly the same tension (ounce in a while they also break), the dimensions of the Injector parts that effect the pressure are also different. Hence the need to use shims to adjust the pressure.

If you ever rebuild your own set keep the parts from each Injector segregated from the others; it makes them easier to shim.
Your Spray Nozzle may not look like the one in the pic; used to show the relationship of the Spring #8 and shims #10.
http://i242.photobucket.com/albums/f...sel911/ZN2.jpg

Pyrometer/Thermcouples are common on large stationary Diesl Engines as well as on large Marine Diesel Engines.

Mystery solved
 

I wanted to update this thread:
Mind you this is an abridged version of a detailed story that relates a nailing problem that would surface slowly after purging air from my fuel system. I thought I had it fixed after I discovered that the injectors were not popping precisely within spec and made the shop reset them all three times. The shop was great, they were consistently nice to me, every time I went in there they gave me free goodies like return line hose, loan of special socket to remove delivery valves, and notepads/pens in addition to pop testing and calibrating my injectors for free three separate times. This shop was a great place to learn, I ran into some "old timers" (including a Stanadyne rep) and these guys felt pretty strongly that air infiltration was my problem.

After many trips to the injector shop, setting the pop pressure at 120, 138, and back to 125 in succession, 5 sets of injector heat shields, countless fuel hose clamp tightening operations, replaced primary filter, many sleepless nights visualizing my fuel delivery circuit, and expecting the inevitable horror of pulling my IP, I have solved this mystery wrapped in a riddle inside an enigma. When inspecting my secondary filter I discovered that as a result of over-torquing the hollow screw to hell and back, the center rubber seal in the canister was chewed up and distorted leaving the opportunity for the return fuel to mingle with the delivery fuel (see my latest thread). While I had the secondary fuel filter housing and canister off, I replaced the three connectors that link the housing/canister to the IP (milky white translucent plastic dealer items w/banjo and compression fittings). Put it all back together and presto, nailing problem solved.

Whether the problem was caused by the filter or by the IP/fuel filter connectors, I don't know but the car runs like a new one now. I wanted to let everyone know that all is well and I am ready to tackle the next problem. I have learned to slow my tortured brain down and expect the simplest solutions to be the secret to success.

LOL!

It might be nice to repeat the name of the Fuel Injection Shop you got such great service from.

Diesel Fuel Injection Service - Portland, OR
:D

Has anyone referenced here all the possible causes of "nailing"? From what I understand, that annoying sound could be caused by (don't quote me, I'm observing, but don't really know):
-injector nozzles with poor spray pattern (dirty or worn)
-injectors that are sticking/not popping correctly
-air leak (where? supply lines?)
-bad fuel
-pump timing (too advanced? too retarded? I don't know)
-weak injection pump (?)
-excess carbon buildup in pre-chambers (?)

I've been trying to figure out how to tackle a periodic, random nailing I'm hearing....it happens most often under light throttle...goes away under heavy throttle, and not present at warm idle. Somedays it's really bad, other days, barely at all. Diesel purge didn't help, so I'm thinking it's time to replace the injector nozzles, but now I'm wondering if that's REALLY the problem (probably won't hurt though).

This is what I think:
I have not understood why yet but apparently some nailing can be caused by too advanced or too retarded IP timing. The Nailing/Knocking sound has to do with too much of the Fuel bruning at the wrong time and or place in the Pre-Combustion chamber.
Burning and the wrong time and place includes most of what you said.

You also left out low/bad compression.

Weak Injection Pump- I am not sure what you mean by this. Worn or abused IPs have erattic fuel delivery and deliver less fuel. I am speaking of the high pressure fuel that goes to the Injectors.

This can be caused by worn Elements/Plungers and Barrels (scratches on the parts due to bad fuel or poor filtration or some alternate fuels) or if the Rollers, Tappets, and Camshaft inside of the IP are worn (the makes the internal timing of the IP).

[On other vehicles with Rotary type Fuel Injection Pumps (like VW Rabbits, my Volvo and vehicles with Standyne IPs) the advance mecanism is built into the Fuel Injection Pump and as the pump wears the advance become retarded over time. The advance pistons are also at the bottom of the IP and that is where any water will also settle. If the water sits long enough the advance piston can stick.]

Aparrently on those IPs with O-rings that leak; when they leak it some time causes nailing (from what I have read in the threads).
Quite a few members have reported that after the O-ring changing job they experienced Nailing until they have driven 200 miles or so. The reason for this has yet to be explained.

I am not sure if a weak Fuel Supply pump will cause Nailing or not. I guess with the OP saying it was his filter there could be something but it is not clear to me what it is.

I have not read of the excessive Carbon explination for Nailing. I would be the result of another problem or problems like Oil burning, bad Injector Nozzle, Ball Ping damaged, damaged Pre-chamber, bad fuel and so on.

When air is in the lines, the spray pattern is disrupted. In my case, I have a rebuilt head (new valve guides, seals, rebuilt injectors) and the bottom end is in good shape. When purged of air, the engine runs like a new one. Slowly, over a period of about 3 or 4 days it would begin to show the signs of air creeping back into the fuel system (nailing, rough starts) but when it runs well, it really runs well, smooth smokeless starts - no nailing whatsoever under any circumstance. That was the real clincher evidence that the problem was air. Even when the car was nailing, I could drive it real hard - 75 or 80 mph on the freeway for a couple of hours always purged it enough to run smooth for a while.

During the head refurbishment, the IP pump sat for about 3 months with the injector lines off. The PO burned B99 for a year, and I used B99 for about 4 months after first acquiring the car. All the fuel lines were 5/16" Gates and were swollen and distorted when I replaced them. The IP poly-connectors to the fuel filter housing had become brittle, but did not seem to be leaking - I replaced them anyway. The IP has no outward sign of fuel leaks. Somehow air was getting in. Many times I pondered the possibility that air was getting by the delivery valve o-rings when the car was off. Possibly negative siphon pressure was slowly pulling air into the pump. However, if that were the case, then the air would be purged after running for a bit. I would be very receptive to any input on this theory.

Air infiltration is a peculiar beast. A ghost with no direct trace to its whereabouts. Whatever I did, the engine is better now - for how long will be the question where all of my (and other's) theories will be tested.

You are doing good work here Sasquat. Keep it up until it reaches a solid conclusion.

It is possible that it takes more than one cause occuring simultaneously to set up a nailing condition such as the intermittent type at low throttle. Revisit your theories with this in mind and see if you get further.

If you would not mind could you please redo the list you started after you get all of the input and try to prioritize it and organize it. You might also add valve adjustment as a potential contributing cause.

This nailing is an insidious condition that can do damage over a long period of time.

Of course he would need to have adjustable lifters, his 602 does not.

This is indeed true but the fact is that if the hydraulic lifters are not to specification his valve train timing will be off. This alone could cause nailing. And further if there is drift in the lifters with time and temperature the nailing will be more difficult to characterize and may not be always repeatable. This thread is now into advanced nailing issues so every possibility has to be brought up.

Dionysius, you bring up a good point - my valves are in essence brand new and I believe that some "settling" in this area is in my future, much the same as rebuilt injectors, delivery valves, etc - all of which present a "break-in" period.

OK, at the behest of Dionysius, I am revising my list that I originally posted from what seems like such a long, long time ago.

My OP was something like this:

"Assuming the following:
- rebuilt, calibrated original set of injectors
- rebuilt head, including prechambers, valve guides, seals
- non-contaminated fuel

With no apparent fuel leaks, can worn delivery vales/sealing washers & rings be the cause of nailing? If so, how?"

My original question was regarding the delivery valves and their respective seals. I have not changed those out yet since I have assigned that task as "last resort" before pulling the IP and taking it to my favorite rebuilder. I lieu of the possibility that the DV o-rings are the cause of my nailing, I have taken it upon myself to go through practically every other aspect of the fuel delivery chain. In essence, starting at the pump and working back towards the tank.

So, my list of assumptions has changed to the following:

- rebuilt, calibrated original set of injectors (proven x3)
- rebuilt head, including prechambers, valve guides, seals (all good)
- non-contaminated fuel (drained tank, refilled w/fresh #2 plus cetane booster)
- good compression and power, no oil consumption
- Return pressure relief valve is good (clamped off return line while idling w/no change in idle quality)
- changed all 3 nylon IP connectors including fresh aluminum crush washers
- replaced all rubber fuel line from the hard lines on the wheel well to the lift pump
- verified and replaced secondary and primary filters
- new nylon IP connectors show visible #2 diesel with no apparent "tiny bubbles" (Where's Don Ho when I need him?)
- no visible fuel leaks on or around the IP
- disabled EGR function to no apparent change in behavior

A bit more detail about this mysterious behavior. Initially, I could disconnect the injector lines and glowplug relay and then turn it over for 15 seconds, wait 1 minute, turn over for another 15 seconds and so on. After repeating this 4 or 5 times, I could reattach the injector lines after this "air purge" (a 602 has no air relief mechanism that I know of) and the engine would run smooth as silk - idle, acceleration, surface street cruising, the whole nine yards. Me and a buddy called this the "honeymoon." Slowly, usually over the course of about 3 days, the rough starting, idling, and nailing would return, i.e. the honeymoon was over.

What I think is left before succumbing to the urge to rebuild the IP pump is:

- Clean tank strainer
- Rebuild lift pump
- change out delivery valve o-ring seals and crush washers.

Did I miss anything?

Bump

anyone?

Well for starters your injection pump itself is okay. The pump has not enough descretion to decide to act up itself after a couple of days normal running. Then you purge the air out and it's okay for a few days again until the fault repeats.

I have not read your total thread though. I will try to make this fast as I have to get back to the cottage or the wife will do Horrible things to my head.:D It seems to me delivery seal leakage will delay or eliminate injector openings. The usable opening pressure of the injector is delayed by those faults. Again they would normally as a fault be quite consistant as this is not a dynamic item. The leakage internal or external will perhaps vary a little by temperature but that should be about it. Normally the pump should clear a modest amount of air and yours does not seem to do this. Instead it seems to accumulate day by day so to speak. Your methodof clearing it can be nothing else I think.

It almost still sounds like the pump slowly is becoming air bound. Perhaps it's time to try other fuel feed devices semi permanently mounted under the engine compartment to try to find the air source. I freely admit these are just suggestions but still where I would start. In my mind there is little present possibility it is the uinjection pump.

One last test to verify the return valve on the pump is not wide open. When engine roughens up clamp the return hose. If after a few minutes engine smooths out deal with the return valve. Air may be able to reverse enter the pump perhaps when sitting if the valve is defective and wide open. Some broken springs or really gunked up ball seats have been reported. Yours is sealed I believe. Keep us posted as this has become interesing to us and frustrating to you unfortunatly.. There will be a light at the end of the tunnel though. :)

Thanks Barry, good luck with the injury-avoidance scheme with the significant other - I have become the artful dodger of frying pans and kitchen knives because of this computer and that car, so I know a bit about what you are saying :D.

I have tested the return relief valve by squeezing the return line with no change to miserable idle quality.

I am in the process of draining the tank and cleaning the tank strainer - it may not be my magic bullet, but it has never been done, so I am replacing the tank nozzle to hard line rubber hose connector ($21 - ouch) as well as the strainer seal. I will report back when this task is complete.

Tank strainer was clean thanks I guess to the miracle of modern biodiesel (I have been burning #2 for the past year, PO burned bio) - seal was flat and rubber hose was very old. Blew pressurized air back through the hard line before reconnecting to the tank. There was a loud pop and a small amount of diesel sprayed all over the differential and axel area from the end of the hard line. It seemed there was a little back pressure in the line before the pop, so maybe I had an goop-infested hard line. I purged the system of air again and its running smooth. We'll see if these changes make a difference.

Let's just hope it's not the training coming down the tunnel though :D

Barry - elaborate on a "semi-permanent fuel feed device."

The only other likely suspects would be:

lift pump
thermostat
return relief valve

Maybe try bypassing the thermostat/fuel heater, i.e. connect the feed hard line directly to the lift pump to rule out those candidates? Everything is new from the tank to the hard line in the engine bay.

When testing the return relief valve performance by clamping the return line - how long should I clamp it off?

Thanks to anyone who may offer to help...

I think what he means is to take a container of diesel fuel and locate it in the engine compartment. Put the inlet feed line to the pump and the return line into this container. This is a proof-positive way of by-passing all of the plumbing to/from tank and tank itself.

Air ingress is sneaky. In fact it can be like a diode in that air can seep in but no fuel seeps out so no visible clues.

Sounds like you have replaced the plumbing. Keep up the good work. If all else fails you may have to look at the IP. The IP is a very low failure source as I know it.

Thanks, I had planned on bypassing all the plumbing to the fuel heater and thermostat and seeing how it ran since the lift pump pulls through all that mess first.

I need to test the lift pump (getting gauge from Harbor Freight on Monday) and if the pump looks good, I will replace the return relief valve. Everything is high mileage, and consequently suspect. :D

In the meantime, I am looking forward to installing my Monark nozzles!

When you say fuel heater and thermostat what are you talking about??
I am not aware of such items in the fuel line. What model have you got???? Is it factory original????
I am familiar with 123 and 116 models and they sure have no fuel heater and thermostat!!!

602.911 (W201)
2.5 N/A

Item # 148 in the drawing is a fitting that goes into the head adjacent to the water jacket to warm fuel in extreme cold weather. Thermostat kicks heater out of circuit at 25° C

Running on external fuel supply to me means elimination of everything before the injection pump to get an absolute indication. Just lettiing the lift pump pick up the fuel from a container still leaves a few things in the supply circuit. More so on your 602. I also understand that under pressure that section of the system should not really ingress air. Yet something is obviously going on.

Unfortunatly this means the elevation of the new very clean fuel supply to the cars roof area if it is to be driven. You need some gravity feed. Or a temporary electric fuel pump if all is under the hood. If the problem can be present with a car at idle just hanging up the fuel supply container should be enough.

I would put a cheap disposable filter in the supply line. When new the small filters flow very easily. This approach should be productive. Otherwise the lift pump and filters plus your heating device remain unknowns if you just let the lift pump pick up the fuel. You really want to be certain of what is going on at this point in my opinion.

I still think it is not the injection pump. Once they develop a mechanical injection pump fault it tends to remain pretty constant. There might be an exception there as well yet that just does not occur to me at this time. The above way is about the only way to be certain in my opinion. You also already blew out the supply line to the tank.

It would not hurt to blow out the return line with the fuel cap off either. Since you could pump up the fuel the feed line was at least partially clear though. It appears the feed line might have been partially obstructed. This may have created the problem by requiring the lift pump to pull harder than normal. Since it did nothing it was not the true issue. Still good you found it as eventually it might have totally obstructed.

Funny, I blew out the return line yesterday :).

I found a couple of small cracks in the secondary filter housing body. Have not dismantled housing to check to see if they have gone through but there does not appear to be fuel leaking from them. The old MB aluminum gets brittle with age.

I personally would not bypass the filters etc by direct connection unless a new separate filter was installed. Chance of some disloged dirt getting into the injection pump. With a filter there should be no issue.

We are lucky. The design of the lift pump is such to allow clamping off the return for quite a period of time. The base pressure in the injection pump will rise to the maximum of the pumps output. Pehaps thirty pounds. Then the lift pump just sits there. Not struggling in some overloaded condition. At idle there would be no long term issue that I can think of.

Actually with a new small filter of reasonable fine mesh your ideal is pretty good. I do not know anything about the fuel heater device or thermostat. But if the device is under more positive pressure than the fuel supply it would force air into the fuel system if a seal where questionable.

It was simply a matter of disconnecting and clamping off the thermo/heater to lift pump line, and then switching the feed line (with primary filter attached) from the thermo/heater inlet to the lift pump inlet. Idling does not show much difference. I will test drive to purge any air and see if there are any different results.

I do not personally liike the way this going for you. If the fuel tank does not have an obstructed vacuum relief line. I am going to assume you have been testing with the cap loose or off? Stuffing a rag in the neck stops most fuel slosh out if tank has any amount of fuel in it by the way. Retained and building tank vacuum over a few days would cause major problems. Just removing the cap now when idle is rough. Idle should restore in less than five minutes if the vent is closed.

Other than that you only have one test left. The totally independent fuel supply source to the pump in my opinion. Normally the pump should clear itself of air once running if in good shape. Plus stay clear of air. For a better opinion on this others should comment that have or have owned your engine type. I do not know it all. Or if you do an absolute test and problem remains Talk to an injection pump place.

One last thought as well. As system was using alternative fuel previously try running the injection pump on it. If problem leaves pump may have excess wear internally. I hope that is not the issue. Probably not in fact but no easily turned stone should be ignored.

Sealed off cracks in aluminum fuel filter housing with RTV.

IMHO the fuel heater thermostat is shot. Inlet (from fuel feed) barb is loose - will have to rebuild - new one is $165 :(. Closed off thermo/heater loop and took it out of fuel feed circuit. Running direct from fuel feed to primary filter to lift pump now.

5/16 NAPA (Gates, I think) fuel hose is cheap and not very durable - let along the wrong size (5/16 - s/b 7.5mm). Had a small piece of German metric stuff and replaced primary filter to lift pump section. Will get more German metric tubing from injector shop on Monday.

I am still not convinced the return relief valve is sound. Engine has 246K on it and this part has never been replaced. Still need to thoroughly troubleshoot that part.

Will be testing lift pump pressure Monday.

Tank does not seem to retain vacuum and removing the cap does not change idle noticeably.

Thanks a ton for all of your input Barry - all has been illuminating. I am getting closer now.

Since this is a tough one I will offer a thought.

What if the thermostat failed and you were over-heating the fuel. This would cause trapped air to come out of solution.

I know it is a long shot but never underestimate the enemy......

I disconnected the thermostat - it seemed to be worn out anyway - I will test and update as circumstances change. Stay tuned! :D

Oh - forgot to mention - I have ordered Monark nozzles and will have the local shop install those soon - should be interesting to see what that change will yield... :guitarist