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.