where do you recommend installing the gauge??

Gauge should be tapped in between the secondary fuel filter and the injection pump input. The gauge itself should be located somewhere in the passenger compartment. I favour the glove box as esthetically just do not like visable add on gauges.

The neatest take off point in my opinion is to drill a banjo bolt and thread it for the take off fitting. There are at least two descriptions of this on site.

Why is it necessary to run it all the way into the cab?? Couldn't one just thread a banjo bolt w/ a flare fitting and thread a gauge to it whenever the fuel pressure is to be checked??

Given those circumstances, that model MB diesel should deliver in the upper twenties range of MPG. Unless you're driving much above say 70 mph.

Checked the spring in the pressure relief valve and it measured 25mm. Still going to do a fuel pressure reading.

Read through some posts today and I think I may have found my problem... looking for some feedback. My temperature gauge reads about 70(I guess thats Celius) and it should be 80-85 correct?? If my engine isn't warming up all the way it will use more fuel if I'm not mistaken. Need to double check my gauge, and if good, replace my thermostat.

Since the effort of a permanent installation is basically not much harder. There is nothing wrong with it as a general monitor of the fuel system condition. Over time it will help as well.

Without a cab location how will you know fuel pressure at highway cruise? This is the first area pressure problems might become apparent if system was starting to obstruct.

Again the 616 engine is one you just do not want to operate at low pressure. I still feel low pressure has made many members charter members of the first rod bearing club. Now to be fair it takes a long time if this is so and the low pressure condition may have been present for years. By the same token letting the condition continue is not good either.

You can get low pressure for example just with partially obstructed fuel filters not changed out. Partially obstructed fuel filters are also proven to reduce fuel milage. With out a gauge you never know what the current operational fuel pressure is in the injection pump.

Overall in one way or another a permanent gauge will save you money and increase reliability. You will see problems coming on well before they become conditions that kill the engine running or starting properly. Call it an early warning system. This area is where the majority of problems are posted about on site. A fairly simple system that you then have an indicator of what is going on.

Diesel 911 was the poster to come up with the drilled banjo bolt approach.Besides being neat it allows easier movement of the system to another car down the road or quick checking or installation other cars. Just pull the required normal banjo bolt and install the drilled one on any example.

Today I installed a T-fitting between the secondary fiilter and inlet to the injection pump and connected my 60 psi fluid filled gauge(couldn't find a 30 psi gauge, but the 60 will work as it has a mark for every 1 psi). It registered 12 psi at idle and maintained this throughout my test drive except for when it was pulling hard at high RPMs it would drop to 10 psi. Next, I am going to install a T-fitting between the lift pump and the secondary filter to see what my drop is across the filter. I don't expect it to be much because I just replaced both fuel filters about 2 weeks ago, but I am going to check it anyways. What else could be keeping me from reaching 19 psi of fuel pressure?? Maybe the filter screen in the tank?? I have already adjusted the spring in the fuel pressure relief valve to 28mm.

Also I never got any feedback as to operating temperature and its effects on fuel milage....any help will be much appreciated.

The spring can measure the same length and still the relief pressure could be low. Probably not but possible. What heat is present on the spring over the years may have had an effect. Or may not. Anyways the lift pump can have a weaker output as well.

Running at 70 degrees C of course could be a thermostat. Could also be the sender or gauge calibration as you suggest. Yes technically the hotter the engine the more efficient the combustion. Less internal wear as well.

If more btus can be released on the same amount of fuel .Or released faster might produce more energy for the same volume of fuel. The engine temperature always has seemed to have a bearing on this. Perhaps not so much with direct injection.

To take this a step further.I have even been wondering a little what the effect might be of using a custom modified thermostat and anti freeze that will not boil until over three hundred plus degrees. The cast iron block and head might allow these early engines to tollerate running at elevated temperatures someday. That might improve fuel milage.

Only time and testing will tell over time. The only really high boil point antifreeeze I know of has some problems. It does not boil till 360 F or some other insane number.

There may be other high temp anti freezes out there. Before even considering this approach a good trouble free one would have to be found. Also its thermal transfer ability would have to be as good as what we have currently I imagine. I only met one person that has used it so far.

Hopefully if practical the higher operating temperature for much increased efficiency is not impractically high. If it was present at 220-250 F head temp would be nice. Also in winter the heater would get quite a boost. Strangely enough the efficiency of the existing cooling system might be better. Greater transfer rate from a radiator at higher elevated differential temperatures to the ambient air.

The basis of this line of thought is the blocks of the older pre tdi volkswagon diesels. Their head temperatures were substantially higher than our older mercedes are. This was created by an intentional design issue. That was made to enable their heaters effectivness.. First diesel heaters that could burn you at the dash ports that I am aware of. These where the first diesels that I ever drove that had heaters that where far better than typical gas cars heaters.

The warmer air with less oxygen may have contributed in some way with the hotter head temperatures on very hot days. One way or another the fuel milage was substantially better. These engine did not boil over either. One could test the theory by using the troubled high temperature anti freeze curently on the market I suppose. The difficult issue may be to change the calibration of the thermostat. Most are controlled by the heat acting on a wax pellet I believe. I think the hoses etc would stand the elevated operational temperatures. The plastic portions of the rads may not like it as much but might be okay.

Who knows? If I live long enough and get the time I might experiment with an older tired 123 diesel with a good engine. The answers would be forthcoming soon enough on an experiment like that.

Now if I have your attention there are some things we seldom examine. They in my opinion are related in some fashion to this area. Adding a turbo to a 123 4 or 5 cylinder gives higher milage and more power. We have never disscussed this at any length but it is obvious the burn efficiency is improved. Actually fuel milage should go down instead of up or remain the same at best. Instead again it improves.

This is undisputed in my opinion. So what has changed. The compressed air being hotter? The combustion conditions changed because of increased turbulance? We all pretty well assume that after a normal diesel burn there is oxygen left over. Is this true across the cylinder internally?

I was under the impression a long time ago. If I could scavenge and increase the temperature of the incoming air charge to those volkswagons. I would have consistantly seen higher mpg. Of course the only free source of additional or hotter air was the exhaust system. The tremendious flow of air required to service the engine meant that source was puny in nature. To meet the needs required something producing much more heat by volume.

So what in your opinions is going on? I am well aware this is just speculation. Yet some of the known facts surrounding the issue are well established. There is less efficiency in non turbo 123 diesel engines than turbo ones for example.

Does the heater work better on the 603s? They increased both power and fuel milage yet are still basically the same crude by todays standards indirect injected diesel engines. Posessing the exact same capacity of three litres.

Well instead of moving the gauge introduction port. Close the return line on the output of the relief valve. This will show you what the lift pump is able to provide pressure wise in the injection pump base. Should rise to at least thirty pounds pressure I think. No harm is done at all to the lift pump with this test as it is a constant presure type I believe for all practical purposes. I further expect from your reported results that only a few pounds pressure at idle is being lost across those filters. You at this stage want to know instead what the lift pump can produce at maximum pressure to the injection pump base.

Normally since you have decent fuel filters. This is verified by the only slight reduction of base fuel pressure under heavy load seen on the highway run by the way. You will have narrowed down possibilities a little with the closing off of the return circuit. If you cannot generate about thirty pounds or above. The lift pump may need a reconditioning. Or perhaps the tank screen filter is poor. A very simple test can eliminate that though after the relief line closure test. You are getting to know your fuel supply system very fast.

There are a few quick things we will have to jointly check soon. The fluid filled gauge is new so for the moment we will assume it is relativly accurate. Or considered accurate enough for the present needs.

The second consideration we will probably need others to give the information. For example the pressure output of a lift pump may be different between the 617 5 cylinder engines and the 616 4 cylinder engines. I think entering new territory here at some point is going to occur. Or a slight proccedural change is a better description.

Testing the lift off point of the relief valve has never been talked about much. I see no reason with your gauge and a three hose arangement should not be valid. A line from a controllable air source. This to a tee fitting. One tap to the gauge and one to the relief valve. Increase air pressure till the relief valve opens. This of course reflected by the pressure gauge not gaining any more. Or do all the test with the relief valve submerged. The relief valve should be submerged in water or whatever you preffer to verify there are no air bubbles escaping at say 10 pounds air presure anyway.

One way or another you are going to have better fuel milage and performance than you had before soon enough. The amount of apparent increase of milage is unknown. Yet just by you knowing you had 12pounds before going to 19pounds people will have to believe the milage increase. Plus most importantly the direct cause. Unless you drive the car harder as it will be a little more powerful remember. I think you will also enjoy the newer idle resulting. If you really get this all right colder weather starting as well as current temperature starting may be improved as well.

As strange as it sounds I hope the lift pump is weak. The valve kit is only about ten dollars. I have an ideal of how to rebuild it to a higher standard than the norm. This just by testing the sealing ability of those valves prior to re installation of the pump. Paying serious attention to detail should pay off.

If they where tight when new and proven tight after rebuild. Plus the relief valve is made tight if it is not already. You may be able to hold pressure in the system with the engine off. I do not know if this was possible or obtainable when these cars where new. Such a situation I believe if possible could lower cold start temperatures substantually. The easier these engines catch the easier it is on the starter and battery even if this does not lower the starting temperature.

A very few of the older 123 engines will come to life when most will totally refuse. . I have never have other than the obvious things like weak starters, poor batteries, weak glow plugs, or compression understood the possible cause until reciently.

That is some of these old cars still might have a fuel supply system that is so tight it retains pressure. If so on the first crank full fuel quantity is delivered to each cylinder at the right time as well. Probably just an unobtainable fairy tale yet maybe we will find out one way or another soon.

So I went ahead and checked fuel pressure before the secondary filter and again found it to be 12 psi at idle. At cruising speeds it increased to 13 psi and dropped to 11 psi at high RPMs and during hard pulls...so effectively I have a 1 psi drop across the filter, which is what I expected.

I than proceeded to clamp off the return line with a pair of vise grips and again check my pressure. Again it was 12 psi, but it was getting dark so I didn't get a chance to test drive it again. I figure if there was going to be any kind of increase I would see at least some while at idle.

Whats funny is my car seems to have plenty of "get up and go" for a 240D. I took my buddy for a ride and he was very suprised and said that it was not far off from his 300D. I myself have been suprised by this car...I mean it does have some blow-by and I have yet to do a compression check(don't have a gauge that will read that high) but it seems to have plenty of power for a 240D. I was expecting it to be a total turd after reading people's comments in regards to these cars.

I would like to hear from someone who has been able to get 19 psi base pressure...will it truly give me power/milage gains??

Also, where can I order a repair kit for my lift pump? For $10 I say it worth a shot!

And now a simple explanation of why your car may get crummy fuel mileage... Have you considered that your odometer is slipping? I've never been able to get my 300D's mileage because of that fact. (2 different speedos too) Check your odometer when warmed up (that's when my slips) against mile markers. If it is less than the markers... then you know the problem. The resolution is to have the odometer gears fixed to the shaft.

__________________
Pete Williams, President emeritus - MBCA-International Stars (Eastern Mich.) Now Wi. & Az.
1985 300D Manual 4 speed Green Hen's tooth
1979 MGB triple black Az car nice
1969 back 1/2 of MGB (Az home built trailer)
1991 Volvo 245 Wagon Nice AZ Restored Gold Brick
1983 Jeep CJ7 37,600 Miles Summer use only

The site sponsers sells the lift pump kits regularily. There are complete long read threads like cervans on the pressure importance and effect in the archives. Sounds like your lift pump is on the way out. It no longer seems to be providing enough pressure to cool the injection pump even. If it were an obstructed tank filter or fuel line I would think the pressure would have further fallen under load than it did.

Even though the benifits are well known there are only a very few members that have put a gauge into the system. This is hard to understand by me. Many have just stretched their springs with no guidance at all.

With a decent amount of fuel prefferrably a full tank. Disconnect the fuel line hose from the steel fuel line. Fuel should flow out at a reasonable rate. . This indicates of course the tank screen is not covered in junk. Or the supply line is not gunked up. Kind of a verifying test.

For example when you send your injection pump out for a rebuild it must include the lift pump. Most injection pump repair/ rebuild shops today will set the relief valve at 19 pounds pressure. Plus make sure the lift pump is providing enough pressure to easily overcome the relief valve.

Right now you are not making enough pressure to even overcome or open the relief valve. So the injection pump is not getting a flow of cooling fuel through the core area. Remember you eventually do not have to stay at 19 pounds when you are capable of getting there. You probably will though.

If your gauge is fairly accurate that lift pump was going to become totally useless at some point possibly in the not too distant future. To make sure your gauge is not way off just remove the hose from the relief valve. No fuel should be passing through it at all right now when ideling. I hope others will post on your thread with their experiences.

Remember there are very few that used the gauge approach so far. Although there was another one only a few weeks ago. He had a signifigant improvement on a five cylinder. The pressure spread from what he had to what you would have was very simular. I will look up his thread and post where it is at the bottom of this page.

Twelve pounds is probably reasonable to what many actually have. It sounds like your friends 300d could stand a little attention as well. There is no way you could do well against my 1977 300d non turbo. I also have two 240ds. Plus a 1983 300d turbo.

It looks like you should question the archives quite a bit unless other individuals come forward.

Is this the clear hardline? If you clamped that with vice grips my guess is it was not totally shut (because it is too stiff) and that's why your fuel pressure did not go up.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Yes, it was the clear line and it was a little stiff, but I made sure I clamped it good. Even if it wasn't completely clamp off it definately should have been enough to see even a minmal pressure increase.

Over the years someone did replace the return lines on the injectors w/ some clear lines. Shouldn't there be a good "flow" of fuel through these lines?? There doesn't look to be very much.

Those lines are nylon, too stiff to be clamped shut without destroying them. Even the tiniest flow through the clamped line will result in no pressure increase.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked