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  #1  
Old 06-23-2008, 07:03 PM
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Injection Pump air leak and miss fire

I have been going through my 85 300dt trying to get it all fixed up for my father to drive, and I have run into a problem that I can't seem to figure out.

When I first fire the car up in the morning or after its had a few hours to cool down fully, it miss fires. At first I thought it was a bad glow plug or two, but I pulled, tested, reamed and reinstalled the glow plugs. The miss fire was still there.

Then I thought well it must be the injectors, so I built myself a pop tester, pulled the injectors, cleaned and upped/ balanced the pop pressure to about 140bar + or - 3 bar due to inaccuracy of the gauge. The book says new injectors can be from 135 to 143bar, so that checks out. They all had very nice spray pattern and pre-injection squirt. Put the injectors back in with new heat shields and still had a miss fire.

OK maybe the pump timing is off, so I set that to 24* and still had a miss fire.

Then I noticed that the clear return line from the injection pump has air bubbles in it and when I rev the engine a lot of bubbles appear. Now I've checked the lift pump and there is no air being sucked in from the line that comes from the tank and there does not appear to be any air in the line from the filter to the injection pump. I even took out the check valve/pressure reg for the injection pump and stretched the spring to 26mm as per the manual, still air. So where is the air coming from.

I've read through old posts but no one really seems to have the same problem. Anyone have any ideas?

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  #2  
Old 06-23-2008, 08:05 PM
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Well the pump is under about six pounds fuel pressure I estimate so it is not likely to be sucking air in through any of its body parts. Actually I believe all components past the lift pump are under pressure. Did you submerge the return line in fuel in a container to verify the transient air? Forget the last statement as you are seeing the air bubbles in the clear return line.

I imagine in your case the primer pump might be a source or perhaps at the output area of the pre filter. It is unlikely to be past the outpit of the lift pump. Since the transient air bubble stream is not visable in the pre filter it tends to narrow things down a lot.
I have wondered about an easy test for the primer pump. The best so far I can come up with is a balloon or condom over the pump handle if it fits the pump shell quite tight. Get as much air out of it as possible. Run the car for 15 minutes or so and recheck for bubbles. I guess if the aerated fuel symptoms where gone you might consider the test conclusive. With your heavy air leak you might almost see the balloon deflate.

This of course makes no allowance for a base air leak on the primer pump. Generally speaking the odds of one there are slight compared to the top section of the hand pump area. Or something like a crack in the output area of the pre filter is the other thought. Could even be a poor hose pre filter seal I imagine.
Correction, I just read on another post the injection pump return relief valve is factory set at 14 psi. I cannot verify one way or another at this point.

Last edited by barry123400; 06-23-2008 at 08:31 PM.
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  #3  
Old 06-24-2008, 11:14 AM
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Well I've gone through the fuel system and the air is coming from the injection pump. Now the bubbles are very small and are now only at when I rev the engine. I'm thinking that the bubbles maybe air that is dissolved in the fuel and when the higher rpm = higher flow rate of fuel, the tearing of the fuel as it goes by the pressure reg (ball and spring) causes the air to come out of solution, thus forming small bubbles?

I have also noticed the the engine smokes a lot more now then it used to. I thought maybe that was because the injectors were at ~130bar and are now at ~140bar and that the injector timing should be advanced slightly to account for the delayed injector caused by the higher pop pressure. So I set the timing to ~26* and still have a lot of nailing and some smoke (blueish black). Anyone else have smoke issues after cleaning and pop testing injectors?
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Old 06-24-2008, 11:23 AM
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a video would help alot. Advanced timing will cause harsh starting conditions. black smoke means partially burnt fuel, white smoke means unburnt fuel, blue means oil. Valve stem seals? your valves could be too tight, and that will cause hard starting too.
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  #5  
Old 06-24-2008, 08:02 PM
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You sound certain enough to want to talk to a pump guy. I still feel air cannot get into the pump when it is under pressure by external leakage. Yet in my mind there is one more possible source. Again I am not a pump guy remember so make allowances.

A leaky delivery valve is what I am thinking as the source of the air. As the pump element pressures up the fuel is passed by the delivery valve. As that piston retracts it normally is going to separate from the small amount of fuel left in the cylinder. As the delivery valve has slammed shut. Creating a vacuum as the small amount of fuel remaining cannot change its volume. This creates an air bubble by intense vacuum.

Normally this would not be a problem. The elements piston at the bottom of it's stroke opens an intake port and the vacuum actually aids filling the cylinder again. It has to be this way or the cylinder would not fill at the operational speed it operates at.

Now if a delivery valve. A fuel check valve is actually what it is. If it has leakage it may in my opinion overcome or reduce the size of that vacuum bubble somewhat in the cylinder from the leaking residual fuel left in the injection line and expell a portion of it back into the main pump body. Hence your airstream from the return line. Instead of aiding in sucking in the next charge a little air escapes outward through the intake port. That is why the amount of air leaving the pump increases with rpm as well.
Also the pump element delivery valve leaking is not going to service it's injector properly giving other symptoms that you may be experiencing. That injector for example may not be able to pressure up quite right.

May induce an actual late individual cylinder injection time. It's going to be late and perhaps weak and that may be your smoke. A semi proof might be to loosen one injector line at a time and see if any given line being loose stops the smoke. If an individual delivery valve can be changed out at home is beyond my knowledge. It is possible to inspect and clean it probably. There is also a locator spring in there to direct it to its seat properly. It might be broken for example.

I seem to remember some time ago a gentleman having some trouble with his. A previous owner had left a spring or two out actually. So the return valve only seated properly in a very random fashion.

Just some random thoughts to get myself out on a limb again. A pump guy could verify my thoughts right or wrong quite easily I would think. You kind of made me do this excercise as I felt you were sincere and if so there had to be an answer that made possible sense to me. It took a little thought.

One thing for sure is if this condition exists the engine is not going to run well with it. It should not take much for better minds to provide a saw to cut that limb I am out on off. Does this kind of thing make any sense to you at all or anyone else?

We have to overcome or increase our knowledge as these antique systems get older and fix the things we can. It is imperative as the first thing to identify them. Forgive my failings at expressing things well. Usually I am typing as the ideals are forming in my mind.

If by any long shot I am right a chemical cleaner run through the pump might clean that individual return valve. I would still loosen each line in an attempt to identify the cylinder before that. At higher revs I am not sure what effect if any would occur with the bubbles with the injector line loose. Perhaps just eliminate them.

This all is a real stretch for me yet I can not think of any other senario at this moment that could generate air bubbles inside the pump. On these older pumps the delivery valve body seal is copper I believe. If it was bad I think the leakage would be external and reduced fuel pressure for that injector. The newer pumps can leak back internally and defeat the element I believe. Anyways loosen the injection lines and rev the engine one at a time is what I would do. Thats of course if this senario had any credability in your mind.

Last edited by barry123400; 06-24-2008 at 10:40 PM.
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  #6  
Old 09-22-2011, 02:49 PM
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conclusion?
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  #7  
Old 09-23-2011, 12:07 AM
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I have almost certainly traced my similar issue to the lift pump. My in-line filter has a very small bubble and does not take on air... but I can hand prime the system and see air bubbles flow out the return line.

This is the second time I have taken this LP apart to refurb, so I bought a complete new unit. Will update.
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Old 09-23-2011, 12:10 PM
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Couple questions that haven't been asked or offered that might help.

How many miles are on this?
Does it have the old style or upgraded style primer pump?
When were the valves last adjusted?

and one more,

Has a compression test been run?

Not saying its any one of those, just that making assumptions can make you miss where the problem really is pretty easily.

Personal experience Loss of power...briefly

Spent a LOT of time with WHunter on the phone with that and it took many months before the problem was found, and it turned out to be something brutally simple that was overlooked by assuming it couldn't be that.

Readers digest version......when you find yourself face to face with a brick wall and can't get past it.....take a few steps back....take a deep breath, and try to look at it from a broader perspective and not hyper focus on one thing. Been there from time to time....and it helps to try a fresh approach when the one you took doesn't make progress.

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Last edited by boneheaddoctor; 09-23-2011 at 12:23 PM.
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