Let me begin by saying that I’ve already searched through past postings, and haven’t found anyone with a problem of this type.

My car is a 1991 MB 300D 350kmi, 5 cylinder turbo charged 602 engine, 50k miles since full rebuild done by Fletcher Jones in Newport Beach, including: a brand new head (all valves and cam), full bottom end, turbo, injectors, and injection pump. The previous owner was a tech there.

The Problem:
It has an intermittent low power problem, that usually occurs when the car is cold, about 30 seconds after beginning to drive it in the morning, but has happened when it is fully warmed up. The car has such low power that it can't accelerate over 20mph on flat ground, and the condition usually lasts for about 30 seconds, and is accompanied by a little black smoke visible from the rear view mirror (about as much as when you really punch it when it is running good). It used to occur only every few weeks, but has lately been happening every other day. I have replaced the fuel filters twice, had it looked at by a few mechanics, and tried different diesel stations but nothing seems to work. The only other problem with the car is a little lifter noise and low power when it is cold, but many have told me that this is normal.
Additionally, while the car is having the problem, there is a slight ticking noise from the engine compartment, and the car seems to vibrate with the accelerator pedal being much harder to push down and almost pulsating. The only possible reason I have heard (from one mechanic) is that the turbo might be seizing against its housing, but wouldn’t this be accompanied by noise, or scarring on the impellor blade and housing (which there isn’t)? In order to stop the problem, I can shut the car down, and then restart it and it will run normally after.
So far, I have replaced the fuel filters, inspected under the valve cover, checked the lifters according to the manual, replaced the glow plugs, checked out the whole intake system for any vacuum leaks or other problems, checked the compression, replaced the air filter, and added synthetic oil. (Some stuff I was just planning on doing anyway.) Also, I looked at the turbo, and noticed a little play in the shaft, but it certainly spins freely, with no evidence of binding.
Where do I go next???

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1991 Mercedes 300D, 350K mi,
1999 Dodge Ram CTD

Check for the EGR valve getting stuck open.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References

is it possible to just disconnect the EGR valve? My understanding is that the EGR keeps cylinder temps down, will disconnecting it cause them to get too hot, or change the combustion timing?

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1991 Mercedes 300D, 350K mi,
1999 Dodge Ram CTD

I don't know about the 91. On 2.5's certainly by 93, and maybe 92 or 91, the computer monitors the EGR and will cut the boost if it is not working. So if you try it and get only 2/3 normal power, then you know the cause.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References

Could you block off the exhaust passage through the EGR? That way the computer would still think it works.

My car was built without an EGR so I don't even have anything to look at.

Edit: ... just for troubleshooting purposes as asked about in this thread. To eliminate the EGR as a possible problem without introducing other problems via the computer compensating.

Not to make this an EGR thread...but doesn't an EGR valve allow exhaust gas to enter the combustion area at highway type driving so that the cylinders are filled with an essentially useless gas that just takes up space, and reduces the internal cylinder temps thereby reducing NOX emissions? If one was to disconnect the valve and drive with it for an extended period, could they risk eating a hole in the top of the piston from high temperatures, or would they simply gain more power from having more room in the cylinder for air? Overall I could care less about the NOX emissions.

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1991 Mercedes 300D, 350K mi,
1999 Dodge Ram CTD

That's why I didn't respond to TwitchKitty. There is so much information about the EGR on this site it's ridiculous. It's all there if you search for it. For now, let's keep the focus on your engine problem. Get a MityVac and use it to test your EGR valve and see if it sticks. This in the absence of anyone suggesting something else that might be the problem.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References

Enrique at Mr. MB Motors in Tarzana performed a flawless rebuild on my 2.5T. He has won praise from many on this board for his honesty and skill. If you remain stuck, may I suggest you take your 300D up to the San Fernando Valley for diagnosis.

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'91 300D 2.5 Turbo 330K
'00 VW TDI Golf, 190K
'67 BMW R50/2
'73 Norton Commando Interstate

I bought the car in LA, but live in the Bay Area (Pinole)...if you or anyone knows of a good mechanic up here, then I would certainly be open to it. Also, I tried an EGR check last night and it seemed to work fine, and wouldn't stick at all after vacuum was relesed. However, I haven't been able to have the problem happen again, and last long enough for me to check the EGR while it is happening. I figure I'll add pressure (to force the EGR closed), and see if it goes away.

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1991 Mercedes 300D, 350K mi,
1999 Dodge Ram CTD

I had a similar power loss problem with my 91 300D, the only difference is mine seemed to run better when cold and get worse as it heated up. The car would cruse ok on the highway, but getting it up to speed was a long ordeal - quite annoying when pulling into traffic! And it idled very rough.

After a throwing a bunch of parts at it and testing everything that you could think of, the advice of another forum member steered me to the injector pump seals. Most of your symptoms make me think it is a fuel delivery/injection pump problem. You may want to find out if the pump was new, rebuilt, or original.

The seals are not very expensive at all and won't take long to replace. Let me know if you want more information.

Fred

1) You can't block the EGR on the 2.5 Turbo engines from 1990-up. The computer knows (from intake air temps, etc) that the EGR is dead, and kills boost completely. You can disable the EGR on these motors but it takes a lot more work - there have been some posts detailing how to do it, use the search feature.

2) The problem sounds like either the computer is getting data it doesn't like, or your injection system has issues (ALDA, etc). The ticking/buzzing and the fact is goes away with a shutdown & restart seems like injection pump or computer to me. Wish I could pinpoint it further.

3) Low power when cold is NOT NORMAL. I'm so tired of people saying that. Yes it will be slightly less than when warmed up, but not to where you're afraid to pull out in traffic. The official MB service docs say that for "low power up to approx 2000rpm" to adjust the ALDA richer (CCW) :

http://www.meimann.com/docs/mercedes/Bosch_ALDA_adjust.pdf

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Sorry I did not make myself understood.
What I posted above is intended to eliminate the EGR as a possible problem without forcing the computer to compensate and possibly introduce other problems as noted in your previous post.

The EGR is not activated on a gas engine until the engine is warmed-up, maybe it is on a diesel. For this reason I doubted the EGR as a likely problem when it was first mentioned.

An intermittant problem like this will be difficult to diagnose. A possible solution will likely have to be tested over a period of time to be affirmed.

The first thing I would look for would be a tiny leak in a fuel line that could let air into the system, tiny, hard to find.

Fred (CSX293): Which injection pump seals are you talking about? Also, would they cause an intermittent problem as they went out?

Dave (gsxr): I'll try the ALDA richer this weekend to see if that cures the low power when cold problem, but would an ALDA cause an intermittent major power loss problem as well? Also, is there any way to troubleshoot or test the engine's computer?

TwitchKitty: I've checked the fuel lines many times for air leaks, but haven't found any as of yet. However, the pre-fuel filter I have is clear, and it has a slight bubble at the top of it...this is normal right?

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1991 Mercedes 300D, 350K mi,
1999 Dodge Ram CTD

The IP seals (full name Pressure Valve Seals) maintain a minimum pressure in the injection lines between shots of fuel. When they fail, they allow this minimum pressure to bleed back into the pump. The next shot is weak because the IP has to both make up for the loss and build up the usual pressure to pop open the injector.

I could see that early signs of failure could be intermittent; heat, proper torque at installation, etc. could all play a role.

The seals and o-rings are about 60 cents each; you need one of each per cylinder. The most expensive part is the special socket for the IP caps and a torque wrench if you don't have one. The correct socket for your car is a Hazet tool - #4556 and is available at the dealer under p/n 617-589-01-09-00 - about 35 dollars. It may be available cheaper through another outlet I'm not sure.

Proper torque and cleanliness is the key. All work can be accomplished with the IP in the car and without removing anything else - just a little pushing of things aside to get the torque wrench in there. I ended up breaking one of the clear fuel lines going to the filter - kind of good actually - they were very brittle and I would rather they broke in the garage than on the road.

I can't say for sure that this is your problem, but mine was a different car after replacing them. Let me know if you need more info. Good luck!

FredP

The ALDA would not cause the major power drop you have. If the computer cut the signal to the ALDA, power would drop to normally-aspirated levels, but not the loss you are getting! If the computer isn't throwing any codes, I don't know how to test it, other than to find another one and temporarily swap them out. I doubt this is the problem but I'm running out of ideas here.

And yes, the air bubble is perfectly normal in the clear pre-filter!