I have a 1994 S350 Turbo Diesel with just over 200,000 miles. I been having a problem with the turbo boost for several years, during which the problem has become progressively worse.

When the engine is cold, it seems to boost normally. After driving for a while (30 minutes when the temp is around 0, maybe 5 minutes when the temp is 40 and above) the turbo stops boosting. If I pull over and shut the car off and immediately restart it, it will boost near normal for a minute and then boost somewhat for the rest of the drive (apparently regardless of the remaining duration).

I've removed and cleaned the EGR valve (it seems to operate smoothly). No help. I've also removed, cleaned contacts, and re-installed the ECU. No Help. I'm wondering if the MAF sensor could be the culprit.

I've taken it to the local Mercedes Dealer more than once but they have no idea what the problem is.

Any suggestions?

Sounds like the comptuer is going into limp mode. As I recall there's a way you can get blink codes out of it, that would be my first troubleshooting step.

-Jason

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

Oh, on a side note, what casting number is your cylinder head? Does your car have its original engine? Any oil consumption?

Replace the vacuum actuator on your turbo with a normal wastegate actuator and it will remove the computer's influence over the engine. You'll also be able to delete the EGR too.

+1. This is what my turbo did as well. The computer sensed a fault in the system and cut all boost, until you cycled the ignition again, and then it would just repeat itself...

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93 300D 2.5 Turbo, Black/Palomino 273K
09 E350 Black/Black 41K

Thanks for the replies,

compu 85: by "limp" I hope you mean limp to the repair garage, as opposed to...never mind. I did not check the engine numbers but I believe its the original motor. I've read the horror stories on this forum about the engine (rod bender) and other major defects affecting this particular model. Its had them all except for the bent rod and cracked head. It uses about 0.5 quarts in 3,500 miles, most of which I think is leaking from unknown locations at the front of the engine. Its been doing it for years. The compression was checked about a year ago by the Mercedes dealer, and was reported to be holding equal compression across the 6 cylinders and was well within specs.

ForcedInduction: I would love to loose the EGR valve. I saw its handy work last spring when I installed new glow plugs. The intake manifold was choked with coke. I tried just blocking off the EGR, but the MAF ratted me out to the ECU which killed the boost.

When you say "Replace the vacuum actuator on your turbo with a normal wastegate actuator", could you be more specific? I know the 350 and 300 are nearly identical, and the older 300 turbo lacked a computer. Are you suggesting using the vacuum actuator and wastegate actuator from the older diesel?

The EGR delete will CAUSE this problem! Do not do it!

Check the obvious...mine was doing the same thing for the last week. I finally got in and checked the EGR vacuum hose. It had failed and was preventing the vacuum signal from getting to the EGR valve. No EGR valve actuation -> not pressure change in the intake -> computer sees a fault and turns off the turbocharger until you get above 3,000 - 3,200 RPM.

The reason it runs fine cold is that the emission programming will not try to actuate the EGR valve until the engine has warmed up -> it does not see the EGR valve problem.

Make sure you are getting a good vacuum signal to the EGR valve.

-Jim

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1995 S350D, Green with black leather interior.
Bought January 2008 w/ 233,xxx miles.
I did 22,000 miles during the first year of ownership.

There will always be something to confound EDS. I'm with FI, cut EDS out of the loop by fitting a proper boost actuated wastegate -

http://www.peachparts.com/shopforum/showthread.php?t=212097

Hardly matters if you disable EGR now since you're not getting boost anyway.

If you want boost in the meantime and have a responsible left foot, connect vacuum directly to the wastegate actuator so it stays closed. There will no protection from overboost, your left foot is all there is to keep the engine safe. I hope you know what that means.

Two guesses on the oil leak - 1) the front cover of the vacuum pump is loose or has a bad seal, either of which requires a new pump; 2) the big bolt holding the belt tensioner arm to the engine front cover has come loose.

Sixto
87 300D

Just a follow up on my problem. I decided to replace the waste-gate actuator and bought an actuator and boost controller. I've been waiting for warmer weather to install the new actuator.

In preparation for the installation, I wanted to check the travel of the existing actuator as compared to the replacement actuator. With the engine cold (for obvious reasons) I found the point where the actuator rod attaches to the waste-gate control arm and pushed it back just to check the total travel. The rod pushed back with mild resistance and pressurized the vacuum system. I could hear moans and groans from various actuators in the vacuum system. I cycled the boost actuator several times listening for any obvious hissing (leaks) but heard none.

Since pumping the boost actuator back and forth several times, the car has run near perfect for more than a month. The boost has not failed even after long distance runs on warm days and the transmission is shifting better than it has in over a year. Obviously, pumping the actuator knocked a piece of crud out of something somewhere in the vacuum system.

I still intend to change the boost actuator over to a pressure regulated type and remove the EGR value (to stop pumping crud through the engine), but in the short term...what can I say, the stupidly simple seems to work. I rank this fix right up there with the infamous GM "tap test" for their engine control computer (I used to own an 89 Corvette).

Thanks for your post. My turbo was not doing what it is supposed to so I tried checking the travel on my actuator. Unlike you, I DID hear hissing. I kept doing it until I located the hose. There was a vacuum line disconnected up by the spin on fuel filter.

My turbo is back and the shifting is, as the Zohan says, silky smooth!

Thanks,

Jim

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1995 S350D, Green with black leather interior.
Bought January 2008 w/ 233,xxx miles.
I did 22,000 miles during the first year of ownership.