At normally aspirated levels how fast would the engine be able to accelerate the car to?

I'm asking because this could rule out my turbo seizing as well.

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

This is pretty easy to test. Unplug the hose to the ALDA, and go drive the car - you should notice a significant power difference, especially above ~2000rpm. Running properly, the 0-60 time is about 12 seconds, with the ALDA off it may drop to 16 seconds or so? If you unplug the ALDA and the car is the same either way, something is definitely wrong.

You can rule out the boost by connecting a boost gauge and going for a drive. I use the pressure gauge off my MityVac, and connect a length of hose out the window, and the hood seam, so I can watch it while driving. You should be getting 10-12psi max.

Even if the turbo WAS seizing, I don't see how it could cause such a power loss that the car won't get to 20mph...??

Alright, I finally got to messing around with the car, and I disconnected the vacuum supply to the butterfly valve at the front of the intake manifold then plugged the line with a golf tee. I got the idea by watching the valve as I revved the engine at idle, and noticed that it was opening and closing (which I didn't think it should). I have yet to drive it like this, but I can't see why it would hurt anything, or even why the valve is there. My question to you guys is: what is the valve for, what controls it, and can I drive it like this. After disconnecting it, I revved it up, and the valve didn't move from the open position, but the engine didn't run any different. I figure that a valve that cuts off intake supply, if applied at low rpm's would keep the engine from being able to go over a certain rpm, and never build boost, which seems to be my problem!

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

that valve doesnt cut off all of the air, just most of it, and if you listen to the engine carefully as you open and close it by pushing on it, you can hear a difference in the engine.

that valve is there to assist the EGR to put more exhaust into the intake, because when the turbo is spooled up, the pressure in the intake is almost the same as in the exhaust manifold. so when the EGR opens, that valve closes. they are both controlled by the engine computer.

its pretty easy to check the trouble codes, the service manual says you need an "impulse counter", you dont, all you need is a multimeter and a short piece of wire (count the impulses with your fingers!). i recommend an analog multimeter or voltmeter, its easier to count the pulses.

there is a thread that details how to do this somewhere, i havent done it in a while, so i dont remember, but youll need a service manual to know what the codes mean (or you could just ask us). im guessing there is a problem with your computer, mine went out on me just recently so the turbo wastegate valve is always open. dealership wanted $2200 for a new computer and to put it in, but im gunna try to find a used one.

hope this helps.

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1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350

Grdenko,

First, so by having the intake air valve disconnected, the computer will act the same as if the EGR was not working, and cut back on power?

Second, would a bad computer act intermittently, and what trouble codes am I looking for? Is there a trouble code for bad computer? Wouldn't the check engine light be on?

One last thing, does your intake air valve open and close when the engine is revved in park?

Thank you for the response, not even my expert diesel mechanic new what the valve was for.

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

youre welcome, and i learned it all right here in this forum.


first, no, i think the computer only cuts back on power if the EGR itself is disconnected, but im not sure. the computer cuts back the power by opening the wastegate valve on the turbo. in my case, the computer is not sending a signal to the EGR (because the computer is messed up so they tell me) so the EGR doesnt open, which makes the computer think that my EGR is disconnected, so it opens the wastegate, which releases the turbo boost pressure. if you have the air intake valve(it has a real name but i cant remember it) disconnected though, and you didnt plug the loose end of the vacuum line, you will lose all of you vacuum which would also cause your wastegate to open.

second, yes, its possible that the computer works right only sometimes but not always. if your trouble codes are reset somehow, the compuer might think everything is allright, but then a few miles down the road, the computer might think something is wrong, and cut power somehow. thats how mine acts. but i dont think you have the same problem as i do. my car is alot slower, but i can still always get it over 20! the same computer controls the fuel injection (i think its actually called the fuel injection computer in the parts list) the computer senses the amount of boost pressure and increases the fuel supply accordingly, throught the ALDA (i think) which sits on top of the Injection Pump (on the 602.xx you cant see it unless you take the intake manifold off). so, the computer does have the ability to control fuel supply to the engine, maybe to reduce it too. the reason i think its the computer and not the ALDA itself is because the problem is fixed when you shut down and restart the car.
youre really looking for any trouble code, i think theres about 15 of them. one pulse alone indicates that nothing is wrong, but i cant remember the others. the code will tell usually tell you where to start looking for problems, but if the computer is broken, the codes might be confusing. like my code told me that my EGR was disconnected even though it wasnt and i had just gotten new vacuum transducers, which only solved the problem for a few miles down the road.
I WISH there was a 'bad computer' code, but there isnt. it would have saved me some money.
My '91 doesnt have a check engine light, so i dont really know about that. i do believe that fi you have a 'check engine' light it comes on if there is a trouble code present.

I just read gsxr's post, and i think hes right, the computer could only cut the fuel supply down the normal-aspiration levels, which wouldnt be that slow. but if the computer really is messed up, who knows.

my air intake valve only closes a little bit in park and only when its way revved up and the engine has been running for a while. from what ive read, thats normal.

before you check the computer, you should check the throttle traveler that connects to the cruise control and IP. it could be that the screw on it has come a little loose and doesnt catch all of the time. i wish i could describe this to you better, but i think you know what im talking about. ive forgotten what all of the real names of these parts are. you should also check the vacuum if you havent already. both of these are easier to check than the codes are, but you need a mityvac to check the vacuum.

sorry to be so long winded, but ive lost alot of sleep in the past over this stuff and i know way too much about it from the hours of research on this forum and in my service manual.

later

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1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350

This is an overlook the obvious type question. What about the screen in the fuel tank?

Is there an easy way to check the screen in the fuel tank, or does one have to drop the tank and peer inside?

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

A simple (SHORT TERM!) test is to swap the feed & return hoses under the hood, as long as you have at least a half-tank of fuel. If the problem disappears, the strainer may be clogged. The return line has no strainer and therefore would allow normal fuel supply. Don't forget to switch the lines back or you'll "run out of fuel" with a quarter tank left, or something like that...

Here is a product to help find minute fuel leaks.

Frequently Asked Questions About Vehicle Leak Detection

Well...throughout trying to fix this problem, I also replaced an injector which was leaking fuel. This was about two weeks ago, and right after I replaced the injector I noticed what I thought was lifter noise had stopped considerably, and since then have also not experienced the major power loss problem. The car also starts alot easier when cold. Could this one injector have been my big problem??? Also, I measured my 0 to 60 time, and when the thing is running properly (or at least what I think is properly) it gets there in 14 seconds... is this what everyone else is getting, I know someone before said 12 seconds?

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

Here's wishing you some luck that you found the problem.

The PO told me to expect my car to occasionally do what you described . That was the only problem he knew about as far as running/reliability. The car did it occasionally until the first time I removed the injectors and lines, at the same time I replaced the primer pump. After I got the air bled out of the system it never did it again. Our cars are very different and comparisons are probably next to useless but in the most general terms.

I have removed the injectors twice since then and until I get the air bled out of the system, it will act like that. Once through the gears and the air is gone.

Grdenko: I have been following this post because I have been experiencing a loss of power on my '95 S350. I have been led to believe that my problem is related to the "Electronic Diesel System Control Module". This the proper name for the engine computer, referred to as EDS or N39. I have been contemplating replacing the computer but I have had some apprehension. I contacted a Mercedes tech for advice and I would like to share it. Perhaps someone can find out conclusively if a used computer can be easily installed. Below is a summation of my comversation with the tech:

[ Yesterday I went to Mercedes and the parts guy said that the '93 and '95 use the same computer. I wanted to swap them and see if I transfer the problem to the '93. As a precaution I phoned joeym7 in Mt Sinai for advice. He said that he has changed computers in all models of 140 cars. He never installed a used computer, only new ones at the dealers. He said that before removing the old computer they plug the "Hand Held Tester" and downloaded info from the old computer, then removed it and installed the new one and fed the stored info into the new one. He said that after 40 drive cycles the computer locks into that particular car and can't be used in another car. If placed in another car the car won't start. This was done to prevent altering the mileage. He isn't 100% sure about this feature on the Diesel computer. If the mileage info is stored in the engine computer then it would also have this feature. The S350 uses a digital odometer wheras the '93 has an analog odometer.

So I guess I had better leave the computer alone and swap out other parts. He said there was problems with the air flow senser and the switchover valves below the air filter on the diesels. I'm glad I called him before removing the computer. ]

I have a '93 300SD that always develops full power and wanted to swap computers with the '95 to see if the problem is with the computer.

I finally fixed my '95 S350. The loss of power was due to the blocking off of the EGR valve. I removed the stainless steel plate and re-assembled it with a new factory gasket. I removed the manifold and crossover back in June for a thorough cleaning. Thats when I installed the blockoff plate. Now the EGR is operational. The car now develops full power at all times. The EDS control module (engine computer) is now happy with the arrangement and doesn't cut off the turbo boost.

Don't be too quick to blame the computer. Good luck with your car, let us know how your problem is resolved.

'93 300SD 70,000 mi.
'95 S350 120,000 mi
'99 S600 103,000 mi.
'93 Chevy Beauville van 237,000 mi.

You can still disable the EGR on your S350, but it takes more work. You have to fool the computer electrically - OR, replace the turbo with one that has a pressure-operated wastegate, not vacuum. Search this forum for details. "OldSouth" (?) did this on his S350 and it worked.