Poor Acceleration

[intruder]

I have a 1991 300E2.6 that has POOR/Deayed Acceleration. The car has 101,000 miles, but has had the problem since 36,000. I've been told it is a clogged cat. The car is a pig it the summer regardless of the engine coolant temp unless the car has been stopped and idling (like at a stop light). My mom's old 240D with 5 passengers heading uphill would outrun unless I engage 1st gear and wait the 3 seconds between giving it gas and the car accelerating.
A running vacuum test shows 15psi at idle with a slight cycle of 1 psi at idle that becomes a steady 15psi at 2000rpm and at 3000rpm. The vacuum shows a very slight increase with rpm stable. Vacuum jumps to 20-22psi as I retard rpm and stays at the higher setting if I stabilize the rpm again.
Any advancement of the throttle (rapid or slow) produces a drop-off to near zero with a very slow rise "normal".
The car was tested in park (no-load) and driving (loaded) with similar results. With the A/C on all numbers were lower and it was slower recovering from throttle advancement.
Any ideas what to check next. The car tends to run at 95-100 degrees C in the summer with peaks near 110 when idling.

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Intruder,
You did not say if you have done a compression check, this with the vaccum gauge reading would tell you alot aboout the problem, 15 is low, and suggest late valve timming, chances are with plugged cat you would have the low power but at idle, the vaccum readings would be normal 18-20.
Unless the cat was totally plugged in which caes you would be most likely getting a backfire through the intake on exceleration.
To compare my 86' 300E 3.0 Ltr. gets 17-18 at idle slow acceleration gets up to 22, quick but not stab acceleration gets up around 25-26 then drops to about 10, then quickly return to 18.
Check the compression on all cyl.
Good luck,
Chris C.

[intruder]

Sorry... I did a compression check about a month ago. Although I don't have the figures here at my desk, I do remember checking it against the manual and all were within specs and within 10% of each other.
Also, I recently replaced the ignition wires and coil. I have not checked the timing but I understand that bad timin might account for the minor fluctuations in vaccuum at idle.

First I am assumming you have completley checked for vaccum leaks, ie. checked intake manifold, pinched lines off etc. If your compression is O.K. then I would start by removeing the air cleaner and run it, check to see that the throttle plate is opening fully, and that it is centered in the body. Did your mechanic check fuel pressure? Also the coolant temp sensor could be faulty, or the wire to the sensor could be broken. I would also check ign. timming, although I don't beleave this can be adjusted? I could be wrong, If the ign. timming is O.K. Then I would go ahead and disconnect the exhaust forward of the cat. And run it to see if cat is plugging up, although you should be able to tell w/ vacum gauge... keep applying throttle and vaccum gauge will keep dropping without recovery until throttle is released if plugged. Check w/ cat connected then w/out.
Usually w/ plugged exhaust you hear a kind of low pitched sound from the exhaust, almost a grrrrr on accleration, like your pulling a lot of weight up a hill, and the engine is giving all it's got but going almost nowhere!
Although if it's been this way for several thousand miles you may not have notice a change.
Hey the parking brakes not on? Just kidding!
Chris C.

[intruder]

thanks Chris... I'll re-verify the intake vacuums and check the fuel and the exhaust manifold. I had heard the temp sensor may be at fault; this makes sense. I used to have an '88 'vette that the ECM had a cold mode and a hot mode. During cold mode the ECM ignored inputs like mass air flow, O2, etc.. I assume that MB is similar in setup.
Since there are 4 wires at the temp sensor, would you know which one is most likely to be the culprit?
thanks alot.

Check for Red w/ Green tracer.
Chris

[intruder]

Chris,
Here's a kicker. Last night I loosened the foward 3 exhaust manifolds (I could loosen 1 bolt on the rear three). There was a definite improvement... the car idled at 16 psi with or w/o A/C on. Quick acceleration only produced a drop to 5 psi vice 0 psi with a rapid return and overshoot past 16 psi and then it settled back around 16-18 psi. Being a pessimistic pilot, I re-tightened the exhaust manfold and re-tested. Now it idled with steady vaccum at 10 psi w/o A/C and at 6 psi w/ A/C. Quick throttle advance produced a drop to 0 psi, moderate rise to about 15 psi, and then back to its "normal" 6-10 psi.
I fairly sure the exhaust system has a clog. So the question is: Why the change in symptoms from just loosening and then retightening the exhaust manifold? I can only suppose that the exhaust manifold gasket slipped, further restricting the exhaust.
Oh. With the air intake housing removed the car was very hard to start and coughed a bit before it would fire up. (this was before and after the above changes) The throttle body diaphragm will fully deflect manually and returns smoothly w/o binding.
By the way thanks for your help.
Inruder

[E. Lee]

My 1988 300E has exactly the same problems. Where exactly is the temp. sensor? How do I check if it's alright? I want to make sure everything is fine before taking out the exhaust manifold (which seems to be quite difficult with all the rusted bolts and nuts).

Thanks in advance.

[This message has been edited by E. Lee (edited 09-01-1999).]

Lee,
The temp sensor your lookin for is the last one on the head. (Closest to firewall). It has 2 leads. To check: Use a DVM, cold resistance should be 2.2-2.8 ohms... at 80 degrees C resistance should be 290-370. This according to the book. Now make sure your checking resistance to groud not terminal to terminal!
If you like heres the resistance to the plugs to ground...KEY OFF! Green w/red tracer 4.37 ohms, other one 1.14 ohms. IT'S THE GREEN/RED ONE THAT BREAKS! Usually within 1-2 inches of the plug
The 80C one is a little tuff, it will be different if the engine is hotter... Just checked mine, needle is about 90C and shows 270 ohms, So I guess you can hook it up, watch the meter, when your in range check the temp...Just checked again, I'm at just below 80C meter shows 384 ohms.
INTRUDER...
Your exhaust has got to be plugged! either the main cat or those pre cats or the muffler. No **** one time I ran into a plugged exhaust problem, relpaced the cat, no change, replaced the muffler, no change, Finally I pulled the whole damn system apart and there it was, the exhaust pipe had a double wall construction, and the inside wall had fallen apart and plugged up the tube! Gezzz, what a mess! Anyway yours is probably the cat. Pep Boys used to sell those "test tubes" a piece of pipe w/ flanges on each end to replace the cat, of course for "test" only. The rusted nuts & bolts are a challenge, just spray em' w/ WD40 a day before let them really soak it up, that will help. I always have new ones ready for install.
See ya,
Chris C.

[E. Lee]

Thanks, Chris!

One more question....I can see exhaust gases coming out from the tail pipes. Actually, I put my foot there and feel the exhaust gases coming out from the tail pipes. I've asked an assistant to increase the rpm..and obviously, more gases came out from the tail pipes. So, my question is: do I have a clogged exhaust system? Will a clogged exhaust system prevent gas from coming out of the tail pipes?

Thanks.

[intruder]

Don't get buried in semantics. Clogged and obstructed are different.