Hi,
I have a 1987 260E with 330K miles. Recently it is having acceleration problems, actually the problem has gardually become worse. The car idles fine but as soon as I depress the acceleration pedal, it becomes rough and the maximum rpm I can get is 2500. I looked at the air meter's plate and noticed that it was fluctuating when the acceleration pedal is depressed or blipped. So I checked the vaccum at idle at one of the vaccum ports on the intake manifold. The vaccum level is fluctuating between 3 and 5 mmHg. And as soon as the pedal is depressed, it goes to zero. This sounds very wrong, because based on the vaccum gauge's manual, the idle vaccum should be between 18 to 22 mmHg. The manual also indicates that this behavior (low vaccum at idle and diving to zero on accleleration) is consistent with a clog in the exhaust system or an overactive EGR. I dont think this car has an EGR so that leaves only one possibility, and that is a clogged exhaust system. The car has recently been burning oil at a quite high level, a quart every 800 miles which indicates problems with the valve seals as well as possibly valve guides. The compression test comes normal for all the cylinders. I have tried plugging all the vaccum ports (even the brake booster line was plugged) and the vaccum level remains the same so if there is a leak it is a pretty big one and I dont think that big a vaccum leak is present anywhere that I could see. Is So the questions are: 1. are there any other possibilities that can make the vaccum go so low ? 2. is there a way to search for the big vaccum leak ? 3. is there a way to check the exhaust system independantly ?
Thanks for your help,
Saumil

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Saumil S. Patel

I have an 280 and the intake vacuum at idle is about 15 mmHg, but i do not understand mmHg, what for does it represent?

mmHg is millimeters of mercury, a unit for pressure

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Saumil S. Patel

Standard sea level pressure is 29.92" Hg., which is 760 mm, and actual or corrected sea level pressure in inches Hg. is a common part of weather reports.

Typical M103 idle vacuum (700 revs in neutral) should be about 15.5" Hg., which is 394 mm.

Automatics idling in Drive will show less vacuum.

Duke

I am sorry, the units in my original post should be inches of mercury and not mmHg.

Saumil

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Saumil S. Patel

plugged cats?

george

Yeah, that is what my inclination is also, the vaccum manual does point to the exhaust system clog. I did the spray test to check for large vaccum leaks, used carb cleaner, the only place where there is any effect on RPM is when I spray it on the air meter plate. The RPM decreases slightly. Could worn out valve stem seals cause such a problem ? I am burning a lot of oil, 1 qt/800 miles. This burning could also have hasten the catalytic converter's clogging.

Thanks George !!

Saumil

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Saumil S. Patel

The oil being burned by your engine has plugged your catalysts. You'll need to replace the cats and have a valve job done to the head to prevent it from happening again in 1k miles.

It was the catalytic converter which was causing the low vaccum and therefore almost total loss of engine power (<1000 rpm at full throttle). I replaced the catalytic converter and the power in the engine is back to normal. Now I will get the valve stem seals replaced. The car has about 350K miles but the peak engine RPM is back to what it was when it was new. Thanks to all for their help. Hope this helps others

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Saumil S. Patel