Benz is running rich, blowing black smoak, sometimes runs fine other times runs rough and struggles to accelerate, any suggestions?

Asked by Apr 18, 2012 at 07:07 PM about the Mercedes-Benz 560-Class

Question type: Maintenance & Repair

I have a '90 560SEL that is running rich, blowing black smoke.  Sometimes runs fine, other times runs rough and is difficult to accelerate.  Economy gauge under speedometer pegs to right when running rough and only moves when you're completely off the accelerator.  I want to perhaps rebuild the carb. but can't find any instructions or helps, and the nearest Benz dealer is 400 miles away.  Any help, or ideas as to what else might help would be appreciated.

1 Answer

12,805

Yes definitely running rich. Don't run it too much or you will plug the catalytic converter. I would get your self a Haynes book for your car and troubleshoot the fuel system. Here is a good and inexpensive way to trouble shoot your engine for most major problems. Get yourself a vacuum gauge at auto parts store (about $18). Here is a good reference for vacuum trouble shooting.......... You should get between 17-22 in. hg. depending on your elevation above 2000 feet MSL or mean sea level. To perform this test hook vacuum gauge directly to manifold vacuum. 1. Low steady needle usually indicates leaky intake, a leaky vacuum hose, late timing, or incorrect camshaft timing. 2. If reading is 3-8 inches low and fluctuates at that low reading suspect an intake manifold gasket leak at intake port or faulty injector. 3. If the needle has regular drops of about 2-4 in hg. at a steady rate the valves are probably leaking. (Perform a compression check). 4. An irregular drop or down flick of the needle can be caused by a sticking valve or an ignition misfire. 5. A rapid vibration of about four in hg. indicates worn valve guides. 6. Slight fluctuation of one inch or so indicates ignition problems. 7. Large fluctuation perform a compression check. 8. If needle moves slowly through a wide range check for PCV issues. Hope this gives you a good place to start and helps you figure it out.

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