Engine Idle issue
 

I have 1984 Mercedes 500SEL W126.

My engine is acting strange these days. When I the car is on idle, the oil gage is slightly moving up and down in between 1-2 and the engine is not running steadily. I changed distributor cap and roter because of that but the problem is still there.

What could be the problem?

My first guess would be the rubber pipe work and the air idle control valve. Check the pipework is in good condition and fitted nicely. Check for air leaks.

What kind of RPMs are you idling at? Does the oil pressure rise when you raise the engine RPM? You are running the correct weight and quantity of oil? Be aware that the oil pressure gauge in the W126 is electronic and the sender is known to fail outright or go erratic with age.

My erratic idle on a 300TE ended up being a combination of an improperly installed injector boot, cracked and leaking idle air control hoses, old injectors and leaking seats, a loose HVAC vacuum hose, a bad EHA and a bad O2 sensor. You can spray around the intake area with MAF cleaner to see if idle changes. If you spray an area and the idle increases, you have found a vacuum leak. If you have no vacuum leaks, check to see is the EHA is seeping and/or you have a fuel smell coming from under the hood when you shut the engine off. If the O2 sensors haven't been replaced in a hundred thousand miles or so, that may well be the culprit. Those are fairly cheap, if you buy the Ford models and splice the wiring harness.

An '84 M117 engine is not going to have emissions controls. No EHA, no O2 sensors, and none of the stuff that usually gives issues on the US models post-'85.


This engine will have the basic K-jet system and has different places to look for issues with idle control.

MB gasoline engines delivered in the US from 1981 onwards have been equipped with either K-Lambda (81-85) or KE (86-91). Both versions have exhaust O2 sensing and feedback control of mixture to provide a stochiometric mixture to the catalytic converter.

500SEL is not a US-spec car. OP never said where they are either, this may not even be a US poster. If they are in the US, it would have had to be converted to have the Lambda feedback circuitry. Assumptions can get you in trouble.

Beginning in 1981 the 3.8L M116 (SL, SE, SEC, SEL) was imported by MBNA. In 81-83 the 5.0L M117 (SL, SEC, SEL) was brought to the US in substantial numbers in gray market vehicles. In 84-85 MBNA got wise and the 500SEL became a US spec import.
The OP's car is a 1984 500SEL.