W124 surging idle

[P Cross]

I'd be grateful if anyone could provide the solution to a problem with my '92 200e W124,the tickover swings by about 75-100 rpm ( roughly 750 to 800 rpm ) when warmed up and slightly more when cold,I don't want to start replacing parts needlessly if I can. I've replaced the spark plugs and airfilter(which made no difference at all). Judging by the diagrams in the Haynes manual it's fitted with the Bosch HFM sequential fuel injection system. The previous owner had'nt changed the airfilter in a while by the look of it. There's a small plug (with wiring into the loom) in the plastic ducting from the airfilter housing to the injection which when removed has what looks like a wire 'A' with a bead on the tip,does this read air flow and if so could it be effected by dirt sucked in through a dirty filter. I thought about cleaning it with carb cleaner but wimped out in case I ruined it. PS Huge forum isn't it,didn't know so much could go wrong on Mercs.

 

[mike65]

The number of posts is more a reflection on the fastidious nature of Merc owners and the fact many cars are now well into middle-age not to say dotage!

As for the hunting idle (I would'nt call it a surge) its often the result of an air leak in the vacume

here's a list of things to check

Ignition wires for shorts or faulty insulation
Ignition system for moisture, dust, cracks, burns, etc. Spray plug wires with fine water mist to check for shorts.

For possibility of misfiring, crossfiring, or cutting out under load or idle. If present, refer to Ignition System.

For warn distributor shaft
For moisture in distrubutor cap

Ignition pickup coil resistance and connection
Ignition coil connections


Spark plugs. Look for wear, wetness, cracks, improper gap, burned electrodes, and heavy deposits

Injectors for leakage. Pressurize system by energizing fuel pump through the under-hood fuel pump test connector.

Fuel system for restricted filter or improper pressure.

For vacuum leaks at intake manifold gasket

Idle air control system.

Electrical system voltage.

Also check this thread - http://forums.mercedesclub.org.uk/archive.php/o_t/t_6380/94-e420-idles-at-1400-rpm.html

Mike.

 

[bundy]

uneven idle

just try removing the multi plug on the idle air valve with the engine running.
if the idle settles down its time for a new valve.