W124 230E Bogging Down When Warm
 

Having just replaced the head gasket on my M102 engine and put it all back together, I now have this problem which I've been trying to work out for a week or two.

1. The car starts instantly from cold, sounds healthy with a nice idle, and revs cleanly. In other words, to all intents & purposes it is as a good Mercedes 4-cylinder engine should be. I think this tells me that fuel pump, fuel filter, fuel relay, fuel distributor, EHA valve, spark plugs, leads, distributor cap, rotor, timing, etc. are all Ok.

2. After a couple of minutes, and as it warms up, the idle starts to hunt up and down, almost stalling but not quite, before revving up again. It also won't rev well anymore, bogging down when the throttle is opened quickly and only revving reluctantly on gentle throttle openings. I've played about with the air/fuel mixture to no avail, and the symptoms are the same whether the air filter is on or off.

It seems that something is not operating as it should as the engine heats up so can anyone make any suggestions? I've thought about vacuum leaks & fuel delivery problems but why would these only appear as it warms up? Any other thoughts would be most welcome.

Thanks,

Andrew

Not much to go on here. But... If your engine has a WUR ( warm up regulator ) valve, it should be checked for proper operation. Warm engine bogging down is most likely running too lean; unless there is a cloud of exhaust, then it's too rich. Look for vacuum hoses not connected or mis-connected. Is coolant temp sensor reconnected after head removal?

Thanks for the responses. First of all, let me apologise for having fumble fingers on the keyboard and pressing a 4 instead of a 2 - the engine is of course an M102 (2.3 litre, 8 valve), not an M104 as per my first post. I've now edited it in case this discussion is of use in the future. The car is a 1985 W124 230E and it's Australian delivered so may not have some of the US pollution control gear.

Thanks for the suggestions - I'm not sure what a WUR so I don't know whether my car has one or not. I've checked & re-checked for vacuum leaks, all to no avail so far, and I'm not sure if a leak would allow the car to run well when cold but poorly once warm.

The coolant sensor switch may be a possibility because all the insulation on the wiring is brittle and I may well have caused it to crack & create a short or poor connection when I moved it all around in taking the head off. I'll get back into the shed on Sunday to investigate this further.

Cheers

For anyone interested, the problem is sorted. I replaced the fuel distributor and throttle body as a unit from a wrecked car and mine now runs as sweet as a nut. Thanks for those few who tried to help.