I have a 1990 300CE-24 200,000km, clean, perfect - except that hot or cold, warm or cold weather, intermittently, car starts immediately upon cranking, then stumbles - sounds as if it will stop (has stalled), runs at slow RPM for 3-6 seconds, then resumes normal slightly quick idle.

If I switch it off & re-start, it will start normally.
Leave it for 10-15 minutes, then stumble idles again per above.

Other symptoms:
intermittent sluggish take off from stop start - hesitates for a second while depressing accelerator - then launches suddenly.
engine stumbles at high revs (4500rpm) while still accelerating at high speed.
I am using high octane premium fuel only. The fuel consumption seems OK, although I've only had the car for a week and don't really know what's normal.

Mechanic has:
1. replaced idle air control valve (tested old one and said it was knackered?)
2. new spark plugs
3. new air filter
4. oil change & new oil filter
5. additive put into oil to free up sticky valves (possible cause?)
6. added injector cleaner & other fuel additive to absorb possible condensation in tank

$1000 later, problem has not gone away, and seems worse.

Any thoughts please?

 

problem ranges from air flow meter sensor, electrohydraulic actuator and fuel system.

 

Thanks sleepwalker.
What is the electrohydraulic actuator?

Do you think from my description that the problem could be a leaky injector(s), noting that the idle problem is most apparent after the car has been sitting around for a while.
That is what the mechanic is checking into next. Is this an easy test, is there a schroeder valve on the fuel rail that can hook up to a fuel pressure gauge and check for fuel pressure loss after pump has shut down.. or is it not that simple to check for leaky injectors? I would hope that the mercedes "expert" working on my car knows how to do this.

It just seems like alot of trial and error. I was hoping to get it diagnosed and fixed quickly without throwing more parts at the car that it doesnt need.

 

doesn't that car have plug wires? sounds like a wire with high resistance, only fails under load. check for burn marks around the plugs and on the wires.

 

Wires were resistance checked be mechanic & checked out OK, so he tells me.
Injectors checked today and found to be OK.

Any other ideas, anyone had a similar problem?