Rough idle still on 1993 320CE, what could it be? Everything replaced

[EVL124]

OK, so my idle is still off, when car is cold its perfect, as it warms up it gets a bit lumpy - mostly smooth with a slight miss, also when re-starting car when warm revs fluctuate quite badly for a couple of minutes before settling back to the almost smooth but slight misfire every few seconds. Here is what has been replaced/tested - really running out of things to do now.

Wiring loom
All 3 coils
3 plug supressors and HT leads
Spark plugs
MAF
02 sensor
Throttle body wiring rebuilt - also other known OK throttles tested to make sure it wasn't internals
Known good OVP relay tested
Known good crank sensor tested
Known good ECU tested
Known good coolant temp sensor tested

Compression check has been done, one pot is about a bar lower, but nothing to be too concerned about.

What else is there that it could actually be??? This is driving me insane!

 

[kth286]

is the purge valve on inner wing ticking at approx 2 beats per sec when engine is fully up to temp.

it may be stuck in open position.

 

[Vlad]

I notice that there is no mention of the throttle body being cleaned...ie the butterfly flap inside can get a bit gummed up which can cause the surging you mention....common issue on the M104 engine

 

[EVL124]

I notice that there is no mention of the throttle body being cleaned...ie the butterfly flap inside can get a bit gummed up which can cause the surging you mention....common issue on the M104 engine

I have done that myself, and two other cleaned 2nd hand units with good wiring have also been tested.

is the purge valve on inner wing ticking at approx 2 beats per sec when engine is fully up to temp.

it may be stuck in open position.

Has also been tested I beleive and swapped out for a good one to make sure...

 

[WDB124066]

Have you had the mixture checked? Might tell you something?

 

[EVL124]

Not sure - car has been with Harry for past week or so, he is methodically going through everything so it will be sorted - just a matter of how much it will sting the pocket.

 

[WDB124066]

What do you think he thinks it is.......

 

[BlackC55]

Was the whole loom replaced?

 

[EVL124]

Yes

 

[kth286]

perished hose under the inlet manifold or headlight adjust vacuum hose.

 

[BlackC55]

Inlet manifold leak?

 

[Ian B Walker]

How about the fuel pressure regulator at the front of the fuel rail?

 

[BlackC55]

It will be worth checking the fuel pressure from a cold start to see if it is constant.

 

[grober]

What plugs are you running and have you got the standard compression ratio? Your profile mentions Brabus and AMG??

 

[Dieselman]

Possible worn valve guides, but do check the intake vacuum.

 

[EVL124]

Martin, looks you are onto it, its running lean, but only on 2 pots, so as soon is is enrichened its smooth, its not injectors though so Harry reckons worn/damaged valve that isn't fully closing.

 

[chris846]

Another suggestion here for worn valve guide, (before reaching the last post). The valve can intermittently fail to seat properly on tickover. Maybe try another compression test, paying attention to how the pressure builds in the suspect cylinder - you might see the gauge needle 'miss' as the engine cranks.

 

[WDB124066]

I see. The others have valid points, check for vacuum leaks too, makes me wonder if they make it run lean, does anybody know for sure??

 

[Paul64]

Hi Guys,

Just sorted this same problem with my car - I replaced all the HT system late last year but omitted to change the electric shields behind the rotor arms.
Remove the arms and then their drives using an allen key and simply slide out the shield.
You will find some carbon tracking marks on the back, guaranteed.
I put mine back together again and it's pretty much perfect again now - perhaps it's the same with the 6-cyl engines as it is with the V8?

Paul

 

[EVL124]

Have well and truly ruled out vacuum leaks, Harry is 100% sure its mechanical, so hopefully its not too major - valve guide theory definitely has merit. Will be sorted soon anyway, at least we know vaguely what it is now.

Also, fuel pressure reg and injection system is spot on.