W202 C200 rough running/won’t idle during cold start warm up phase

[LawrenceMB]

Hi all.
I’m sorry if this has been covered before:
I’ve just bought my Son a 1998 W202 C200 (m111 engine) for a 1st car for his birthday (get him onto the brand early - so hopefully he won’t he won’t just sell my old mercs when I’m gone!) and at cold start it runs REALLY rough and won’t idle. You have to play with the throttle and get it to about 1500 rpm to keep it running until the engine temp gets up to about 60 degrees C and then it runs beautifully (except at VERY full throttle using kickdown to 1st on the open road when it starts to run rough and intermittently clears depending on exact throttle position in the 75-100% throttle range (hopefully he’ll never notice that full throttle fault and will never get there! But it is something I notice on test drive when checking it out - which at the time made me think throttle body potentiometer or something?).
So I don’t know if that’s connected to the fault during cold start warm up fault above but this is the fault I’m more worried about sorting before I pass it to him.
It’s a NZ new spec car so will be the same as UK new in regards to emissions equipment fitted, etc (the all seem to be the same spec I’ve noticed)
Cheers and I hope it’s starting to warm up there in Blighty
Thanks
Lawrence

 

[Dodipops]

Try it with the air mass sensor disconnected, does that smooth it out?

 

[LawrenceMB]

I will try tomorrow and come back to you. Thanks mate

 

[noogieman]

You're a good father buying your son a Mercedes.
It means you care about your son.

 

[W1ghty]

Can you call AA/RAC if you have breakdown cover ?

 

[LawrenceMB]

Yeah he loves it too! He was so surprised. Still quite solid models those w202 ones. Front and side airbags, AC works good for clearing the windscreen and quite narrow A pillars on w202 too so not massive blind spots. Plus it means he’ll get a taste for my W202 C36 so will hopefully keep it in the family in years to come! So not purely selfless reasons!!
And yes he just turned 16 so can get junior AA membership (no RAC here in NZ).
But I don’t think they’re my answer to fixing this fault

 

[noogieman]

A new Bosch air mass sensor would probably do the trick.
This part is known to fail.
Buying from dealership is steep, buy genuine from autoparts store for lot less.

 

[LawrenceMB]

Ok. Thanks noogieman, I actually tried unplugging it last night to see if it improved the issue as suggested by Dodpops above, but it still had the same issue on its cold start, although I do wonder if it was ever so slightly improved (it could've been just my wishful thinking?) but it still wouldn’t idle without me getting the refs up above about 1200-1500rpm.
Should I have had a more definitive result by unplugging it and trying the cold start?
Thanks to you both
Lawrence

 

[noogieman]

This car has it been well maintained?

Spark plugs and wires are they OK?
Coil packs are they OK?

Any vacuum leaks from dry rotted hoses around intake system?

Lots of stuff can go wrong on an old car.

 

[Will]

I would get it plugged in for diagnostics.

If it’s running this poor it shouldn’t be difficult to identify what’s wrong otherwise you stand the chance of introducing more issues/variables and shotgun part replacement can get expensive!

 

[jdrrco]

Did a new Air Mass Sensor fix the problem?