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1996 S124 E220 - 1985 VW T3 1.9DG Camper - 1998 BMW E36 328i Cabriolet
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662 Posts
Discussion Starter #1
This post is to be added to the pile of 'poor idle' threads that can be found on this forum - perhaps it might help others solve a very common and often an extremely persistent problem with their W124.

This post relates to 4 cylinder M111 engines like mine, and the 6 cylinder units built post '92 which also have VVT (variable valve timing.)

Some get lucky with a simple fix, and of course the simplest checks should always be top of the list anyway. Try a new OVP relay, a quick clean of the throttle body, cleaning of dirty electrical grounds, replacement of old fuses, a new battery, new plugs and wires, a test of the MAF sensor, a new vacuum hose or two, replacement of any damaged or bare wires - any one of these may help or even cure the problem.

If not, it gets difficult and frustrating - and possibly expensive, too. My car has had all the above attended to over the past year and ran very well, but still refused to idle smoothly. In fact, while being good practice for general maintenance, none of the above made any difference at all to the rough idle. I still had a shaking cabin at stand-still and difficulty making smooth low-speed maneuvers due to mildly fluctuating revs.

This problem has defeated many good mechanics - there are so many potential causes, ranging from degraded wiring or bad sensors to throttle bodies and the fuel system, plus engine mounts and many other parts. Diagnosis time will be expensive in a shop, or endless at home, and the danger is that you'll lose patience and start throwing parts at it which as we all know is the speediest route to poverty. Many owners simply end up living with the problem, and then completely forget what a smooth MB is like.

I finally asked my favourite mechanic to investigate further while the car was in for a maintenance service recently. He was initially reluctant, knowing that he may have to bill me for time and parts despite making no improvement at the end of the day. But then, as he was leafing through my service records, a smile crept over his face. 'Come back tomorrow' he said, and he disappeared under the hood.

Next day I was back in the car and firing it up, fearing the worst. But no more shaking, no more surging revs - there was a 90% improvement, and that's before fitting new engine mounts. And the fix? Not one I'd heard of, but my mechanic had been down this road before with 124's, and according to him, it's an issue that's probably affecting many of the remaining examples still on the roads.

As we all know, head gasket failure is common, and most of our cars will have had this job done - mine has, in 2010, and this is what grabbed the mechanics attention in the records. Apparently getting the valve timing right (after replacing the gasket) is tricky, and certainly likely to fox any but those specifically experienced with these engines. Through long experience, Joe the mechanic suspected bad valve timing as soon as he noticed the head-gasket change, and he was proved correct. So for the price of a couple of hours of labour he had set it up properly again with no expensive new parts fitted 'just to see', or endless diagnosis. Also, while he was in there, he was able to inspect the injectors and seals and fit new seals for the timing and cam covers - all good solid preventative stuff.

Fuel economy is marginally improved as you'd expect, and the engine is noticeably smoother through the rev range, but the smooth idle is where the real improvement is felt.

All in all, a great result for me, but I can't help wondering how many other cars with replacement head-gaskets are running around with the same issue. If you just cant get yours to idle nicely, check out the valve timing (with someone who really knows their 90's MB's) and you might finally get that smoothness back...

Good luck

Tim
 

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1993 300E, 2003 996 Turbo X50
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624 Posts
Scary that whoever replaced it didnt notice timing was off. At least you found someone who knew what it was.
 

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Registered
1996 S124 E220 - 1985 VW T3 1.9DG Camper - 1998 BMW E36 328i Cabriolet
Joined
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662 Posts
Discussion Starter #3
According to the invoices, the head gasket was replaced at a Mitsubishi main dealer (who had once been an MB dealership). However, you'd expect a good standard of workmanship from any big workshop on such a seemingly run-of-the-mill job. I think the point of the story is that setting the MB VVT can easily catch out the uninitiated.

I wonder what the previous owner thought about his new rough idle after he got the car back after the gasket job? I'm surprised he didn't hand the car back to the mechanic immediately. He sold it quite soon afterwards, so perhaps he just got scared to dig himself in deeper chasing faults.

I certainly found plenty of other issues that needed attention despite the relatively low mileage. It's been a year since I bought it, and after the last visit to the specialist and batch of small DIY jobs I can finally say for the first time that there's nothing at all currently wrong or in need of replacement. I'm sure this state of grace will be very temporary, but a couple of completely trouble-free months would feel good...
 

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1995 e320 wagon
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165 Posts
Wow, someone was definitely asleep at the wrench when they did that one. Retarding intake cam in its adjuster is a pretty well documented step when setting timing on the vvt engines. Glad its finally sorted out!

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