1976 to 1993 Gas Engine Shakes at Idle ( Runs Rough )

4 key performance related issues to consider. Do proper troubleshooting first BEFORE buying parts

Below you will find a typical email we regularly receive concerning idle performance on gas engines. More often that not a customer just wants to know what part to buy to fix the problem. If your older gas engine, starts hard, idles poorly, or accelerates with hesitation the problem may or may not be solved with replacing a part or parts. The problem needs to be approached first with a thorough understand of how to troubleshoot the cause or causes BEFORE you throw parts at the problem. For instance you don't want to order a new fuel pump just because your engine is hard starting. After you read the email below Kent will post some tips on how to go about troubleshooting rough idle problems on your own. 

Problem & Solution

Common among these chassis:

My merc is a manual, petrol W124 E220 from 1992 and has 145500 KM, in
very good condition (no rust, almost full history etc...) I really enjoy driving it around and wouldn't trade it for any recent
model. However something has been bothering me ever since I acquired
it. It suffers from slightly 'bouncy' idle, just like this
https://www.youtube.com/watch?v=P4Qr1hxIU4M only slightly faster and I
get the annoying "shaky cabin" as a result (not super shaky, but still
enough to notice). I'm very attached to this car and have already brought it to a
Mercedes repair shop and they haven't been able to identify the
problem, all they could do is stress that the car has no problem when
driven (no power loss, no jolts, no abnormally high fuel consumption
or anything of the sort). They have tested and ruled out the

- Spark plugs
- Fuel pressure
- Intake manifold sealing
- Air mass sensor

I know from the history that the fuel pump was changed not long ago
before I brought the car, but the Mercedes guys found nothing wrong
with it. And here's also something that may or may not be relevant: the bouncy
idle happens only when the car is fully stopped, ie as long as the car
is advancing (even slowly) in neutral gear, the rpm is perfectly
stable, but as soon as the car stops, it starts bouncing like in the
video... Have you already seen this situation? Is there anything I can do about it?

Thanks a lot,
Lucas P.
Hi Lucas, Your problem is not uncommon especially as these cars age. Sometimes it is not so easy to find the real culprit. Don’t let any mechanic tell you it is ok to have a bouncy idle as long as it runs ok the rest of the time. Should not be so on your engine.
Your problem could be caused by one of four things.
  • 1. low compression on one or more cylinders
  • 2. breakdown of spark on one or more cylinders
  • 3. malfunctioning idle speed controller
  • 4. uneven or incorrect fuel mixture
I would start by doing a compression test on the engine. If your compression is not equal that will cause the rough idle. Once you know your compression is good
you can move on to the following:
Spark next: you said you replaced the spark plugs but nothing about the wires or the distributor cap. I have seen a lot of problems with the caps due to age and wires should be changed every 80,000 miles. Has you engine been put on a diagnostic scope for check for breakdown of ignition spark delivery.
Fuel next: once you have determined your compression and spark are ok I would think about your fuel delivery system. Start with the idle speed controller. Make certain there are no cracks in the intake and exit rubber hoses. If you have a 1986 to 1993 C.I.S. model you will need to understand how the idle speed controller works. See my video below for rough startup and idle troubleshooting. 
If all the above pass ok, then it may be time to think about your fuel injectors. I have a number of videos on Youtube talking about the problem of poor spraying injectors. I have a tester kit that will allow you to remove test and clean your fuel injectors. The injector seals could also be leaking. This is common because of the age (not mileage) of those critical seals. If one or more of these seals are leaking air can be sucked in and the engine will idle unevenly. See related products below:
Let me know what you discover. I could be helpful to other customers.
Kent Bergsma