W124 Idle
 

Hello there, I recently took my 1989 300E W124 to the mechanic. I got it back and everything seemed great it drove great it was silent and it all was well. What I've noticed and I told my mechanic was that my cars rpm will vary depending if the car is cool or hot. Meaning when I turn on the car it is all good rpm are great and the car is driveable. After like 15 minutes of driving the cars rpm start to drop. They are supposed to be around 9k rpm while in Parking or Neutral and around 7k to 6k when in Drive or Reverse. Like I mentioned after about 15 minutes of driving my car starts to drop the rpm to the point where it just cant stand being on and it shuts off.

The rpm while in Parking and Neutral drop to 5k rpm and while in Drive or Reverse they are super low, the car cant stay on and it just shuts off. Is there anything the mechanic could have missed? Is it a quick fix? Ive been without my car for a long time now and I just want it up and running.

I can have the car on if I press on the gas pedal and stand on the break at the same time. That way I can get the rpm up to 9k rpm. Another thing I noticed was that my car takes a while to turn on! If I barely tap the gas pedal it turns on right away.

Any quick solutions for these problems?

You've got the CIS-E fuel injection system on your car.

As these systems age, small vacuum leaks occur and the idle can drop + you'll get rough running at idle. When the car is cold, there is an enrichment circuit, which masks the lean out condition.
If you put your car in gear, hold the brake, and lightly step on the accelerator until you see 650-700 rpm (not 6k or 7k as you mis-stated), does your car smooth out?

There are two quicky adjustments that can be made to compensate for the air leaks.

One is to adjust the CIS unit itself. There is a tower between the air intake plate and the CIS head which when turned clockwise, will richen the mixture. I think it's a T-15 torx. Some still have the adjustment hole capped off, once opened, you push down against a spring, then turn slightly, maybe an eighth of a turn, and see what effect it had on the car.

The EHA can be enrichened also. Remove the EHA (two bolts), be careful system is under fuel pressure. There is an adjustment under a screw that you can turn CW to enrichen.

If you don't have a decent understanding of the CIS system, you should have this done by someone who knows how the system works. It's easy to overdue, small adjustments make a difference. Too much and your fuel mileage will drop precipitously. Do some research online and you'll find lots of info on these two adjustments.

This is a band-aid approach to a system problem, but it's a low cost adjustment that can make a big difference in the idle quality.

I just did this on our 560SEL, made a significant improvement. I'll probably tweak it a bit more when I see my buddy with an O2 sensor next week.

Jim