92 400E fast idle - where to start ?
 

I've searched the archives, but have not seen a post relating to the idle problem I seem to have. Car is a 1992 400E w/126,000 miles. Car just started exhibiting a fast idle problem. Car starts and idles fine until engine begins to warm up. After engine has warmed to 40-60c, engine now idles at 1200-1400 rpm when in park.

ASR light is not on.

Any ideas on where to begin looking would be appreciated...

Thanks,
Ray

check your OVP relay - in my car (85 190E 2.3) the fast idle went away when a new relay was put in.

I don't believe that the 400E has an OVP. I think that the idle is controlled by the throttle valve actuator, about $900 from the dealer. I just went through this repair on my 94 E420, and can tell you that the throttle actuator is pretty well buried in the engine. However, my car had a very low idle, to the point that it would periodically stall, so your problem is quite different. Is it possible that the throttle cable is out of adjustment? There is a small, black threaded knob that goes around the throttle cable which can be turned in either direction to put tension on the cable, or in your case, relieve excess tension. Worth a try, but probably not the solution, since it would happen whether the car was hot or cold. It almost sounds as if the car is staying in warm up mode, where the idle is high and the car shifts at a higher rpm to aid warm up.

First, let me correct a mistake in my original post:

I should have said that the ABS light is not on, not the ASR light. I am not even sure if my car has an ASR light. I have read in many other posts, as well as in Steve Brotherton's DIY engine controls article that an erratic idle condition could often be traced back to a bad OVP relay or OVP relay fuse. Even so, I state again, the ABS light is not on.

So after reading Screedchan's update, I am unsure as to whether or not my car has an OVP. Plus my idle condition is unlike any I have read in other posts. I do not have a rough idle or stall problem, nor do I have a hot or cold related starting problem - I only have a constant fast idle after my car warms up.

Questions:

Does my car have an OVP ? If so, where is it located ?
Where is the idle control valve located ? Could this be the culprit ?

Thanks for any and all help,
Ray

Fast idle
 

Ray;
Remove the black cover at the pass. rear of the engine compartment, you'll see a control unit with several 10amp atc fuses.
Replace all the fuses and start the car,your problems will probably be over.
M.F.

Please post if fixed
 

Ray,
Please post a reply if Mike's fuse replacement fixes your problems.

Mike,

Thanks for the quick response - I'll pick up the fuses this evening and update this post thereafter.


Thanks again,
Ray

As suggested by Mike Freeman, I just removed and checked the four 10a atc fuses. They all checked good.

Any other suggestions/ideas welcome,
Ray

Note that Mike said to "replace all the fuses" . MB fuses are known to have intermittant problems, so may look good and test good but still be bad . . .

FWIW, on the Ritter/Easley list, Marshall Booth recommends EVERY fuse in the car be replaced as a matter of course when your car hits its 10th birthday, since that's when these funny intermittant electrical glitches start to show up.

Might help, might not, but I'd go to the dealer and get some fuses and try it if it were me.

Thanks for the info, Steve - you're probably right. I'll pick-up the fuses tomorrow and replace them, although I don't think this is my problem.

I'll update this thread with the results....

BTW Steve, does the 400E have an OVP ?

Just curious,
Ray

400E does have a equivalent fuse, but it dosen't have an OVP relay, the fuse is one of the four fuses in the computer compartment. It protects ABS circuit & throttle controller & Cruise control unit.

When that fuse is opened, you will have ABS warning light on all the times, cruise control won't work and your engine rpm won't pass 5500rpm. This happened to me once, 50 cents for a 10 amp fuse solved three big problems, this was the most successful repair I have ever done on my cars.

In your case, that fuse should be OK, since you ABS is still functional, but you can try to replace all the other three fuses since they are cheap. I can't explain your fast idle problem, because myself is experiencing some erratic idle when cold also.

Fuse update - I just replaced the four 10a fuses in the computer compartment.

The high idle problem did not go away.

I should probably also point out that the wiring harness has not been replaced on this car. I have ordered the harness from PartsShop and it should be here in ~2 weeks.

Could this be causing my high idle problem ???

I am open to any other suggestions in the interim..

Thanks,
Ray

Ray,

When was your car built? Be advised, only those '92s built after (I think) 8/92 were equipped with the self-destructing wiring harness material. If your car was built prior to 8/92, I'd be disinclined to start with the harness.

Good luck!

Michael,

I'm not sure when my car was built, but I had a MB indy look at the harness and he said it was definitely defective. He wanted $720.00 dollars P&L to replace it. This sounded HIGH to me, so I thought I would try and do it myself.

Ray

Open your driver's door-there's a placard near the bottom of the B pillar with your manufacture date.