93 300E - Rough idle after 02 sensor, air mass sensor, OVP relay, wiring harness, etc
 

My 93 300e 2.8 still revs at idle, more than a year after I bought it. This site has been great, and I've tried everything suggested here, but to no avail. I'm hoping someone out there might have some more ideas.....

The Car: 93 300E 2.8. M104 engine. 170K miles
The Problem: Revs at idle, from about 600rpm up to 1200 or so. Its an intermittent problem. Doesn't matter if its hot or cold. Recently it also started idling rough.... the car now shakes at idle. Does this intermittently too.

What I've tried:
- New OVP Relay
- Checked Wiring Harness (was replaced at 99K and is OK)
- New Air Mass Sensor
- New O2 Sensor
- Checked for vacuum leaks, both visually and with starter fluid
- Checked coolant temp sensor; its OK

What I haven't tried:
- Replace engine speed sensor (I can't find this guy!)
- Replace throttle actuator (very expensive, and the car runs great on cruise control, which everyone says is one of the first symptoms of a bad actuator)

My questions for our resident experts:
1) Should I try the throttle actuator? Do these things go bad on the M104 engine?
2) Where is the engine speed sensor? How easy is it to replace?
3) Anything else I should be thinking about? A through search of the archives doesn't turn up anything, nor does a search of the tech manual.

David

I would suggest dist. cap; rotor, but this vehicle may be a bit more advanced and not have these - not sure. What about spark plugs, plug wires?

No, I replaced the plugs only six months ago with bosch platinum 4's. I didn't do the wires, but they looked OK visually. No distibutor cap and rotor in this baby.... if there was, I probably would have replaced it a long time ago. :-)

the spark plug boots do cause problems even if they are visually ok. the ones under the coils particularly are suspect - not the coils themselves but the boot underneath. they cost less than $10 each if i recall. BTW, platinums plugs are considered a no-no on these engines. get the factory spec Bosch copper ones F8DC4 at the dealership (hard to find elsewhere). they are a fraction of the price of platinums and are tried and tested by almost all m104 owners on this forum...

Sounds to me like your throttle actuator is bad, or you have a vacuum leak that is causing the throttle actuator to "hunt" up and down for the right idle speed.

I had a similar problem to yours, but mine was much worse. My idle oscillated up and down so badly the car was not driveable.

Turns out, I had a vacuum leak at the intake manifold that was causing the problem.

We found this out after trading out the mass air sensor (didn't work), then the throttle actuator (didn't change anything), then to the dealer where they noticed the vacuum leak at the intake manifold seal. Yes, I checked for a vacuum leak at the intake manifold, but couldn't find it.

I kept the old throttle actuator, which is still good. Our cars might take the same actuator.

It costs $961 through Parts Shop.

If yours is bad, I'll sell you mine for $350, which is the price my mechanic will buy it off of me when he needs it for a future customer.

Really? The platinum 4's aren't recommended for this engine? Thats surprising.... why not? Guess I made a mistake there.

Still, it was revving before I replaced the plugs, so I doubt that is the source of the problem. I'm still curious about my original question... does the throttle actuator go bad on these engines? Most of the post's I've read look like suginami's.... "I replaced the throttle acutator and it didn't change anything. Then I replaced XXX and it fixed it.

Yes, platinum plugs are not recommended for these engines. The techs that help out on this site won't help in diagnosing a problem unless the platinum plugs are replaced with the plain old copper core non-resistor type plugs. I think it has something to do with the electrode on platinum plugs being very, very small, and the electrode on copper plugs is much fatter.

I've searched before and asked the same question, and the throttle actuators are a common failure item on these engines, just like the mass air sensors.

There are potentiometers that go bad and wear out over time, as well as the harnesses going bad on these actuators.

One more interesting tidbit: switching the a/c compressor on (or off) will smooth out the idle for a few seconds, then it goes back to revving. The tech manual says there is a compressor cutin signal to the computer that increases the engine speed by 50rpm with the compressor on.

Would that point to the actuator? I was thinking the cutin signal might give the actuator a few seconds of stability before it goes back to having problems?

I am on a mission to have everyone on this site who has something other than copper plugs to change back to them. Suginami is right. Everything else should come AFTER this deficiency has been corrected.

One of my "lessons learned the hard way"
 

The spark plug connectors have a resistence of 1000 ohms. Platinum plugs are resistor plugs so that doubles the resistence to 2000 ohms. It will run but not well. Most auto parts stores I deal with list the platinums as a direct replacement. Shows how much they know about MBs.

Re: One of my "lessons learned the hard way"
 

They make more money selling platinum plugs!:D You put them in at $5 a pop, then you get a poor running car and have to buy all this other stuff to correct it:) Sounds like good business;) Great platinum plug posters in the stores too!


The platinum plugs will progressively get worse the longer you have them in these early engines so it is concievably that your car will run worse now than it did when you first put them in. A vacuum leak can really make it even worse.

Idle Problem Solved
 

All-

Just a followup post to close this thread out: I replaced the throttle actuator today, and it FINALLY solved the problem. Figures it would be the expensive part. The idle is now pretty stable -- it sometimes hunts just a bit when switching from D or R into P or N, but stabilizes pretty quickly. I assume this is normal.

Special thanks to Arthur Dalton and Paul S (suginami) for thier help on this one.

If you've got the same problem (doesn't everybody?!) on the M104, try the throttle actuator. I'm going to disect my old unit when I have some time, and see if I can figure out exactly what went bad. If anybody's interested, I'll post details about what I find.

dordal,

It would be interesting to know what you find as it may be possible to fix these for certain types of problems. Post pictures of the disassemble unit if you can.

philmartin