Could anyone please help point me in the right direction?
I have a 1993 190E 2.3L, and have been having intermittent low idle (400 - 500 rpm)for a while, and have been tracking the occurrences more carefully. (This is related to my post earlier in the year about the "Weird Idle involving Overvoltage Relay")

This low idle condition occurs when the engine is hot (>95C). I reach this low idle much sooner in warm weather (being in Vancouver, that is around 20C and sunny)than in cold weather (10 - 15C, rainy and cloudy, of course), presumably since warm weather does not allow the engie to cool as much as I drive.

After low idle in the city, a highway run seems to cool the engine back to 85C, and the car idles normally (800 to 900 rpm) for another short drive. As I sit in traffic again, the engine warms up to between 95C and 110C, and low idle begins again.

I have stalled twice in one day after particularly warm weather, and a long day of city driving. I removed the fuse on the overvoltage relay to "limp" home without letting my engine cool down. The engine idled at 800 - 900 ( I thought that without the OVR, the idle should be higher since no fuel corrections were performed without the OVR) I replaced the fuse at home, and have not stalled again, although the low idle occurs)

I noticed another post about an air slider in another model of Mercedes, but am not sure if my model has an equivalent or where to look for it.

My specific question is "What sensors/controls are involved in idle speed and air intake/supply at the higher engine temperatures?" Is it a defective thermometer that is causing this, or something else?

Thanks in advance for any replies & suggestions.

The air plate position sensor and the coolant temp sensor have alot to do with this system. Additionally, I would check the idle air valve. The easiest way to check this is when it is failing. This is when the technician would want to work on it.

Have you replaced the OVP relay? If not, start there.

------------------
Benzmac:
Donnie Drummonds
1991 GMC Syclone
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

when was the last time you had a tune-up?
plugs, air and fuel filter, etc. i would also start there. fuel filter makes a hell of a difference.

I agree with Donnie, start with a new OVP relay.

But if you want to figure it out here's where to start. Get a cheap radio shack multimeter with a 2amp scale (volt-ohm-amp meters).

Rig some wiring so that the idle control valve (sometimes called air-slide, idle motor) current will run through your meter. Then observe the readings. You should find readings around 6-700ma at idle. If you turn on the A/C you should get about a 50ma increase in current.

Now for the fun story on OVP problems. You have found that disconnected the idle raises. You might also know that the OVP relay powers the controller. You might deduce from this that if the OVP didn't power the controller that the idle valve would react like one that had been disconnected. In some cases this might happen, BUT!! , that ain't the story we want to hear. The one we want to hear is the one where the engine dies.

Well here is that story and I have witnessed it repeatedly (was very confusing for me for a long time - how loss of OVP could cause the motor to run slower than when disconnected altogether).

You will notice that the current flow is always above 600ma. If you use long wires as I do, then you can watch the activity as you drive (don't do this on the street - pay attention). The current often goes to 8-900ma when the system is off idle. This is so that if the throttle slams shut the idle is high and can be slowly brought down. Now comes the problem. When the OVP has a problem it doesn't just disconnect, it acts as a high resistance in the circuit, which limits the current that the controller can apply. During these times the current will drop to 4-500ma for slow running or even lower which causes the engine to die.

This bothered me for a long time; if you follow the logic, lower current equals lower speed; zero current should be very low. Thats not the case at some low current the idle valve returns to its disconnected state; which is slightly more open than standard warm idle.

Fun story, no?

[This message has been edited by stevebfl (edited 07-29-2000).]

Just an update:

Work has me busy virtually seven days a week, but the problem was bothering me, so I dropped it off at the dealer to troubleshoot instead.

It turned out to be the $35.80 CDN for the idle speed test and $35.55 CDN for an EGR.

Thanks for all the input. Earnest.

what are those numbers for? mere tests or replacement parts and labor?