I have searched the posts, but have not come up with my exact situation.

Our 1989 190e 2.6 started stalling when warm and "running away" (high idle). I did the following:
-cleaned the Idle Speed Controller,
-checked the OVP relay (it is clamped shut so I did not open it),
-resoldered some joints in the Fuel Pump Relay,
-cleaned the tracks on the Air Meter Potentiometer,
-checked operation of the Idle Microswitch,
-checked the Throttle Position Switch, and
-replaced the Fuel Filter.

The thorttle linkage is not sticking, and the cruise control operates properly.

It now idles really high, about 2000 rpm. Removing power from the ISC drops the idle back down to about 900 rpm. The ECU is opening the ISC, intentionally causing high idle. I do not think that there is an air leak, as the throttle plate gets sucked down as the ISC kicks in. The microswitch and the TPS are closing properly.

Any suggestions would be most welcome.

Andrew
1989 190e 2.6

there are some techincial bullentins on this subject i can summarize them for you if you want

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

Thanks; that would be very helpful.

In one of the posts that I found, one of the experts said that, in a situation like this, the ECU must be operating on a false input (or absence of a true input), so that it sends the wrong signal to the Idle Speed Controller. I think that the key to my diagnosis is to track down that false input.

Then, of course, the repair has to cost less than the value of the car.

Andrew
1989 190e 2.6

Well, I spent the day going through the diagnostic procedure on the Mercedes CD. It pinpointed the oxygen sensor, and indicated that the rest of the fuel injection system was OK.

I did not believe that the oxygen sensor could have such a large effect. An old post (by Larry Bible, I think) suggested monitoring EHA current while feeding different inputs into the oxygen sensor wire. Sure enough, grounding the wire gave my exact symptoms, while a slight positive voltage cured them.

So, Í'm off to see Paul in the morning, to pick up an oxygen sensor.

I find it amazing that the rest of the system works so well, after 20 years and 240,000 miles (380,000 km). These are tough cars.

Andrew
1989 190e 2.6

Of course, it was not the oxygen sensor. The oxygen sensor was just giving an out-of-range reading, because of another problem. However, I now have a new oxygen sensor, new injector seals, I know that I have no air leaks, I own a Cis-E fuel pressure guage, and I have a much deeper understanding of how the Cis-E system works..

Here are the symptoms:

Cold start is perfect. High EHA current, droping quickly at first, then slowly (as intended), maintaining perfect idle mixture as the engine warms up. It reaches operating temperature and idles well, but the Cis-E signal indicates that it is not in closed loop. All is well.

Then, you touch the throttle. The system goes nuts. The idle oscillates from 800 to 1500 rpm. The air flow meter plate is opening and closing as the amount of air going in changes.

Here is the answer:

It was the potentiometer on the side of the air flow meter. It had a worn spot at idle. As long as nothing moved, it was OK. As soon as the air flow meter opened a bit, it generated erratic signals, and the computer did not know what to make of it.

This is the potentiometer that was available for $50 a few years ago, and is now only available as part of a $1500 assembly. Fortunately, there are auto wreckers.

The old gal now resumes her quest for the 500,000 km grille badge.

Andrew
1989 190e

Thank you for reporting your saga to its conclusion
It will be useful to others that have yet to find themselves in your position.

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1993 190E 2.6 135k
1989 Ford Thunderbird SC 5 Spd 79K