W124 Idle Surge. Possible Fix? Coolant Sensor?
 

Hi all.
I've been having problems with my 94 E200 Wagon.
It always surges at idle when warming up, and can be very hard to start when warm/hot. It was also running a little roughly and had a poor idle. I replaced the OVP a week ago, and the car ran so much better. It always starts really well from cold.
BUT, I still had the idle surge and hard starting occasionally when hot. Like it would not start sometimes until it cooled. Anyhow, my wife came home a couple of nights ago from work, and she had problems all that day. I went to check on the car, and it wouldn't start. I then checked all the wirings to sensors and injectors. Tried the car again and it started fine. But I found that when I nudged the wiring to the coolant temp sensor on the thermostat housing, a second later the idle surged! I tried this 10 times or more and it kept doing it. I checked the resistance on the sensor and found it to be Meg-Ohms!! Haynes says should be about 350 Ohms at 80 C. I could see the pins were tarnished. I cleaned them up and checked the connector wiring back to the ECU which was OK. The car started fine. When I removed the sensor with the engine running, the idle surged! It may be too early to say, but the car runs fine. We have started it from cold several times and no surging, and my wife reports it runs really well. My theory is that the coolant sensor made only intermittent connection, when the ecu detected this, it richened the mixture. This caused the surge until the lambda probe detected this and told the ecu to run lean. When starting from warm/hot, the ecu figured the coolant temp was minus 50c or whatever, and made the mixture too rich.
What do you guys think? Am I chasing a wild goose?
Sorry for the length or this post, I was just so pleased that the surging seems fixed!

Barry


:confused:

Had the exact same problem with mine, only mine was actually an intermittent sensor, and since I replaced it I haven't had a single problem with it since (4 years).

I love this car. Since that one incident, I haven't had to do a thing to it except oil changes, etc. That's pretty good for 4 years (knock on wood).

If the surge ever reoccurs, check all vacuum hoses. A leak anywhere in the system can bring symptomatic surge issues...as it did with mine!

e320 1994 coolant temp sensor
 

I have a surging idle on the a above car. I thought I knew where the temp sensor was but could someone fill me in. Also am having trouble finding a replacement part. Any help with that ,too? May try the O2 sensor also. where can I find info on checking codes on this vehicle. I've heard about box in the engine compartment where lights flash