Rough Idle Conundrum
04-02-2019, 09:43 AM
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1992 300E 3.0L
Rough Idle Conundrum
My 1992 300E 3.0L (256K) is giving me some idle issues. I read the threads on various MB forums on the W124 idling issues, none seem to exactly describe my situation.
I check the OVP by bench testing using a multimetre, all pins 87E and 87L are 12.5 volts. I tested for continuity ohms from the female harness at the relay to the idle air control valve harness and found one of the 87E pins to be 6.45 Ohms of resistance. I assume that is the ECU between the OVP relay and the ICV.I am not sure how I can test whether the ICV is actually getting voltage delivered.
The ICV works fine. I cleaned it and bench tested to make sure the armature worked. After cleaning and reinstallation, I started the car and it ran at high RPM at idle, about 1800-2000 rpm for about 2 minutes and then kicked down to idle at 700 rpm, then got a choppy idle and near stall.
This morning, the car started immediately and ran at 1100 rpm at idle, then decreased to about 750 rpm as it warmed up and had an engine flutter. Driving it gave no issues. Returning home, the engine idle varied from 1000 to 550-ish, rolled between the two, while the brake was engaged and car in drive. It did not stall but wanted to.
I am thinking SENSOR. Likely O2??
SYMPTOMS:
There are no warning lights on at the dash when running, the ABS light usually signals if the ICV is bad, but the idle goes bad at warm-up.
The car runs well at speed
It stalls sometimes with foot on brake at stop lights. Starts up right away.
The ICV works- turns with 12 volts applied when out of the engine
The Relay is OK - fuse good too
The rotor and distributor cap are worn but functioning
Sparks are ok but worn
O2 sensor is likely original
What should I do next?
I check the OVP by bench testing using a multimetre, all pins 87E and 87L are 12.5 volts. I tested for continuity ohms from the female harness at the relay to the idle air control valve harness and found one of the 87E pins to be 6.45 Ohms of resistance. I assume that is the ECU between the OVP relay and the ICV.I am not sure how I can test whether the ICV is actually getting voltage delivered.
The ICV works fine. I cleaned it and bench tested to make sure the armature worked. After cleaning and reinstallation, I started the car and it ran at high RPM at idle, about 1800-2000 rpm for about 2 minutes and then kicked down to idle at 700 rpm, then got a choppy idle and near stall.
This morning, the car started immediately and ran at 1100 rpm at idle, then decreased to about 750 rpm as it warmed up and had an engine flutter. Driving it gave no issues. Returning home, the engine idle varied from 1000 to 550-ish, rolled between the two, while the brake was engaged and car in drive. It did not stall but wanted to.
I am thinking SENSOR. Likely O2??
SYMPTOMS:
There are no warning lights on at the dash when running, the ABS light usually signals if the ICV is bad, but the idle goes bad at warm-up.
The car runs well at speed
It stalls sometimes with foot on brake at stop lights. Starts up right away.
The ICV works- turns with 12 volts applied when out of the engine
The Relay is OK - fuse good too
The rotor and distributor cap are worn but functioning
Sparks are ok but worn
O2 sensor is likely original
What should I do next?
Last edited by Lowlanderg; 04-02-2019 at 09:45 AM. Reason: missed something - fuel filter is new
