Service Engine Soon Problem

[vette-dude]

My Service Engine Light comes on whenever I drive my 91 L98 Vette to work in the AM ( 5 Am to be exact). Let me explain. It only comes on when the temp outside temp is below 65 and it comes on at the same place on the freeway. If I shut the car off and restart, it is not on and doesn't come on again till the next time I repeat the drive the next day. If the outside temp is over 65 it does not come on. I noticed that the light seems to come on when the temp gage reaches the 170 to 180 degree normal operating temp on these cool mornings. I would test for a code but the light does not come on in the warmer temps of the afternoon drive home. My thoughts are that the coolant in the radiator stays cool and when the thermostat opens the influx of this cold coolant causes a glitch in the comparison loop feedback between the temp sending unit and the gage. The Computer then throws the light as it gets confused. Is this possible? The car runs fine with no problems even though the light is on till I turn off the ignition and restart. I don't think it is anything to worry about but just need confirmation that I should ignore it. Thanks.

Randy

 

[vader86]

EGR system

It opens when the coolant reaches 170F and the engine is at part throttle. Common on hwy driving. The ECM reads a loss of vacuum in the EGR.

You would need to replace the solenoid/switch, whatever is on the 91, and then check the pipe for blockage to totally eliminate it.
It can be tough to track down sometimes.

 

[rrhayden]

You need to check for codes even though the light is not on. The ecm will store them for later retrievel. The reason you need to check is that the ecm runns a fixed program on start up, and at 170 degrees switches to "closed loop" meaning all of the sensors. It is easy to check, and will eliminate a lot of guess work.

 

[vette-dude]

Thanks to you guys. I will try to pull the codes this weekend. Will post results.

Randy:BOW

 

[KOPBET]

vettedude,

FWIW - check this thread:

http://www.corvetteactioncenter.com/forums/showthread.php?s=&postid=243526#post243526

 

[vette-dude]

Thanks KOPBET. Sounds like I have an EGR problem. We'll see if the codes show that. It is a little strange as I can let the car warm up in the garage or dirve around the city till it is warm and then take it out on the Highway and the SES light never comes on. I really am beginning to believe that there is a crashed UFO buried on that stretch of freeway and it emits a signal between 5 and 5:30 AM everyday and makes the ECM go nuts. :crazy :crazy

Maybe I should just go to work later in the day!!

Randy

 

[vette-dude]

Just an added note. Looked over car last night before running the codes and the coolant reservoir was almost empty with the car at normal running temp. Added some coolant and on the drive in this AM no SES light. Maybe this was all it was. I'll drive it a couple of days and see what happens.

Randy