bsbones

1986 C4 my service engine soon light comes on after I have been driving for about a 1/2 hr and will go out after another 1/2 hr. I have not noticed any change in performance, If I stop and shut the engine off it will go out but again it will come on. Can you run a scan test if the light has gone out or does it have to be on to test. Has anyone experienced this intermittent condition. Stock engine no modifications.

vetteoz

Pull the codes , any generated but not current will show up

http://www.corvettephotographs.com/c4vettes/codes.htm

kimmer

As said pull the codes but my money is on the EGR.

Cliff Harris

I'll bet you get an error code 32 -- EGR.

It means that the EGR system is not working. When the ECM commands EGR, it turns on the EGR solenoid, which applies vacuum to the EGR valve. When the EGR valve opens, hot exhaust gases flow through the EGR tube that goes over the top of the passenger side valve cover. In that tube is the EGR thermo switch, which closes when it gets hot from the exhaust gases flowing through the EGR tube. If the ECM does NOT see that switch closure it sets error code 32. The complete series of events has to happen to make the thermo switch close, so you need to check all the links in the chain.

TA