my check engine light goes on and off, only stays on for a couple second, especially cruising at about 80, not it stop and go traffic, the only problem with the car is the idle air control control might be a bit dirty b/c the idle fluctuates just a bit, and the car idles at about 1000 rpms. its a 87 5.0 by the way. motor runs perfect except for a small misfire, possible vacum leak( pretty sure i fixed it) and an exhaust leak ( dont see how it would apply at all) what do you guys think? any input very much appreciated. thanks

 

Sounds like an O2/MAF Sensor to me. Try cleaning the sensor along with IAC motor while you are at it and see if that helps at all. I use CRC TB Cleaner spray and it works well on the MAF wire and the IAC. Also, try to get a scanner to check codes. If you have a CEL then you must have a code associated with it. Good luck!

 

Try this, run the engine at idle and remove the vac line that goes to the EGR valve, take a long vac line that you have laying around and put it on the EGR valve and suck on it. If the engine stumbles the EGR is not plugged. If the engine idle does nothing then you have an EGR issue. The EGR valve opens at cruise and if the PCM doesn't see that, then it will flag the check engine light to come on. That 5.0 HO is know for plugging. Also you say you may have an exhaust leak? Check your cross over tube in the back of the engine, those are also know for breaking and causing an EGR not to function properly because that is were the EGR is getting it's gases.