Check Engine light

[adale]

My '93 Navajo keeps having the Check Engine light come on but only after it has been driven a couple of hours at 70+ mph. After you shut the truck down and restart the light goes away and does not come back during normal shorter trips, like less than the 2 hours. The OBD I system does not have any codes set. This has been going on for about 2 years and I can't find the problem. Milage is not suffering nor is general driveablity. Truck starts and runs strong. Any ideas????. I heard the O2 sensors might be getting "lazy" but I don't know and their output is changing. They are orginal and have about 97K on them.

 

[MrShorty]

No codes in continuous memory? Might check Keep alive memory circuit for power. Also check for good computer grounds. Might also check the check engine light circuit and make sure it doesn't have an intermittent short to power. Anytime that check engine light comes on for real, there should be a code stored in continuous memory.

Just had an idea. Can you force it to set a code in continuous memory? Try this:
Unplug the MAF (or any other of a number of sensors)
Start the engine, verify that the CEL is on while the engine is running.
Stop the engine and turn the key off
Setup for self-test and run sefl-test. There should be an MAF code in continuous memory. If you've left the MAF unplugged, there should also be a KOEO hard fault code.