tbwf150

I recently began having a "check engine" light issue on my 1990 F-150 4.9L truck. I could be traveling at any speed over 50 mph on the freeway and my "check engine" light comes on. If I reduce my speed for a moment, the "check engine" light goes off. I continue on and a short time later my "check engine light" again comes on. Once again, I reduce my speed for just a moment, and the "check engine" light again, goes off. This continues on for a while, then stops doing it. Then it begins to happen again. Sometimes it will go off by itself even if I don't reduce my speed. Seldom happens at speeds under 50 mph. Although, it has, but not often. Mostly happens at speeds over mph.

JerseyGray

Check your oil

tbwf150

Thanks for your reply. I change engine oil every 2,000 miles. I'm 100 miles short of the 2,000 miles. I checked the oil and it's not low. I'm using 5W-30 oil and a Napa Gold filter.

sdmartin65

Most likely a vacuum leak or faulty EGR. The EGR needs good solid vacuum to operate, it's always applying or releasing vacuum at cruising speed and will set off the CEL like you describe when there is a problem.