I have tried searching but my time on the computer is limited today and to fix this problem, so sorry if this has been answered else where. I have a 2003 Evo that keeps misfiring causing the Service Engine Soon light to appear. This only happens when I go on long drives. When I return to my usual every day city driving, the light disappears and the car no longer misfires. When the light came on the first time it was recommended to me to change my spark plugs. So I did that and then took a long trip the very next day, and once again the Service Engine Soon light came on telling me the same. I have no clue why this is because it seems to be on just long trips that this problem happens. Please help I have a long drive ahead of me very soon and Id like to fix it before I go. Thanks in advance for what ever info you have.

 

Did you pull the code? Maybe an 02 sensor issue.

List your mods and kind of tune you have.

 

should be in "evo engine/turbo/drivetrain" topic.

 

if its not your plugs, the next thing to do without actually diagnosing the problem would be plug wires.

 

My Evo is currently stock except for a clutch and flywheel with a stock tune. The guy who read the code didn't give it to me but said it was a misfire and that my plugs may be fouled so replace them. So I replaced them and it took slightly longer for the misfire to happen. Sadly it seems when I return from these trips the parts stores are typically closed and the light is on the next day in the morning(before they open) but disappears on the way home from work with no misfires, so its really hard to get. I wish I could get the code, it would help a lot. Plug wires was the next thing I was going to try but figured Id run it by you guys first.

 

It would mostlikely be a fuel or air issue.

I had and issue where the front honeycomb in my MAF box came out and crumpled up into a little ball causing my car to misfire because of the lack of air.
So easiest thing to do is just take off the filter and just check, cant hurt to look.

Hope you find something!

 

I hear this is a common problem. With a proper tune, it should go away. If you still have your cat, I don't know why you'd get a misfire, but who knows.