1999 4.8 EGR bypass problem

I wanted to bypass my EGR to get more horsepower, but didnt have materials to plug hoses, so i unplugged the valve from its harness. I took it for a test drive, and it seemed to have more power but it sounded like it was running too rich. I got back home and noticed the service engine light was on, which didnt surprise me, thats what i get for unplugging crap. So, I plug the valve back in, start it, and the light is still on. But then i leave the truck alone for about 12 hours and i get back in it to go home, and the service engine light is now OFF? why would it be on and then off? is there still a fault code in the ECU? or was it just the computer re-adjusting to having the valve hooked up again? im really freakin confused, i would love it if someone could explain it.

-Nick

Well,

I'm no GM expert, but like you I was messing around with stuff one day on my 99 4.8, and I got the check engine light. :banghead:

I restored it back to the way it was and the light stayed on. I drove the truck for an hour or two, and after a relatively aggressive run, the light went out and did not return. I think it takes a little time to reset? Also, I think you can unplug your battery for a couple of minutes and get the same effect.

Hope that helps!

the light come on becouse there was a problem obviously.. but then you corrected the problem so the light went off.. how or why im not exacly sure.. but if you hook it up to a scanner that code will probably show up in the hitsory codes but can be cleared. but you would never know that code was there unless you look for it in history youd never know the code was set. i dont really have a better explanation on this im not expert just trying to share some info i learned.. hope it helps.
Chris

yeah i also am not a gm expert obviously, i figured it just took a little time for the thing to realize the problem was corrected, hence the light for only a short period of time. At least you two have had it happen as well, so that means its probly not abnormal. Thanks guys!

It's normal for that to happen, some codes take different events and times to reset, whether it be so many ignition cycles or so many minutes. I remember a post about an EGR mod from a S-10 forum where they put a washer in the EGR pipe to restrict it. I am sure they had to place it at the intake where the egr pipe enters