Ok, I've got a 2005 GMC Sierra with the 4.3 Vortec. Whenever I go uphill and try to maintain highway speed (60-65 mph), my engine bogs down and my engine light flashes maybe 15 to 20 times then goes off when the road levels out and RPMs return to normal. Recently, the engine light stayed on for a day, and when I went to have it checked out it went off on its own. I have been told that when the engine light flashes that it indicates a misfire, but I'm not sure what could be causing this. Does anyone know the most common causes of this?

Everything seems fine until I am going uphill, then it goes to hell. I put in a bottle of Seafoam the other day but that didn't seem to help. Any suggestions?

 

have it scanned for codes.

 

I tried to but when the light is not on there are no codes that show up. It has only stayed on one time and not long enough to go have it checked...

 

The light doesn't have to remain on. The computer stores a history of the codes, when they tripped and the conditions present when they tripped. Granted, you may have to hit the dealership or a good repair center to have them pulled but they're still in there.

 

Most likely leaking intake gaskets. This is common with these vortec motors. But dont get the cheap new ones, get the better ones that are thick, or you will have the same problem again soon. The cheap ones are $30, the better ones are bout $75. Very easy to change.

 

Thanks! I will look into the intake gasket situation and see if that helps.