1999 blazer gas cap

I heard something about this long ago on the show. My blazer will run poorly and loose power after driving 5 or so miles with the gas cap on.But runs fine with it off.But this activates the check engine light. Any guesses?

As you have found out the tank should not be vented to the atmosphere. Could you post back more difinitive description of just what happens when the car runs poorly? If the customer insisted I would drive the car with the cap on fully and a fuel pressure gague installed, you are implying that the fuel pressure is dropping with the cap on are you not? On the other hand are you ready to turn the car over to a mechanic and let him pursue his own diagnostic plan?

I did take it to mechanic 160 dollars and a new O2 senser later it happened again. Its more like it bogs down not wanting to go. I did notice alot of vapor pressure when I remove the cap .Way more than seems normal.Is there a evap control of some sort that can fail and not set a code?

I hate to ask, but have you bought a new gas cap (the right one for your car)?

Nope same oem cap Im pretty handy with these things but never seen this before .One time I loosened the cap and the truck cut off that second and a big poof of pressure came out.And check engine light came on again. But then started right up and drove fine for a few miles. So I took the cap off as an experiment. And it runs fine…Someone told me there is a seloniod of some sort in line on the evap loop. Anyone know anything about that?