Engine runs poorly until I loosen the gas cap. Check engine light bank 1 and 2 lean condition

My 2002 Chevy Tracker recently has been running poorly after consuming a 1/4 tank of gas. The check engine light comes on the scan tool reports P0171 and P0174 bank 1 and 2 lean(it’s a 2.5 liter V-6). If I remove and replace the gas cap it runs better but not perfect. Once I use another 1/4 tank the symptoms return. I have noticed the cap is hard to remove lately. This has been going on for two weeks Its been to 2 garages one of them told me my catalytic converters are plugged up and wanted 2500 to fix I told them not to do it because I truly don’t think this is my problem if I can remove the gas cap and it runs better it indicates to me that for some reason air can’t get into the tank as fuel is used.

I agree with you, it may not be the cat. When gas it used up while driving, air is supposed to be allowed to enter the tank to take the place of the gas that is no longer there. If this doesn’t happen, a vacuum is created in the gas tank which will prevent the fuel pump from pumping gas to the engine.

It’s like if you have a half full beer bottle. Turn it upside down and all the beer runs out. But if you put the end in a bowl of water, the beer magically stays in the bottle, seemingly defying gravity. When the bottle is in water, air is prevented from taking the place of the beer as it drains out, so a vacuum is created, holding the beer in the bottle.

I expect there is a problem with the gas tank evap system. Ask your mechanic to double check that before even considering to replace the cat.

What he said. I’m guessing the evap system is plugged somewhere. Do you ‘top off’ the gas when you fillup? That can clog things up.

I used to when I learned that I shouldn’t I stopped. However that was only about a year ago.

I agree that the evap system could be plugged up somewhere but in my experience with a couple of similar problems like yours…the fix was a new gas cap. I would just buy a new gas cap and see if your problem clears up. If it does then you have saved yourself a lot of time and money.

NOTE: Do not try to mess with the old gas cap vent. The vent has a one-way release valve that is pressure-activated and must operate as designed.

I agree with @missleman. I just diagnosed a bad fuel pump in my Explorer, but thought I had the same problem with the gas cap. The evap system draws a vacuum in the tank to pull gas fumes out so the fumes don’t over-pressurize the tank or escape to the free air. The air to replace the spent fuel is brought in by the cap through a spring-loaded one way valve. If the valve is inoperable, the tank will pull too much of a vacuum and prevent fuel from being pumped out by the pump. Try replacing the cap first.

I sure hope it’s the gas cap or the evap system. If I could vote for it, I would. After all: Messing with the gas cap should have no effect on the catalytic converters. It just wouldn’t be fair, as my legal team, Bigg, Frack and Diehl tell me. Legally any way.

I had an 83 Corolla that would run fine for months when I would reinstall the gas cap after I lost power and had to pull over. I did that three times mainly because that’s the only thing I knew how to do without a tool box. Worked every time.

I tried a new gas cap no change.

More than likely, you need a new purge valve in the evaporative emissions system (which is fairly cheap), and/or a new carbon canister (which is expensive).

Since carbon canisters are so expensive, you should really try to find a mechanic who knows what he is doing with evaporative emissions systems. You really don’t want to start throwing parts at the problem when those parts can cost $300.