Trying to get the last of the issues with my son's Protege5 resolved before I turn it over to him at the end of the summer. Shortly after we got it, I noticed occasional hesitation, usually under part throttle. Shortly thereafter, it threw codes for random misfire and lean F/A mixture. The random misfire was cured with new coils/wires and the F/A mixture by replacing the intake trunk between the airbox and throttle body, which had a crack hiding on the bottom side on the far side of the MAF, allowing unmetered air. It was also throwing a cat code (which may have been secondary to the lean mixture issue) which we "fixed" with an O2 spacer. Collectively, those fixes solved probably 95% of the hesitation issue. Every so often it would re-appear briefly, but I couldn't draw any correlation until recently. It seems it only appears after a fill up, moderates as some of the fuel burns off, then disappears by 3/4 tank. I'm stumped. Any wild or no-so-wild theories?
How much are you filling it? If you're forcing the tank full, you could be pumping fuel into the evap system, and when the purge solenoid opens at part throttle the engine is getting a slug of fuel.
My WAG: Wonder if a clogged fuel tank vent could be allowing a bit of a vacuum to develop? As the tank level drops and there is a larger volume of air the effect is less noticeable.
I wrote a reply about checking your purge valve. Did I post it to another thread, or forget to post?
Anyway, make sure your purge valve isn't leaking. Disconnect the hose to the canister and plug it while the car is misbehaving, and see if the problem goes away.
I'll second the purge valve. When they get stuck open, and you fill the gas tank the vapors go right into the intake manifold. It usually just makes it hard to start and goes away after you use some of the gas in the tank. Basically the fuel takes up the space in the tank the vapors would be in and it forces the vapors to the charcoal canister. if the purge valve is stuck open it just goes into the intake and makes it hard to start or hesitate.
