The car: 2003 Baja 5 speed, non turbo , 69K miles
The problem: Feels like a misfire or bucking under cruise. Mostly near 3K rpms. Otherwise seems to idle OK and runs fine under acceleration.
Check engine light does not kick on.
Problem seems to get worse at below 1/4 tank or oddly enough right after topping the tank off.
This started about 2 years ago. I initially thought the E10 in the tank had absorbed some water. Since then I've been using ethanol treatment and even tried running the tank near empty before topping the tank off which hasn't seemed to help.
Just something quick to test, I loosened the gas cap at 1/4 tank when the bucking started today. I did not heard a swoosh but the car seems to run just fine with the fuel cap loosened. And the check engine light never kicked on.
Just wondering what might cause this. Because I did not hear negative or positive pressure when I loosened the fuel cap, does that mean there is nothing wrong with the fuel evaporative system?
Am I even on the right track?