Runs better with fuel cap loose

[jseabolt]

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?

 

[auto2]

have you changed the gas filter?

[jseabolt]

have you changed the gas filter?

 

No I have not but once I get a chance I'll swap it out. This may sound crazy but I didn't really think changing fuel filters frequently was all that nessessary these days. With plastic or lined fuel tanks and assuming there is less trash in fuel these days. On the other hand, people didn't have to deal with ethanol years ago either.

 

Another thing I should check is the throttle cable. I can recreate the condition by simply taking my foot off the accelerator under cruise. Maybe the throttle cable has stretched and I need to take some slack out of the cable.

[MilesFox]

Perhaps there is trouble with the venting system, purge control solenoid.