CEL coming on when gas hits 1/2 tank?!

[masq]

It's the strangest problem that car owners on other auto forums I found through a Google search seem to have, but never received any good answers for. Every time my 2002 C2 cab hits about the 1/2 mark on the fuel gauge, I get a CEL for P300, P304, P305, P306.

Any ideas on what might cause this?

Edited January 19, 2010 by masq

[wross996TT]

...last time you changed plugs and coil packs?

[masq]

...last time you changed plugs and coil packs?

450 miles ago. :)

[PAULSPEED]

There is a container near where you fill up the tank. You can

get to it by removing the wheel and removing the plastic

wheel wells. Mine pops on every now and then. I

bought an o-ring for the gas cap that helped a little.

Paul

[anothervr6kid]

O2 sensor, pre cat if I remember right. It all three cylinders come on at once it is usually an O2 sensor.

[masq]

There is a container near where you fill up the tank. You can

get to it by removing the wheel and removing the plastic

wheel wells. Mine pops on every now and then. I

bought an o-ring for the gas cap that helped a little.

Paul

A loose tank would cause the cylinders to misfire on one bank only? Not doubting your experience, but I'm not sure how it's mechanically relevant?

[masq]

O2 sensor, pre cat if I remember right. It all three cylinders come on at once it is usually an O2 sensor.

Isn't there a dedicated P code for bad O2 sensor? I've been having CEL issues for the past 3 months which led to new plugs, packs, and replacement of lifters. During the entire time, we never saw a code for O2 sensors.

So my question is if you think replacing a $150 dollar part would be a wise? I don't know if the P car oxygen sensors are known to go bad without throwing out error codes...

Thanks for your help everyone.

[jetbox]

O2 sensor, pre cat if I remember right. It all three cylinders come on at once it is usually an O2 sensor.

Isn't there a dedicated P code for bad O2 sensor? I've been having CEL issues for the past 3 months which led to new plugs, packs, and replacement of lifters. During the entire time, we never saw a code for O2 sensors.

So my question is if you think replacing a $150 dollar part would be a wise? I don't know if the P car oxygen sensors are known to go bad without throwing out error codes...

Thanks for your help everyone.

Hi,

I am by no means an expert but my experience may help. I had a tank vent valve go bad which caused the l/h bank to run slightly lean giving some fault codes. I believe the vent valve purges fuel vapour fumes from the fuel tank (carbon canister). Could this be related to the 1/2 full tank and cel you are experiencing?

Those that understand the system better will be able to explain more. But I think there could be a link

Hope this helps

Jetbox

[masq]

Great explanation. Thanks.

Is this a problem on the 996's?

[phillipj]

It is not uncommon ;)

[masq]

I wanted to close out the issue by posting the resolution I found.

Without having the tech tear apart the engine again, I decided to try a bottle of Techron with a fresh tank of gas. For $13 bucks, I finally resolved the misfiring issue that had been plaguing me for months!

It's been a month, and no misfires.