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Charcoal canister leaking fuel

07-25-2010, 08:25 PM
I have a problem similar (at least with the same area).

I have not run the code reader on it yet, but my carbon canister is filling with fuel somehow. The 99 Jimmy will have sporadic missing or revving and the exhaust seems really rich. Again, sporadic. We may have narrowed it down to happening soon after filling COMPLETELY up with gas. after filling the other night, the problem started, revving and dying. We notticed fuel coming from the Vent off the Carbon Canister. A couple months ago we had fuel coming from the front of the tank (I believe this to be the ball valve from the fuel tank that feeds toward the carbon canister, unconfirmed). We, being poor and the part was dealer only attempted the JB-Weld fix. Thanks in advance for any help.

07-26-2010, 03:00 PM
Fuel leakage from the canister is caused by over-filling the tank. When fuel is pumped into the tank, it's around 55 degrees, it rapidly warms up and expands in your fuel tank. There is an area on the top of the fuel tank that is designed to accomodate for fuel expansion however, if you keep clicking the pump when refueling, that area is filled with raw fuel. When the fuel expands, it goes into the canister. The canister is designed to capture fuel vapor only, if raw fuel enters it, the canister becomes contaminated and must be replaced.

07-26-2010, 10:49 PM
Thanks for this info. That is very likely what happened to us. The fuel guage is broken so were in the habbit of filling it to maximum capicity. And the most recnt event was right after filling it up.

Is it possible that over filling the tank to the point described would cause coughing or revving and dying.

07-27-2010, 03:27 PM
Absolutely! You likely have other issues as well: poor fuel mileage, rough idle, hesitation, stalling, hard starting, Service Engine Soon light on, etc etc.

07-28-2010, 01:56 PM
Thank you so much for the help.

I'll give a brief description of the issues and see if you maybe have any other tips.

It is a 1999 GMC Jimmy, 4wd, v6, 200,000+ miles. The fuel guage hasn't worked in awhile and our 16 year old ran it out of gas several times.

A few months ago the engine started reving very sporadicly. Usually at lower speeds. Driving normally and it would start revving up to about 2000 rpm and down to nearly dying. We have to feather the accellerator to keep it running. If it dies, it usually starts again without much issue. It runs very rich when its revving. Check engine light on and off again.

We notice fuel leaking from the gas tank. Drop the tank, replace the fuel filter and see the leak comming from a white nylon nipple near the front of the tank (fitting on top). This is the longer, skinnier tank. We attempt a JB Weld repair on this fitting (it seems not to leak anymore). I am told this is probably a ball valve from the carbon cannister (unconfirmed).

Later the check engine light comes back on, we run the code on it, TPS sensor code. Replace TPS sensor and clear codes. Check engine light comes back on a couple of days later and is on now. We havn't run new codes on it yet.

Last week, we OVERFILL the gas tank (probably) two times, both times the reving occurs. Fuel leaks from the vent off the carbon canister (attached on frame above spare tire). We disconect the sensor for the carbon canister on the intake for a couple days. Clean the filter in the vent. No problems for a few days. Reconnect the sensor on the intake. No problems on a few short trips.

Sorry fot the long description but Thanks so much in advance for any tips!

(p.s. I plan on running the OBD codes again, but i'd imagine i get one from disconnceting the sensor along with possibly some others)


07-29-2010, 08:14 PM
As for the fuel guage: Most likely the level sender is faulty. Before doing anything else, check the fuel pressure and leakdown. Ignition in RUN position, engine off, fuel pump running, fuel pressure must be 60psi to 66psi and must remain above 55psi for 3 to 5 minutes after the fuel pump shuts off. If it's within specs, drop the fuel tank down far enough to access the electrical connector at the fuel pump module. Unplug the electrical connector from the fuel pump module. In the harness connector, check the resistance of both black wires to a good, clean body ground, should be less than 5 ohms on each wire. With the connector still unplugged, briefly turn the ignition to the RUN position, (DO NOT leave the ignition in the RUN position.) the fuel guage should indicate "FULL". If the resistance and gauge function tests pass, the level sender in the fuel tank module is faulty. It can be serviced separately from the fuel pump, but the entire module must be removed from the tank to do it.

If you have had sensors disconnected when the ignition is in the RUN position, a DTC will be set, the SES light will come on, and the DTC will remain in the history file until cleared with a scan tool that is capable of clearing the history file. The code readers that the auto parts stores use can not clear the history file. Disconnecting the battery will not clear the history file. It can only be done with a capable scan tool. After the history file is cleared, perform a "drive cycle" Then check I/M readiness on scan tool, if all monitors have run and completed their tests and the SES light is not on, you're good to go.

NOTE: The EVAP monitor uses the fuel gauge output signal during its test, so, the fuel gauge must be repaired before the EVAP system monitor will run its test and complete it. The saturated/contaminated canister is the main component in the EVAP system, so it will most likely need to be replaced before the monitor will complete also.

08-03-2010, 09:05 PM
Thanks so much for your help. I did run the OBD2 code reader on it tonite. For your information, the service engine soon light went out today. P0420, Catalyst system below threshold (Bank 1) was the saved reading.

The Jimmy does have a exhaust leak which I'm hoping my be the problem.

I'm ordering the Fuel sensor sending unit and putting it on. Changing the filter on the carbon canister. Then going from there.

Any other tips you might have would be greatly appreciated & I'll post my results here.

Thanks again Old Master

08-04-2010, 03:30 PM
An exhaust leak in front of the oxygen sensor that is located behind the converter, can/will set a P0420 DTC.

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