Gas caps - how do they fail?

[crw]

I'm wondering how to detect a bad gas cap and what effect they might have when going bad.

This happens to be for a car with a direct injection engine, so, higher pressure.

A few months ago in the MINI I started getting a light for the gas cap. (The MINI has a dedicated light for the cap.) I fiddled with it, tried to get it cleaner and make a better seal. Eventually the cap light stopped coming on, but now I get a P0300 and P0313 (low fuel) light. I'm always wary of these stupid things turning into wild goose chases, but this one was no exception:

New plugs, check. Took off ignition module and tested it, check. New battery, check. (The old one was 7 years old.) Many uses of Techron and B-12, B-12 and Seafoam administered through the PCV, I have a tubing which makes it straighforward and easy to do. Etc., etc, I can't remember all the things I have tried......

Which brings me back to the gas cap. For this car in this town, I'd have to order it. But I want to know how these fail and if I could expect it to solve my problems. (Not really problems, mind you. The car runs like a banshee, until the code is thrown and you can feel it "de-tune" ever so slightly. Gets great mileage with or without the code.)

On a side note I'm getting of tired of modern cars and warning light nonsense. I can go through the diagnostic checks and everything checks out clean. But the code keeps coming back.

 

[crw]

I forgot to mention, I put in a new fuel filter, which on the MINI is kind of like open heart surgery......

 

[The_Jeff]

You're barking up the wrong tree.

Whatever your fuel delivery pressure is has nothing to do with your EVAP/ vapor pressure, which is about 1/2 a PSI.

There's a valve in the cap that's supposed to open past a predetermined point, so if there's a TON of vapor pressure it lets a little out, or so that if there's vacuum from using up gas it lets some air in. The EVAP system would prefer to hold all the fumes, and let them soak into the charcoal canister, which dries out by a hose leading off to the intake manifold when all is well and computer driven solenoids allow it.

The EVAP system pressurizes the air space in the tank and the fume pipes off to the charcoal canister as part of the OBD check. Although you can have a bad gas cap, you can also have a rusted line or split hose or bad solenoid in the charcoal tank... it takes a "smoke test" to find this at times. They put smoke in your gas tank and see where it comes out.

Caps fail either by having too weak a seal/spring and not letting the EVAP system pressurize, or by having too strong a seal and going "whoosh" when you gas up, potentially starving the system for fuel as the pump not only makes normal pressure but is fighting vacuum on its intake side.

 

[crw]

Yeah, I do not have any sort of "whoosh" when I remove the cap. It's probably the wrong side of the system to look at for my P0313.

 

[Nick1994]

A lot of gas caps have a rubber seal. Just buy a new one and you'll know if that's the issue or not.

 

[Yah-Tah-Hey]

Just replace the O ring.

 

[djb]

The vacuum/pressure release is a sealed internal assembly.

You can replace the external gasket that seals against the inlet. BMW doesn't list a part number, but it's the same as the gasket on the oil filler cap. Which also doesn't have a part number, but oil cap is much less expensive than the gas cap, and it's an easy swap to do a test.

A fuel delivery error code isn't going to cause the ECU to go into limp-home mode, so you should look elsewhere for the problem.

 

[Errtt]

Here (Calif) smog test techs also take the gas cap and hook it up to the machine. Has some adapter and hose.
I had one fail once, told the tech "hold on" and I ran across the street and bought a new cap, ran back and even though it was new, he still tested it. It was okay like all else, so it passed smog testing. I didn't get any check engine light, nada & nada II

 

[01rangerxl]

Before replacing the cap, find out where the purge valve is and follow the vacuum lines to make sure nothing is disconnected.

My truck has a "check fuel cap" light. It came on a few years ago and I initially put a gas cap on without doing further diagnosis. That was not the issue. It ended up being a vacuum line for the purge valve that had worked its way loose. I plugged it back up, and the check fuel cap light has not come back since. I don't think I even cleared the code, I just plugged the line back where it was supposed to go and the next time I started the truck, the light was off and stayed off.