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Loose gas cap at fault or possibly sabotage?

I’m unsure of what I should do concerning a recent ‘check engine’ light(yesterday) that’s come on. There’s also a digital read-out that said to tighten the gas cap.

Like a lot of cars, I have a lid over the gas cap that ‘clicks’ when it’s fully shut and flush to the car. I’m certain I did this after my last fill up but found it unsecured yesterday morning. I didn’t see any granules or other instance of a substance that may have been added to my tank but with the latch unsecured and the light coming on I have to wonder. I, or someone else could have bumped into or whatever but who knows?

Now here’s the clincher. my cap hasn’t securely fit on the tank opening for some time now(I know, one of those things that’s easily replaced but always gets lost in the back of your mind). I t would go on and no matter how much you turned, it wouldn’t ‘lock’ into place. As a quick fix my dad put some kind of paste-like substance to ensure air didn’t get in(this was some time ago) and I’ve been operating like that since. As little as I have to drive, vehicle issues were never as a high a priority as I’m thinking they should have been before now. Given this incident, I bought a new one(locking) and put it on.

Could the lack of a seal cause not just the digital read-out but also the ‘check engine’ light to come on and remain on even after the new cap is secure?

My mind immediately goes to the worse possibility but don’t know if there’s any way to try to narrow things down a bit. Any help would be appreciated.

If you are in the US AutoZone will read the codes that turned on the check engine light . Post them here and what this vehicle is and you will get better answers . Or if you are not able to work on this thing and I don’t think your father is either just find a shop and have it fixed.

The fault may take some time to go away. The test your car takes to set or make the light go on or off only happens under certain conditions. The test requires ambient temperature to be warm so this time of year it could take awhile. You could unhook the battery for 30 minutes. If after that the light eventually comes back there is a leak somewhere in the EVAP system.

I do have an Auto Zone in my city and the day off so I’ll make that my first stop this morning.

Depending on what they say, I’ll try the battery as well. I’d heard that some systems go by a number of miles driven while others use a cycle rotation of how many times a vehicle has been started so that might factor in somehow.

First, many gas caps don’t “lock.” They’ll just click as long as you keep turning them. 3 clicks means it’s sealed. I would wipe off whatever goop your dad put on it.

Second, if it’s indeed an evap code, often it’s caused by the purge valve failing if the cap isn’t to blame.


The way I read it, the Original Poster purchased an aftermarket locking gas cap.

So, two codes came up pointing to EVAP with a possibility of it being a valve. The associate suggested lightly sanding around the gap to remove any rust that might be present, wipe clean and apply some vaseline around the rubber within the cap to help with a more secure seal before driving it for around 50 or so miles.

I did that and I’ll have to keep an eye on whether or not anything else comes up in the meantime.

Thanks for the quick responses and suggestions :smiley:

Good advice, worth a try, could work!
If there was any rust/corrosion on the filler neck where it makes contact with the gas cap that will cause a code for EVAP LEAK (large or small) and can trigger a “loose gas cap” or “check engine light” warning.

I lived in the rust belt during winters and every few years my Bonneville will indicate a code is present and I will get a P0455, I think it is, for large EVAP leak. I used a technique very similar to the one suggested by the parts store employee. It works and the car will go another 4 or 5 years before it repeats. I spray my filler neck and cap seal with WD-40.

Be careful when sanding, sparks at the open gasoline tank filler are not your friend. I stuff a rag in the neck (large enough not to go through into tank) prior to the procedure to keep debris from entering and then thoroughly wipe it out.