Check gas cap light and check engine light on

Tiny
KLDD1935@GMAIL.COM
  • MEMBER
  • 2009 BUICK LUCERNE
  • 124,000 MILES
Check gas cap light keeps coming on even after having it replaced. And engine light came on after removing cap to fill up with gas. Repair shop cleared codes and same thing happened again when removing cap to fill up with gas.
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Monday, September 20th, 2021 AT 9:11 AM

2 Replies

Tiny
ASEMASTER6371
  • EXPERT
Good morning,

Can you give me the code number that is set in the system?

The gas cap light will come on when there is a big leak in the evap system itself.

https://www.2carpros.com/articles/evap-system-code-repair

https://www.2carpros.com/articles/how-emission-control-systems-work

You would need to smoke test the system to determine the failure.

Roy
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Monday, September 20th, 2021 AT 9:29 AM
Tiny
AL514
  • EXPERT
Hello, the reason this light comes on in the first place is that the ECM (the cars computer) has run a self-test and found a large leak in the Evaporative Emission System (Evap).
This system allows the fumes from the gas tank to be taken into the intake manifold in a regulated amount and burn with the fuel in the cylinders. So it may not be the gas cap that's having the issue. It can be any of the Evap systems components from the gas tank, or the vent valve (which opens to atmosphere to allow you to fill your gas tank and also to help with purging the fumes).

It can be the Purge Valve itself (which is the actual valve that takes the fumes from the gas tank into the intake manifold). Or it can be a number of different vent lines coming from the gas tank, the Evap Canister (which stores the fumes). So you see its a pretty big system. The reason its not setting a code for the Evap system itself, is that this leak is a very large leak, so the ECM assumes its the gas cap since that's the largest opening in the system.

If you open the hood, since this is such a large leak, you may be able to find a broken hose or one that came disconnected due to engine vibrations. Evap leaks can sometimes be very hard to find though as well.
Sometimes a technician will have to use a leak detector, or a smoke machine to find these type of leaks.
Pressure testing at 1 psi is also an option.
I personally use a smoke machine to find these leaks, even then they can be very hard to find,. But that's what's happening.
Doing a visual inspection under the hood is all you can really do if you haven't gone through this experience before,
Here is a guide with a video to help you if you want to try to find the leak.

https://www.2carpros.com/articles/evap-system-code-repair
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Monday, September 20th, 2021 AT 9:52 AM

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