Can A Custom Gas Cap Cause A 'service Engine Soon' Light?

35thGTLimited · Jul 28, 2016

Just bought this car two days ago (2000 Mustang GT Convertible), it has a custom fuel cap, and the light has been on since the very beginning, to 'Check Fuel Cap', though I've ignored it, planned to look up how to turn it off later. Also the battery light comes on sometimes for a few minutes when I first run the car, assuming this is from sitting (It sat at least 6 months, possibly a lot more) and I'm putting in a new battery and terminals tomorrow.

But this morning, a 'Service Engine Soon' light came on, I'm wondering if it could be related to the battery/charging issue or even just the fuel cap? A quick google search suggested it can definitely be the fuel cap.

Been out of the game for a while, would appreciate any advice at all. Also nice to be back, been about 8 years since my last Mustang.

I think the dealer reset the codes, tried to pull one over on me, oh well, my problem now. Thanks!

Az Pete · Jul 28, 2016

Yes, the cap an cause your issues. Have the battery tested to verify it being good.

35thGTLimited · Jul 28, 2016

Thanks for the reply I really appreciate it. So I just went out to clean my terminals and test it, and noticed that the short like, 3 inch ground wire that runs from the negative terminal to the chassis, like near the rad, was completely sheered off.. I fixed, cleaned everything up, and went for about a 25 minute drive, highway, also a good ten minutes of idling, the battery light went off and stayed off for the first time since I bought the car. Battery reads 12.4V and 550 crank amps (should be 600 I think) and the alternator is charging nicely at about 14.45V. I slammed it with all the accessories, everything, light hasn't come back on. Still gonna replace the battery since I found a sick deal locally for $40, why not, but I think I solved my problem.

The cap I dunno what to do, it's entirely custom, lemme grab a pic for you maybe someone is familiar. Is there a way to mess with the sensor so it thinks it's closed all the time? I want to keep this custom one, just needs touch up paint work. I've never used a custom fuel door of any kind on any car, not familiar at all with how it senses it being open or shut.

Az Pete · Jul 28, 2016

The original fuel cap screws into the tank filler neck. This ensures a good seal. The reason a cap throws a code is it fails this sealing task which limits the fuel systems ability to function properly. Normally takes a couple of start cycles for the system to reset.

How does that fuel cap seal to the filler neck?

jeno_labanca · Jul 28, 2016

35thGTLimited said:
Thanks for the reply I really appreciate it. So I just went out to clean my terminals and test it, and noticed that the short like, 3 inch ground wire that runs from the negative terminal to the chassis, like near the rad, was completely sheered off.. I fixed, cleaned everything up, and went for about a 25 minute drive, highway, also a good ten minutes of idling, the battery light went off and stayed off for the first time since I bought the car. Battery reads 12.4V and 550 crank amps (should be 600 I think) and the alternator is charging nicely at about 14.45V. I slammed it with all the accessories, everything, light hasn't come back on. Still gonna replace the battery since I found a sick deal locally for $40, why not, but I think I solved my problem.

The cap I dunno what to do, it's entirely custom, lemme grab a pic for you maybe someone is familiar. Is there a way to mess with the sensor so it thinks it's closed all the time? I want to keep this custom one, just needs touch up paint work. I've never used a custom fuel door of any kind on any car, not familiar at all with how it senses it being open or shut.

Thats just exterior you need to replace the cap

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35thGTLimited · Jul 28, 2016

Ooh, okay, total brain fart here, confusing fuel door with fuel cap, my bad, yeah I'll do that first thing, looks cheap enough. I'll have my buddy check it over when we do the battery and new terminals tomorrow, thank you. I just assumed the cap was fine, was focused on the door because it's not stock. Thanks again
Edit:

Az Pete said:
The original fuel cap screws into the tank filler neck. This ensures a good seal. The reason a cap throws a code is it fails this sealing task which limits the fuel systems ability to function properly. Normally takes a couple of start cycles for the system to reset.

How does that fuel cap seal to the filler neck?

Under the fuel door the cap looks totally stock, like the image posted above by Jeno.

Az Pete · Jul 28, 2016

Yep.....that cap is most likely your issue.

35thGTLimited · Jul 28, 2016

Thanks a bunch guys for taking the time to reply. I'm gonna do the fuel cap and battery tomorrow, hopefully this resolves my issue! Will post results/findings.

Az Pete · Jul 28, 2016

With that ground wire issue, check all ground wires you can find under the hood. I think there are a couple near the headlights.

35thGTLimited · Jul 28, 2016

Az Pete said:
With that ground wire issue, check all ground wires you can find under the hood. I think there are a couple near the headlights.

Okay, we're going early tomorrow, I'll make sure to mention it to my mechanic, and also look around a bit myself. Thanks for the reply!

Can A Custom Gas Cap Cause A 'service Engine Soon' Light?

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