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AFR goes super rich when fueling.

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Hi everyone. I am a novice. I only tune my own car, so I don't know if this is normal.

Whenever I go to a gas station for fueling. At idle, my AFR gauge shows 14.7 well until the fuel being pumped into the tank.

Then the AFR will jump to under 10 until fueling is done and the cap is on. Then it'll need at least 5 minutes or a few kilometers of driving for the AFR to slowly craw back to 14.7 at idle. During fueling, in my computer I can see both the vacuum and rev drop a little. But it still stay within the same grid.

What could be causing this?

My first thoughts would be-

1) Breather pipe on your fuel tank is blocked, but I would expect the fuel pressure regulator to still maintain a steady pressure even when the tank can 'breathe' and if the pump was struggling with the fuel cap on then I would expect the normal AFR to be a bit unstable.

2) Your intake air temp settings are causing it to go rich through heat soak when the engine is sitting idling but this would not be affected only by the fuel cap being removed.

I could be way off the mark but I'm sure someone else will come along and give some more ideas.


That's certainly a weird one! No, it's not normal for this to happen.

If your AFR is affected from the time you remove the fuel cap then I'd suggest that the problem is in your fuel system design and specifically the venting of the tank. I'd be fitting a fuel pressure gauge to the engine and watching what the fuel pressure does when you remove or fit the fuel cap. Even this scenario is a little confusing as if the tank is pressurising, I'd actually expect the AFR to go lean when you take the cap off. This may be off track but based on your explanation I'd certainly begin my investigation here.