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E36 M3 (1992-1999) {Euro - S50 B32 321hp @ 7400 rpm} {U.S. - S52 B32 240 hp @ 6000 rpm}
Total Produced: 71,212 - Years Produced: 1992 to 1999


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Old Sun, Feb-03-2019, 06:31:01 PM   #1
RND1
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Default Does fuel filler neck have a check valve?

I never resolved a fuel leak issue I was having when cornering hard at an autocross event. The fuel filler neck (No. 6 in diagram) seems to be intact as I'm not getting any fuel under the car. It's running up the fuel neck, out past the cap, and down the side of the car.

I thought all fuel necks have a check valve so the fuel can only run one way? Anyone know if our cars do? The fuel filler neck is a $250 part so don't want to buy one unnecessarily. I'm hoping there's a check valve that I can purchase independent of the pipe, but I don't see one in the diagram below:

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Old Sun, Feb-03-2019, 06:41:23 PM   #2
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Default Re: Does fuel filler neck have a check valve?

Just the door at the top where the filling nozzle pokes through. I don’t recall anything else.

Are your evap lines hooked up? They vent the tank of pressure build up.
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Old Sun, Feb-03-2019, 07:41:10 PM   #3
RND1
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Default Re: Does fuel filler neck have a check valve?

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Originally Posted by Braymond141 View Post
Just the door at the top where the filling nozzle pokes through. I don’t recall anything else.

Are your evap lines hooked up? They vent the tank of pressure build up.
Yes, the car is stock. Doesn't seem like it's building excessive pressure, at least gas cap removal seems normal (no big pressure release). I'll start tracing everything between the fuel neck, tank, and charcoal canister to see if anything obvious jumps out at me. Thanks.
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Old Mon, Feb-04-2019, 04:12:16 AM   #4
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Default Re: Does fuel filler neck have a check valve?

No check valve, sounds like you need a new cap…but you should be getting evap codes if it was venting and able to leak.


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Old Mon, Feb-04-2019, 12:00:26 PM   #5
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Default Re: Does fuel filler neck have a check valve?

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No check valve, sounds like you need a new cap…but you should be getting evap codes if it was venting and able to leak.


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I swapped the cap with another E36 M3 at the event to see if I had a bad cap, but same issue occurred. I do have an evap code, but the car is in storage for the Winter, and I don't recall what it was now. I'll be pulling it out in Spring and will read the codes again.

If I have an "EVAP Gross Leak detected" code, what's the best way to track this down?
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Old Mon, Feb-04-2019, 12:08:43 PM   #6
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Default Re: Does fuel filler neck have a check valve?

If there was a checkvalve, you couldn't top it up all the way up and through the filler neck.
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Old Mon, Feb-04-2019, 04:39:30 PM   #7
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Default Re: Does fuel filler neck have a check valve?

Check the fuel vapor collector tank inside the right rear wheel well (behind inner fender cover). The upper nipple often breaks. Can be repaired with a short piece of brass tubing & epoxy.
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Old Mon, Feb-04-2019, 04:40:57 PM   #8
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Default Re: Does fuel filler neck have a check valve?

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Originally Posted by M_Parallel View Post
If there was a checkvalve, you couldn't top it up all the way up and through the filler neck.
Why not? The check valve just prevents fuel from going back up the filler neck from the tank, but doesn't prevent you from filling it all the way to the top.
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Old Mon, Feb-04-2019, 05:15:22 PM   #9
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Default Re: Does fuel filler neck have a check valve?

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Originally Posted by craigjimc View Post
Check the fuel vapor collector tank inside the right rear wheel well (behind inner fender cover). The upper nipple often breaks. Can be repaired with a short piece of brass tubing & epoxy.
Thanks - will do. If mine is broken, would this cause the fuel to run up the fuel filler and out the door?
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Old Mon, Feb-04-2019, 06:02:16 PM   #10
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Default Re: Does fuel filler neck have a check valve?

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Thanks - will do. If mine is broken, would this cause the fuel to run up the fuel filler and out the door?
You have a leak that needs to be fixed. That’s more of an indication that that might be your issue than nothing at all.
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Discussing Does fuel filler neck have a check valve? in the E36 M3 (1992-1999) Forum - {Euro - S50 B32 321hp @ 7400 rpm} {U.S. - S52 B32 240 hp @ 6000 rpm}
Total Produced: 71,212 - Years Produced: 1992 to 1999 at BMW M3 Forum.com (E30 M3 | E36 M3 | E46 M3 | E92 M3 | F80/X)