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Gas leak/smell in Vapor tank fun

Posted: Apr 06, 2021 8:52 PM
by passta
Having an issue where I was constantly smelling gas in the trunk and cabin with the windows down on my M635. I noticed one of the vapor lines at the gas tank end, was weeping, so I cut a chunk off and clamped it back on, no more leak, but still the occasional smell heavily concentrated in the trunk, and its been progressively getting worse. All the fuel lines and pumps are brand new, except the vapor lines. Checked the in tank pump, no leaks, gasket is fine. Checked the main pump by looking as best I could with the car running, no gas smell and no obvious leaks from that area, just in the trunk.

Today I filled up the tank, in hindsight I wish I had not done that, and the smell was still there and seemed to be getting worse. I got home from station about a 1/2 mile away, and opened the trunk with engine running, and gas was steadily dripping out of the vapor tank in the trunk onto the carpet. Shut the car off immediately, and the gas was still leaking. I was going to disconnect the battery, which is in the trunk, but I didn't want to risk a spark, so I let it be for the moment. Something had to be pressurizing the gas tank to force gas into vapor tank I thought, since gravity couldn't do it, so I opened the gas cap and air rushed out and quite a bit of gas came bubbling out the neck.

I took the vapor tank out, emptied it of gas, and cut about an inch off each line at the vapor tank end and re-assembled. Let it run, and it was building pressure in the tank again, and I could see gas start to drip from the vapor tank again.

I'm thinking I have a blocked vapor line that is causing a pressure to build up in the tank, forcing gas into the vapor tank when enough fuel is in it? My next step is to push some compressed air thru the each vapor line 1 at a time and see if any of them are obviously blocked, and also order replacement lines.

Does that sound like the best course of action? Am I missing something else I should check? I don't think there is anything wrong with the vapor tank itself and the pipes at the tank are not rusted or bent in any way. Also, do the 3 vapor hoses go to specific spots on the tank and vapor tank end? Or does it even matter? I numbered them just in case but was curious if the 3 mattered, the 4th goes to the vent that goes down by spare tire underneath.

Re: Gas leak/smell in Vapor tank fun

Posted: Apr 07, 2021 9:41 AM
by SlickDizzy
As part of general maintenance on my car I replaced all the vapor hoses as I was also getting a fuel smell in the trunk. This solved the fuel smell issue, but then my car developed a very strange problem where after 3-4 hours of continuous driving, it would act starved for fuel and stall out, unable to restart. This stumped me for a while until I decided to try popping the gas cap off and was met with a substantial “whoooosh”, after which the car fired up and ran just fine.

I eventually determined that the metal hard line on the trunk bulkhead that the vapor tank vents to was clogged, and since there were no other leaks in the system after replacing the other hoses, it was causing enough pressure to build up in the tank that the fuel pump could not overcome it. I replaced the metal hard line and all was well, no more issues.

Tl;dr: check the metal hard line the vapor box connects to that runs along the trunk bulkhead. If it’s clogged, the fumes will vent any other place they can instead of through that line like they’re supposed to.

Re: Gas leak/smell in Vapor tank fun

Posted: Apr 07, 2021 12:13 PM
by Shawn D.
I 2nd checking the vapor tank hard line. Note that it goes forward to the charcoal canister, so if you've modified that circuit or it isn't working, that could be an additional issue.

Re: Gas leak/smell in Vapor tank fun

Posted: Apr 07, 2021 4:15 PM
by Mike W.
I've had terrible problems with those vent hoses leaking over the years, I'd just replace them rather than just the ends. I've had to do that I think on every 70s-80s BMW I've had and that's a bunch. Now gasoline is fairly expansive with heat, but it also seems odd to even have fuel in the expansion tank. So I'm a little puzzled there, but maybe it is just normal expansion. You noted it's a M635 which was a euro car but you didn't give location. If it's here in the US I'm assuming some sort of federalization was done on it upon importation. What they did varied enormously so you need to just give it a close look. They might have run a hose up front, they might have put a valve in somewhere, they might have left it as is, no telling. It is possible the hard line is crimped as others have suggested or even just blowing that line out with air, from the expansion tank side might fix it.

As far as hose location between the two tanks, I don't think it matters, except I remember 2, 6MM and 2, 8MM lines and everyone else says they're the all the same size. Since the hoses are low on the expansion tank, if they are porous, and while the ends tend to be the worst I have seen the whole hose porous, they would leak before it came out the overflow, if it's the original configuration.

Re: Gas leak/smell in Vapor tank fun

Posted: Apr 08, 2021 10:21 AM
by passta
I took all the lines loose and blew compressed air thru them, the 3 to the tank all were clear, nothing came out the other end. The hard line also sounded to be clear but I was not able to get under the car to check below. I did not notice any air coming out of the hard line anywhere in the trunk. Also blew out the vapor tank to make sure there were no clogs on that end, got more of the gas that had gotten in there out but otherwise all the vents were clear. Put it all back together and started it up, no pressure build up in tank, and no gas smell or gas pouring out of the vapor box. Drove it around the house for 10 minutes stopping to check it every few minutes, no smell, no leak, and no pressure build up in the tank. I'm guessing it was something in the hard line that cleared out with the air blast that I just couldn't see. I ordered new hoses anyway, so far so good. Thanks for the replies and suggestions everyone!