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Excessive pressure in gas tank & gas odor

18208 Views 18 Replies 6 Participants Last post by  marti5
Has anyone had this problem? I have a 1995 Bmw 525I. Love it and I've only had minor problems with it.
However yesterday when I was driving home from Tucson Az. I noticed a gas smell inside the car. I had just filled the tank in Tucson before heading home. When I got the car in the garage at home and got out--it sounded like the gas tank was boiling. It was making a loud burping sound--like a pressure build-up trying to release. I opened the gas cap cautiously and a lot of air was coming out. In fact the air didn't stop coming out for several minutes. After the air finally stopped I looked down the gas filler hole and noticed that an O-ring had slipped sideways and was keeping the little spring door open a little. I got it back in place. I haven't driven the car since. Am I driving a bomb? I've checked all under the car and took the back wheel and mud guard off to inspect the expansion tank for leaks but no signs. Has anyone else had this problem? I found only one mention of this problem on another site and the guy said that when he asked his Beemer mechanic of 28 years about it--the mechanic was stumped. Never heard of this problem in all his years.
I forgot to mention--the day this happened--it was over 100 degrees.
Besides this problem, the car seems to be running ok--maybe a little sluggish but not major. I'm stumped too!!!
Thanks for any suggestions or further clarification.
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Had this happen on mine all sorted now only took 25 minutes, its for models a couple of years newer than yours but you may have a similar set up:
Thanks for the reply,, but my problem is just the opposite.
My gas tank doesn't have a vacuum--it's under pressure. When I opened the gas cap it filled the garage with gas fumes. Had to open all the doors and gladly the wind was blowing that day.
One thing though, I remember now that the bubbling or boiling sound was coming more from the left side near the rear wheel. My Bently Manual doesn't show anything there. I'm going to remove the left rear wheel and mud guard this morning and see if anything is under there that is plugged. I'll let everyone know.

is yours and M50 engine? I too think it's the Charcoal canister..but's a BMW anything goes.Sure am Interested to know what the problem is.

thanks for your input. I've just recently found out about the canister..have to by pass the canister until I get a new part. Car runs more effectively now.

There must be a vapour escape pipe blocked or some valve that lets the vapour escape so pressure wont build up, some of my friends have drilled a 2mm hole in the cap but Im sure there must be another vent somewhere.
You do have a similar set up as I have, as I do have a charcoal caniser and a vent pipe #7 here:   BMW E34 525i ACTIVATED CARBON CONTAINER/TUBING
I still haven't found the charcoal caniser and vent pipe yet. It's not on the passenger side of the car under the fender well---the same place the expansion tank and I couldn't find it under the hood anywhere. It doesn't show where it is my Bently Manual. I guess I'm going to have to take of the left rear wheel and mud guard and see if it is there. Just had too much to do this morning.
Thanks Marti for your input--I use that OEM Beemer site a lot.
I did drive the car today several miles and when I stopped--I opened the gas cap and--"no pressure in the tank". Of course, it's not that hot today.
So--will check for the canister this evening.
I finally found the charcol canister. It's in the engine compartment just behind the driver's side headlight.
I also found that the symptoms of a bad canister is--"too much pressure in the gas tank and gas smell on hot days". Go figure.
I'm going to order the part from Bararian Autosport for $80.00. Hope that's what the problem is.
The following link is the only other reference I found that resembles my problem.

Charcoal Canister Location/Replacement [Archive] - bimmerfest - BMW Forums

When I get the part and get it on and it works I will let everyone know.
Yes it's the same setup, mind you I'm in the tropics, it gets pretty hot sometimes..and yes those days the engine sucks. But now, since I've bypassed the system it runs much better,not a permanet solution ! - Once you get the new can (it should be made of metal) for the E34s then I think you need to set Co2 again if it runs rich.

FYI - my mechanic told me that he had an E36 that actually blew off the connection hoses at the gas tank line feeding into the fuel line.Kept doing it a few times until they replaced the canister.Then the problem went away.

Don't drive a BMW when it is 100 degrees out. I live in Vegas and when it is 100 degrees out, I drive my nissan and the bmw sits in the garage. These cars weren't built to handle heat. No car is, but especially these cars. Why abuse an expensive car?
Don't drive a BMW when it is 100 degrees out. I live in Vegas and when it is 100 degrees out, I drive my nissan and the bmw sits in the garage. These cars weren't built to handle heat. No car is, but especially these cars. Why abuse an expensive car?
Marti5 do you agree with this comment? I live in Australia and it can get pretty hot down here in the height of summer, I have just purchased an E38, should I be looking to get a heavier duty radiator or additional electric cooling fan/s?
The vehicle build sheet says something along the lines of hot weather pack, does that address this issue?
Well 100f is only 37c, so I cant really see a problem driving it as for uprated rad and fan, I dont see why this is necessary. Ive just been over to Belgium and it was 34c over there and all was fine, I believe Germany got up to 36c in a couple of places and thats where they are built. Also Im sure Dubai gets a lot hotter than that and they are driving them around all the time over there.
Thanks Marti5, I was hoping that was the case. We do get days in the low to mid 40c on the odd occassion, but it is more the exception than the rule.
Driving in those temperatures won't cause any harm with upgraded cooling. A car with hot weather pack should fit the bill just fine.
Hey guys--& girls too if any--
Thanks for your input.
I still haven't fixed my problem. I did find the charcoal canister finally but didn't have the time to do anything about it. It's buried behind the left front headlight and under the power steering reservoir. I finally got it off today. I wish BMW manual would say how you can tell if the canister is bad or not. I can blow through either one of the tubes on top and nothing is plugged. As far as I can tell, there is no way this could be causing the back pressure in my gas tank.
The other day—I had to drive about 70 miles to Tucson again—on a hot day—and when I got there I could smell gas again. I got out and opened the gas cap a little and “nothing”. However, when I pushed my finger inside an opened that little metal door inside the fill spout—gas came blasting out of the tank and wouldn’t stop. I had to quickly put the main cap back on to get it to stop. I waited a few minutes and opened the gas cap just a little this time and the pressure vented for 5 minutes—and still didn’t stop. I finally gave up and drove home, hoping I didn’t blow up.
I’m going to bypass the canister today with a copper loop and drive for a while and see what happens.
The only other thing I found as part of this venting system is—on the outlet side of the canister—the hose goes to some kind of check valve with an electric wire attached. But I can’t find anything online or in my Beemer Manual as to what it is or how to check if it’s bad. When I blow through that hose going to the check valve-- it’s letting air through and if I suck on the hose the valve is closed.
Does anyone have any other suggestions? If I ever find what has gone wrong I’ll let everyone know.
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A charcoal canister should never go wrong, your problem will be the venting pipe or even a bad lambda probe as its all controlled from the ECU. take a look on Online BMW Parts Catalog
Well—I finally figured it out my excessive gas tank pressure problem.
I took the right rear tire off. I removed the stone chip plastic guard from the fender well and exposed the fuel expansion tank. I traced the line from the charcoal canister up by the left front fender-- back to the top of the fuel expansion tank so I was sure I had the right hose. I then removed that hose from the expansion tank and put the end of it in a jar with a white paper towel. Then I removed the same line from the charcoal canister in the front of the car. Next I used my air compressor and blew air from the front to the back and into my makeup jar with the paper towel. I couldn't believe the crap that came out of the line, plus what looked like a small rock. “Where did that come from”? The line was plugged but its clear now!! Anyway—that fixed the gas tank pressure problem. I don’t have the gas tank pressure problem or bad gas odor anymore.

1995 BMW 525I
Thats excellent news and I like the way you sorted the problem, I will keep that for future reference.
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