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very high fuel tank back pressure BUT WHY?
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butterman.b
Novice
Feb 26, 2012, 10:06 AM
Post #1 of 14
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very high fuel tank back pressure BUT WHY?
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the other day my buddies and I were happily towing a small empty trailer down the highway in my 1991 Crown Victoria (all stock 302v8 AT etc.) when we started to smell gas fumes. We had filled the tank (and topped off) just 30 miles earlier and so we thought there might be a leak. we pulled over and popped the trunk and a cloud of gas fumes rushed out (you could feel and smell it and almost see it) I then went to the side of the car and opened the gas filler door and there was raw gas inside the door. I turned the cap and it popped off into my hand, a blast of gas fumes came out of the filler neck and lasted for almost 10 seconds before it stopped. Not just your average little whoosh of back pressure but a full 10 seconds of pressurized gas blasting out of the filler neck with small bubbles of liquid fuel mixed into it. I have to admit we were scared to get back into the car and drive away. But we continued on our trip and stopped again about 12 miles up the road. Pulled the gas cap and the pressure was there again but not nearly as dramatic (only 12 miles so..). We went another 30 miles and stopped again pulled the cap and bam another major sized blast of pressurized gas came out, close to being as bad as the first time. We turned the car around and headed home figuring we would just have to stop ever 20 miles or so to vent the pressure. Strangely the next two times we stopped to vent there was no pressure! none whatsoever. Then I decided to see if the tank being full would cause the problem to reappear so pulled into a station and filled it right to the brim. I drove home from there 35 miles and no pressure build up whatsoever. I am stumped I have been in the car business 35 years and I can't remember ever seeing something like this before. What do you think might have happened?
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Feb 26, 2012, 10:34 AM
Post #2 of 14
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Re: very high fuel tank back pressure BUT WHY?
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Evap system is clogged. These cars also subject to fuel tank, neck and rust so need to find and fix what and where it's doing wrong,
T
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butterman.b
Novice
Feb 26, 2012, 10:49 AM
Post #3 of 14
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Re: very high fuel tank back pressure BUT WHY?
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Are you referring to the black coffee can sized canister that mounts under the hood? Do you have any idea why the problem suddenly went away?
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Tom Greenleaf
Ultimate Carjunky
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Feb 26, 2012, 11:38 AM
Post #4 of 14
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Re: very high fuel tank back pressure BUT WHY?
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Hose or line to and from could have been clogged and if working now and things look ok there isn't much to do,
T
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nickwarner
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Feb 26, 2012, 11:50 AM
Post #5 of 14
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Re: very high fuel tank back pressure BUT WHY?
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The buildup and then sudden release multiple times could possibly have knocked out whatever was plugging the system up. As Tom says, if its working now you'll be hard pressed to find a problem.Should the fault reappear it will need to be diagnosed while faulty to find the offending component.
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Discretesignals
Ultimate Carjunky
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Feb 26, 2012, 12:28 PM
Post #6 of 14
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Re: very high fuel tank back pressure BUT WHY?
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If you fill the tank to the brim, there can't be no pressure because you have more liquid in the tank than vapor. That's why modern EVAP system diagnostics won't run unless the tank is between 15%-85% full. It needs the vapor in there to check for leaks.
It's fine now till the fuel level gets low. Check your tank vapor lines and canister for restrictions as the others stated.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Feb 26, 2012, 12:44 PM)
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butterman.b
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Feb 26, 2012, 12:33 PM
Post #7 of 14
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Re: very high fuel tank back pressure BUT WHY?
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sounds logical and accurate. I think I will remove the canister and have a look anyway. maybe blow out a few lines with compressed air just to be sure But all in all I think your right and and the car is safe to drive. Thank you all for taking the time to look at the post and think about what happened. Bill B
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Discretesignals
Ultimate Carjunky
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Feb 26, 2012, 12:40 PM
Post #8 of 14
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Re: very high fuel tank back pressure BUT WHY?
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Be careful blowing lines out. If you go that route make sure they are disconnected from the tank. Don't blow compressed air into the tank because the oxygen in the air could possibly create a flash fire. That is why most leak detection equipment uses non flammable nitrogen. Its even worse with the E85.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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Hammer Time
Ultimate Carjunky
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Feb 26, 2012, 12:45 PM
Post #9 of 14
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Re: very high fuel tank back pressure BUT WHY?
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You can also blow your tank apart with as little as 20PSI.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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nickwarner
Veteran
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Feb 26, 2012, 12:46 PM
Post #10 of 14
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Re: very high fuel tank back pressure BUT WHY?
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These lines have a lot of rubber hoses that do deteriorate over time. Its the same as rubber vacuum hose and is dirt cheap as its sold in bulk at auto parts stores by the foot. As you take it apart they will likely fail or most likely have to be cut off anyhow. Get your sizes and buy a few feet at the parts house and replace them all. For the age of the car may want to replace all rubber hoses to head off future issues. Be sure to do one line at a time to avoid mixups. That rubber is over 20 years old. Last time I trusted a brand new piece of rubber I got 18 years of child support payments.
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Discretesignals
Ultimate Carjunky
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Feb 26, 2012, 12:57 PM
Post #11 of 14
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Re: very high fuel tank back pressure BUT WHY?
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If you replace the vapor hoses, you wanna get the ones that are rated for ethanol. Back then they didn't have the alcohol drying out the hoses.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
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butterman.b
Novice
Feb 26, 2012, 1:24 PM
Post #12 of 14
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Re: very high fuel tank back pressure BUT WHY?
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great reminders. I will consider all of these points. I have a tendency to forget how much has changed since I learned how to fix cars in the early 70's. About that 20 psi to blow up a fuel tank; I had to laugh back in the late 70's I owned a junk yard and we used to occasionally pump air into some of the newer tanks just to see if they could take it and needless to say they could not. We got some very weird looking shapes out of those "experiments"
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Tom Greenleaf
Ultimate Carjunky
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Feb 26, 2012, 3:47 PM
Post #13 of 14
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Re: very high fuel tank back pressure BUT WHY?
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A side maybe for plugged lines: Where I am (MA) and area has an insect I'll call a mud wasp and they pack literally mud which is so air tight it's blocked off like you intended to do that and they will go after fuel items and do. Got the T-Shirt. If that stupid and possible those lines that are plugged if by something like that you just pinch them on the crusty parts and it falls out. A way out there maybe that does happen where I am,
T
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nickwarner
Veteran
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Feb 26, 2012, 5:00 PM
Post #14 of 14
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Re: very high fuel tank back pressure BUT WHY?
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No matter the cause, I'm still stuck with the damn child support.
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