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Q: Gas tank "thumping" noise on 1997 Pontiac Bonneville

I had Code P1675. Mechanic found a broken wire between the purge solenoid and the computer and fixed it. C.E. light is out. Also had a P0440 and had to replace the charcoal cannister due to overfilling gas tank and canister filling up with gas. I replaced the purge solenoid because mechanic said it most likely drew raw gas into it instead of just fumes like it's supposed to and ruined it. The problem now is this, when I start the car and drive 2-5 miles I hear a "thumping/banging" noise in the gas tank area. Two to three minutes after shutting off the engine and letting the car sit, I hear the "thumping/banging" again. Sounds like someone beating on the gas tank. I also replaced the gas cap (4 times) and the tank filler neck. What problem(s) could be causing this?
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It sounds like your vent is plugged up or broken, so when you drive you create a vacuum and the fuel tank is being pulled in, thus the banging I just had a car with this issue, and I had to pull the tank out of the car and had to the show the customer before he would believe me, but seeing IS believing as they say. When you shut off the engine the vacuum decays and thus the tank reverses its process. Try driving your car with the fuel cap off and see if the noise goes away. That will prove it!
For the past 3 days, I have been driving with the cap loose and the tank does not thump. Took it to back to the garage today to get it checked. Asked the mechanic if it could be that the purge solenoid is not shutting off and he said no because the system is preasurizing and if it was a purge solenoid or a vent the C.E. would be on. The wire that was broken was for the vent solenoid control circuit and when he fixed the wire code P1675 went away and the C.E. light went out. I know however that this doesen't mean the vent is not bad. He said that the emissions systems are so precise in cars that the car will do self checks to make sure it is running as close to specs as possible. He went on to say that the gas I had in the cannister could have done other damage to hoses or something else further back from the canister and without him knowing, maybe the vent to. This will be the first time he checks for a reason the gas tank is "thumping." Maybe he missed it and the vent is damaged because of the gas that was being pulled from the canister. I read the vent is on or near the canister. Hopefully if that is the case he will find it. I will tell you this, if he doesn't find this problem I am kicking that car to the curb. I've had it a little over a year and it has been in the garage 5 times. I only paid $1700 for it and have nearly that much in it. Guess that's what it is when you buy a used car. Thank you for the answer and if that turns out to be the case I will let you know. If the mechanic doesn't find the problem, I will tell him to take a good hard look at the vent. Even if he says its working, I might just tell him to put a new one on it because of what you said. I have read this flexing of the tank can split it open, and I don't doubt it,. I sure don't want to find out. Signed baldhead 1968
Is the vent you are talking about on the gas tank itself? I want to make sure you are not talking about the vent solenoid and I understand you correctly. Before we got the car the gas tank was changed due to leaking. (signed baldhead1968)
WOW, My 97 Buick was making a thumping sound in the tank and I had a small leak in the tank and the gas gauge quit within a couple of weeks. I replaced the leaky tank and the sending unit/gas pump and guess what. The gas gauge still does not work (stays at full like before) and the thumping is still there. Think the is related to the same problem? I thought the float was making the thumping and need adjustment. CADPLAN
Cadplan, this sound you are having could be related. My Bonneville is still in the garage being checked. When and if I find out what is causing the banging I will post it. Hopefully it will fix your problem as well. I did read about a guy that had a 1998 or 2000 Chevy Luminia that had this problem and said he changed his gas tank three times and it didn't help. Don't try new gas cap either. Tried that 4 times including a dealer gas cap and it didn't work. Parts guy did tell me that GM has been having trouble with their gas caps on newer vehicles to. Keep watching & when and if I find out anything good I'll let you know. (Signed baldhead1968)
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