Oldguy48 Posted May 12, 2012 Report Share Posted May 12, 2012 My wife and I did a bit of cruising today with our P15. The car ran great, and everything was OK, but I stopped for gas. When I removed the filler cap, there was a noticeable hiss of pressure being released. Added fuel, and continued driving for about another 40 or 50 miles. When we got home, I removed the gas cap again, and again I noticed the hiss of released pressure. The gas cap appears to have a spring loaded valve in it so as to not create a negative pressure in the tank, but should there be some sort of vent to prevent a positive pressure build-up in the tank as well? I suppose the pressure could have resulted from the warm outside temperature, but I've never observed this issue in any other vehicle I've owned. Thanks for any words of wisdom ! By the way, the car is equipped with an electric fuel pump. Quote Link to comment Share on other sites More sharing options...
Don Coatney Posted May 12, 2012 Report Share Posted May 12, 2012 Pressure, or negative pressure? Quote Link to comment Share on other sites More sharing options...
Oldguy48 Posted May 12, 2012 Author Report Share Posted May 12, 2012 It was positive pressure, as air escaping from the filler neck when the cap was removed. Quote Link to comment Share on other sites More sharing options...
TodFitch Posted May 12, 2012 Report Share Posted May 12, 2012 It was positive pressure, as air escaping from the filler neck when the cap was removed. I don't see how pressure could build up in the tank. Assuming the vent in the cap was blocked as fuel is removed it will create a void which means low pressure. Having a electric pump shouldn't make any difference. Quote Link to comment Share on other sites More sharing options...
OldDad67 Posted May 12, 2012 Report Share Posted May 12, 2012 My wife and I did a bit of cruising today with our P15. The car ran great, and everything was OK, but I stopped for gas. When I removed the filler cap, there was a noticeable hiss of pressure being released. Added fuel, and continued driving for about another 40 or 50 miles. When we got home, I removed the gas cap again, and again I noticed the hiss of released pressure. The gas cap appears to have a spring loaded valve in it so as to not create a negative pressure in the tank, but should there be some sort of vent to prevent a positive pressure build-up in the tank as well? I suppose the pressure could have resulted from the warm outside temperature, but I've never observed this issue in any other vehicle I've owned. Thanks for any words of wisdom ! By the way, the car is equipped with an electric fuel pump. Sounds like you don't have a vented cap. What may happen is that you will starve the fuel pump of fuel due to the vacuum created in the tank by the cap and you will come to a complete stop for lack of fuel unless theres just enough leakage past the cap seal. If it were mine I would drill a very small hole in the valve in the cap to be sure it is venting correctly. Someone else may have a better idea. Quote Link to comment Share on other sites More sharing options...
deathbound Posted May 12, 2012 Report Share Posted May 12, 2012 If your vented cap is not functioning properly, the vapors may be expanding at a greater rate than fuel being removed from the tank via the fuel pump-mechanical or electrical?????? Try a new vented cap or known functional used one. Quote Link to comment Share on other sites More sharing options...
james49ply Posted May 12, 2012 Report Share Posted May 12, 2012 all newer fuel caps are vented to allow air into the tank, but not out of the tank (EPA pollution control:mad:) the newer cars have a charcoal canister to absorbe the fumes before venting then when the car is running it dispenses the stred fumes and fuel to the engine. remember that alcahol will boil of at low temperatures and will pressurize your tank. best solution is to get an origional style cap Roerts carries them and they have locking ones too. Quote Link to comment Share on other sites More sharing options...
Captain Neon Posted May 12, 2012 Report Share Posted May 12, 2012 Ethanol boils at 173F. It is that hot out, one has bigger problems to deal with than a vented gas cap. Quote Link to comment Share on other sites More sharing options...
greg g Posted May 12, 2012 Report Share Posted May 12, 2012 (edited) Remove your cap, replace it with a shop cloth fixed with a zip tie or a rubberband. Drive a test route. There should be no pressure of vacuum in the tank as the cloth will allow the tank to be vented to the atmosphere. If this works out, get a new vented cap. My feeling is that the tank is building vacuum as the pump pulls fuel from it. What you hear when you loosen your cap is air rushing in to replace the volume of fuel that has been removed while driving. This is the same concept as a gas can that gluggs while filling your lawnmower if you forget to open the vent plug. If it was pressure in the tank you could feel it pushing on the cap when you release it. There is really no way that pressure can be built up in the tank while fuel is being removed. The only way it might build pressure is if you fill the tank with ground temp (55 degree) gas on the morning of a day that is going to get hot and the fuel expands as it warms squeezing any air that might be left in the tank. A vented cap will address both these situations. Edited May 12, 2012 by greg g Quote Link to comment Share on other sites More sharing options...
Don Coatney Posted May 12, 2012 Report Share Posted May 12, 2012 I am in agreement with what Greg said. Others who do not use an electric fuel pump and do not have a vented gas cap have shared how there car would suddenly quit running as if it were out of gas after driving a few miles. When they removed the gas cap they would hear a rush of air. Then the engine would start and run well for a few miles until the same thing happened again. Installing a properly vented cap fixed the problem. As you are using an electric pump it apparently will keep on pumping with a negative pressure in the tank and your engine did not starve for fuel. I suggest you find a vented gas cap before your tank collapses. Follow this link for an interesting story. http://www.watertowndailytimes.com/article/20090424/NEWS03/304249965&cid=0&ei=alPySZSRAofQjAfxkcmBDA&usg=AFQjCNFuofa7YVqzzmngLIpAwFqttWoj0w Quote Link to comment Share on other sites More sharing options...
Oldguy48 Posted May 12, 2012 Author Report Share Posted May 12, 2012 Gentlemen, Thanks to all for the replies. I'm going to find the old gas cap and compare it to the new one on the car now. And I'm going to keep a close watch on the issue until I know exactly what's happening. Quote Link to comment Share on other sites More sharing options...
DJ194950 Posted May 13, 2012 Report Share Posted May 13, 2012 I too have experienced the PRESSURE release when i stopped to put gas in. Once was within 1/4 mile of leaving of my house and a short warm up at home. The second time was after a 20 min. drive to a friends, later driving home and stopping for gas on the way home. Without at doubt it was pressureized. I had bought a new chrome cap on Rock Auto web site. Never had the problem before, BUT had never driven the car much at all before. Venting problem??? Don't know yet, but has not caused any problems.YET?? Quote Link to comment Share on other sites More sharing options...
Rusty O'Toole Posted May 13, 2012 Report Share Posted May 13, 2012 The tank will build up pressure due to heat expansion and contraction. It is vented so it will suck in air at night, when it is cool and the gas and air contracts. But in the heat of the day when the gas and air expand the pressure cannot escape. I have seen this before. You need a vented cap or vented gas tank. Quote Link to comment Share on other sites More sharing options...
DJ194950 Posted May 13, 2012 Report Share Posted May 13, 2012 I too have experienced the PRESSURE release when i stopped to put gas in. Once was within 1/4 mile of leaving of my house and a short warm up at home. The second time was after a 20 min. drive to a friends, later driving home and stopping for gas on the way home. Without at doubt it was pressureized. I had bought a new chrome cap on Rock Auto web site. Never had the problem before, BUT had never driven the car much at all before. Venting problem??? Don't know yet, but has not caused any problems.YET?? Quote Link to comment Share on other sites More sharing options...
greg g Posted May 13, 2012 Report Share Posted May 13, 2012 These tanks have but one connection to the atmosphere the filler neck. It must have a vented cap, to allow for the build up of vapor pressure due to heat cycles and to allow air to replace the volume of fuel pumped to the engine. Simple as that. Quote Link to comment Share on other sites More sharing options...
blucarsdn Posted May 13, 2012 Report Share Posted May 13, 2012 (edited) I have told the following story about modern fuel and ignition systems vs. old car systems many times on several of the old car forums I frequently visit. Several years ago I was traveling between my home in California and my daughters place in Montana.. I had pulled into a road side rest because it had started to snow and I was not sure if I should continue. As I was taking my dog for a walk I noted a '55 Chevy pickup that had pulled in to the rest area.. A young lady got out of the truck and lifted the hood.. I walked over to her, asking if she was having trouble with the truck and offering her my help. As I got real close to the truck I noted that it had an engine that was 'WOW'. The young lady told me that the truck was giving her and her co-workers fits. She had on a mechanics uniform with the name of a local dealer, the uniform had patches that stated she was a certified ASE Mechanic.. I offered my help, telling her I had experience with older vehicles, however, upon a closer look at the engine I noted that it was much newer that a '55. I also noted that it had a HEI ignition.. The lady went on to tell me that the truck would run for awhile then stumble and/or quit.. She had to have the truck towed several times. As I looked the installation of the engine over I noted that the lead from the HEI ignition ran from the distributor to the original 'BALLAST RESISTOR' on the firewall... I explained the function of a ballast resistor to the lady, explaining to her that the HEI was designed for a full 12v, where-as the resistor was supplying less than 12v... This situation could result in a condition that could mimic a loss of fuel. I put the wire from the ignition onto the primary side of the BR, thereby bye passing the BR.. I next noticed that the truck had a nice new shinny chrome fuel cap. I walked to side of the cab, removed the cap and heard the sound of air entering/leaving the tank.. A review of the cap noted that the cap was unvented. I explained to the lady the problems that could arise from vacuum being created in the tank, which could prevent the pump from drawing fuel from the tank.. I replaced the unvented cap with an aluminum soda can that I cut down, thereby eliminating the vacuum and preventing the falling snow from entering the tank. I instructed the lady to drive the truck around the road side rest to check it's function. After several loops around the rest area she returned with a big smile on her face.. The lady exited the truck, came over to me, throwing her arms around me she planted a big kiss on my cheek.. Payment in full had been received.. The young lady did tell me that she felt very stupid... She was a ASE Certified Mechanic in new car dealership and was totally clueless to the issues I had pointed out to her.. I told her that I was very much aware of her lack of knowledge... She had to rely on the info in a computer, that only pertained to modern fuel injected vehicles, where-as I rely on knowledge learned from practical experience gained from several years of automotive experience... Bill Edited May 13, 2012 by blucarsdn Quote Link to comment Share on other sites More sharing options...
Captain Neon Posted May 14, 2012 Report Share Posted May 14, 2012 Which is why UTI grads almost always have a job upon graduation. They haven't forgotten to teach their students how stuff works, esp. at Hotrod U, and not just how to hook up a computer and techniques to change the bad part. Quote Link to comment Share on other sites More sharing options...
Eneto-55 Posted May 14, 2012 Report Share Posted May 14, 2012 If you want to determine for sure if it is pressure, or vacuum, stretch a piece of a ballon over the filler neck then go for a short drive. Neto Quote Link to comment Share on other sites More sharing options...
OldDad67 Posted May 15, 2012 Report Share Posted May 15, 2012 Which is why UTI grads almost always have a job upon graduation. They haven't forgotten to teach their students how stuff works, esp. at Hotrod U, and not just how to hook up a computer and techniques to change the bad part. The difference between blow and suck is not "Rocket Science", at least not yet. :D Quote Link to comment Share on other sites More sharing options...
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