John S Posted May 3, 2015 Share Posted May 3, 2015 Gas tank has pressure in it when I remove cap. 1970 Monte 350/300 hp the car does not have emissions. The tank does not have any vent lines on it. I have a vented cap replaced twice but pressure still builds in tank. Normal? Or do I have wrong cap on. John S Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted May 4, 2015 Share Posted May 4, 2015 you should have a vented cap, if it's not venting, it's bad or try to clean it so it vents Quote Link to comment Share on other sites More sharing options...
John S Posted May 4, 2015 Author Share Posted May 4, 2015 I should not have any pressure in tank after driving for a while. I have 3 vented caps, two of them new. I guess I will take a close look at them see if there is a way to clean it. Seems improbable to get two new faulty caps but not impossible. John Quote Link to comment Share on other sites More sharing options...
Montemedic Posted May 4, 2015 Share Posted May 4, 2015 Many owners of the early cars with non vented systems have the same issue. Even with vented caps they can still build pressure. 1-2 1/8 inch holes drilled into the gas cap seem to work well with no leakage of gas and cures this problem. Quote Link to comment Share on other sites More sharing options...
MI_Custumz Posted May 4, 2015 Share Posted May 4, 2015 Ok, stupid question. How do you know it's under pressure? Does it sound like a soda can or bottle being opened (well, kind of) ? Does the pressure go away overnight if it sits? Quote Link to comment Share on other sites More sharing options...
Rob Peters Posted May 4, 2015 Share Posted May 4, 2015 Ok, stupid question. How do you know it's under pressure? Does it sound like a soda can or bottle being opened (well, kind of) ? Does the pressure go away overnight if it sits? Yes, you would hear it. In reality you could hear pressure OR vacuum. It could be vacuum if you drove awhile and stopped to fill-up. Vacuum due to the displacement of the gas you have used up and pressure due to expansion of the fuel in the tank on a warm day. Â Yes, it is possible it could bleed off overnight. rob Quote Link to comment Share on other sites More sharing options...
MI_Custumz Posted May 4, 2015 Share Posted May 4, 2015 Thanks. That's what I was thinking, but not enough coffee this morning to be sure. Quote Link to comment Share on other sites More sharing options...
AL M Posted May 4, 2015 Share Posted May 4, 2015 Is this a replacement tank? If so, this tech article saved me. It starts on page 3 so make sure you read the first 2 pages. In addition, the only thing I would do differently to this cap is drill a single hole with a very small .030 or .040 bit. These small bits won't let fuel out and will allow the tank to vent. I used a 3/32 and a tiny bit of fuel comes out of hole. http://www.chevelles.com/forums/showthread.php?t=186510#/forumsite/20668/topics/186510 Quote Link to comment Share on other sites More sharing options...
AL M Posted May 4, 2015 Share Posted May 4, 2015 This info should go out to all in this club who have a 70 and looking to replace with the stock factory tank. Spectra premium industries is in the process of looking into this issue. Until they fix it I would recommend that all 70 owners get a 71 tank and use one of the tank vents to allow the pressure out and get a non vented cap or modify this new cap. Steve Levanti warned me about these issues but I didn't listen. Had all the pressure and leakage issues. Then found that tech info on TC and got lucky because the cap really is a cure all to this problem. Quote Link to comment Share on other sites More sharing options...
John S Posted May 4, 2015 Author Share Posted May 4, 2015 Where on the cap would you drill the hole? I am assuming above center but not high enough to drill through gasket. Is that correct? John Quote Link to comment Share on other sites More sharing options...
AL M Posted May 4, 2015 Share Posted May 4, 2015 John, go back to the link and see the pics. The op drilled 3 holes. Theres no need for the outter holes so the middle hole us where u need to be. Eye it up. Quote Link to comment Share on other sites More sharing options...
John S Posted May 4, 2015 Author Share Posted May 4, 2015 I have a different style cap, I have  replacement for OEM style. I have a couple of caps to play with. I'll figure it out. John Quote Link to comment Share on other sites More sharing options...
AL M Posted May 4, 2015 Share Posted May 4, 2015 Is your tank a repro? Quote Link to comment Share on other sites More sharing options...
John S Posted May 5, 2015 Author Share Posted May 5, 2015 I can't say for 100% certainty but I've had the tank out 3 times and it does not look to be reproduction. I did check the "check valve" in the cap and it does seem to work properly. You can blow thru it with "lung pressure" so it is not stuck. John Quote Link to comment Share on other sites More sharing options...
AL M Posted May 5, 2015 Share Posted May 5, 2015 Well if it is a repro and from canada its most likely a spectra and none of the auto part store caps are going to work. Trust me I have many that im using as paper weights. Now if its an origional tank im stumped. Lots of luck in working it out. Ive given you all the info I have. Trust me though, if all else fails just buy that cap. Its only $10 Al Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted May 5, 2015 Share Posted May 5, 2015 took my 70 for a ride last night and tested it, it was holding pressure  new aftermarket tank new VENTED cap  I drilled a hole in it for now until I get one of these Quote Link to comment Share on other sites More sharing options...
AL M Posted May 5, 2015 Share Posted May 5, 2015 The problem with drilling in the origional 70 vented cap or replacement is that even though you releave the pressure u still get a lot of spillage. Sam where did ya get the tank from and what brand is it? And also start with a small bit like I said earlier 0.30-0.40. 5/32 and 1/8 even 3/32 is too big Fuel will come out of hole but just a lil. And remember go back and look at those pics. The middle hole is all you need. Keep us posted! Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted May 5, 2015 Share Posted May 5, 2015 I have to find one of those caps, that link you posted didn't show any Quote Link to comment Share on other sites More sharing options...
AL M Posted May 6, 2015 Share Posted May 6, 2015 CST 5092 local autozone If the link doesnt work. Look up "gas tank pressure problem solved" on Team Chevelle. Its easy to find. Quote Link to comment Share on other sites More sharing options...
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