Gas Tank Vacuum - Wrong Gas Cap?
Thread Starter
Pro
Joined: Sep 2010
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From: Hutto Texas
Gas Tank Vacuum - Wrong Gas Cap?
I was filling up my '81 over the weekend and noticed it took a long time to for the tank pressure to balance out. I thought it was under pressure but in fact it was under vacuum. I see the tank expanding back to its original shape around the neck while it hissed. My '81 has been converted over to Edelbrock Pro-Flo EFI and the charcoal canister has been removed. From look at my gas cap it appears to be a vented cap. My question is, if it is vented why would the tank be under vacuum? Does vented caps just allow air to escape and not come in?
Race Director
Joined: May 2006
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From: Dayton, Ohio
The cap is not vented or you wouldn't have had a vacuum buildup. You've somehow probably due to mods blocked off your tank vent.
EDIT- With some egg on my face I have to report after reading the 81 manual it says the cap is in fact vented both ways. Dang I should have known this because I checked on this about a year ago.
If you have excessive buildup of vacuum or pressure the cap needs to be checked.
EDIT- With some egg on my face I have to report after reading the 81 manual it says the cap is in fact vented both ways. Dang I should have known this because I checked on this about a year ago.
If you have excessive buildup of vacuum or pressure the cap needs to be checked.
Last edited by ...Roger...; May 25, 2011 at 12:21 PM.
Race Director
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From: Dayton, Ohio
Going back in time we all remember the vented caps in the mid years and the vented caps up to 70 but then the vent was put in the top of the tank now it seems to have come back to the cap.
I'll do a little research and see if I can find the different vents and years.
I'll do a little research and see if I can find the different vents and years.
Race Director
Joined: May 2006
Posts: 16,528
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From: Dayton, Ohio
OK it looks like
up thru 69 the cap did the venting
70-74 the vent was built in the top of the tank,cap was sealed
75-77 I'm pretty sure the cap does the excessive venting but I don't have a manual to prove that
78-82 the cap does the excessive venting ( i'l post a pic from the manual in a min)
up thru 69 the cap did the venting
70-74 the vent was built in the top of the tank,cap was sealed
75-77 I'm pretty sure the cap does the excessive venting but I don't have a manual to prove that
78-82 the cap does the excessive venting ( i'l post a pic from the manual in a min)
Last edited by ...Roger...; May 25, 2011 at 01:10 PM.
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Race Director
Joined: May 2006
Posts: 16,528
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From: Dayton, Ohio
OK I'm not as brain dead as I thought. I always thought the vapor canister and line vented the tank and it does,the fill cap vent must be there in case the canister plugs up or a line gets pinched.
I dug a 78 tank out to make sure.
In the case of the OP he has plugged the canister line leaving the cap to vent the tank but the vent won't operate until the pressure or vacuum becomes excessive. OP you need to hook your vapor canister line back up. IMO
edit Mike you beat me to it.
I dug a 78 tank out to make sure.
In the case of the OP he has plugged the canister line leaving the cap to vent the tank but the vent won't operate until the pressure or vacuum becomes excessive. OP you need to hook your vapor canister line back up. IMO
edit Mike you beat me to it.
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Pro
Joined: Sep 2010
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From: Hutto Texas
Thanks for all the info guys. I still have the original canister so now its back to the AIM and see if I can track down all the lines that go to it. I had a feeling that might be the issue and as always CF members come through.
Drifting
Joined: Jul 2002
Posts: 1,995
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From: Garland Texas
Roger,
I'm not finding anything on the 81 cap venting to atmosphere. Excessive tank pressure vented thru the cannister into the float bowl of the carb which is vented to the top of the air horn.
Mike,
You're close. The CCC used an electronic purge valve on top of the cannister to control the fuel mixture. When the CCC opened the purge valve to draw in vapors from the cannister it also changed the dwell on the M/C solenoid. This was only used on the 81 and the 80 california 305. When the pressure relief valve in the cannister went bad excessive tank pressure will push the charcoal pellets into the float bowl and clog up the carb.
Hardway,
you need to reinstall the vapor cannister like Roger said but not the one you took off. Find an earlier cannister 71-76 and plumb it into the pcv system just as the factory did. You won't be able to plumb the original into your new system. This will solve your problem.
Mike
I'm not finding anything on the 81 cap venting to atmosphere. Excessive tank pressure vented thru the cannister into the float bowl of the carb which is vented to the top of the air horn.
Mike,
You're close. The CCC used an electronic purge valve on top of the cannister to control the fuel mixture. When the CCC opened the purge valve to draw in vapors from the cannister it also changed the dwell on the M/C solenoid. This was only used on the 81 and the 80 california 305. When the pressure relief valve in the cannister went bad excessive tank pressure will push the charcoal pellets into the float bowl and clog up the carb.
Hardway,
you need to reinstall the vapor cannister like Roger said but not the one you took off. Find an earlier cannister 71-76 and plumb it into the pcv system just as the factory did. You won't be able to plumb the original into your new system. This will solve your problem.
Mike
Last edited by tracdogg2; May 25, 2011 at 01:41 PM.
Thread Starter
Pro
Joined: Sep 2010
Posts: 745
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From: Hutto Texas
You guys are a wealth of information on this topic. Below is a NOS canister on Ebay. Looks like I would only be using 2 lines, the one from the fuel tank and another to a vac. source on the intake manifold. The guy has a good picture of the instructions as well.
http://cgi.ebay.com/ebaymotors/NOS-1...item4aa4d8692c
I may be able to use my original bracket as well.
http://cgi.ebay.com/ebaymotors/NOS-1...item4aa4d8692c
I may be able to use my original bracket as well.
