weird issue with fuel tank pressure and venting...I think

[1972BlueC20]

I've discovered a new weird issue since getting my truck back from a new engine install. The shop that did the install is going to look at it for me and most likely fix it, but I'm hoping I can figure it out myself as I like tinkering with the truck, after all that's why I bought it.

Some of you might have been following my ongoing saga of issues over recent weeks, but the latest weird thing is puzzling me and I'm hoping you guys can help!

So I was having issues with dropping fuel pressure and air bubbles in the fuel filter after the truck was idling for a while. I noticed that my return fuel line was left open at the engine side but still hooked up to the tank. I also was running a non-vented fuel cap for the last 6 months with no felt problems (on old original engine).

I plugged the return inlet on the tank itself, and I installed a new vented gas cap.

The next time I started the truck it ran fine, but after I shut it off IT WOULD NOT RESTART! I tried a few times but nothing.

I decided to see if the gas cap was causing the issue and I went to take the cap off and there was TONS of pressure that released.

After taking the cap off the truck started right up!

I put the old cap back on, and so far it's starting and restarting fine.

Any ideas what's going on here??

[BLT gmc]

I guess I do not fully understand why you would have pressure in the fuel tank if the new cap you bought was really a vented cap. Is it possible the new cap is non-vented?
Is the tank in your truck in the original location and is it EEC or non? Bruce

[1972BlueC20]

Originally Posted by BLT gmc

I guess I do not fully understand why you would have pressure in the fuel tank if the new cap you bought was really a vented cap. Is it possible the new cap is non-vented?
Is the tank in your truck in the original location and is it EEC or non? Bruce

The tank is original non eec tank. It's a 1972 C20 3/4 ton.

I suppose the cap could be messed up or not truly vented. It said vented online when I looked up the part #, but was very hard to get on and off. Very tight seal.

[Bigdav160]

Thank You EPA! The new "vented" caps are not truly vented. They hold up to 3 psi

Perhaps the vacuum valve in the cap is not working?

[1972BlueC20]

Originally Posted by Bigdav160

Thank You EPA! The new "vented" caps are not truly vented. They hold up to 3 psi

Perhaps the vacuum valve in the cap is not working?

So what's the way to overcome this? Drill a tiny hole in the cap?

[BLT gmc]

OEM cap from one of the vendors/guys who part trucks, maybe. Bruce

[Lee H]

The return line lets excess gas/pressure produced by the pump return to the tank, this improvement for EPA regulation almost eliminates vapor lock. Not saying that's your problem but I think your mismatch of parts may be contributing. Would also speculate the cooler weather my be compounding issues.

[1972BlueC20]

Originally Posted by Lee H

The return line lets excess gas/pressure produced by the pump return to the tank, this improvement for EPA regulation almost eliminates vapor lock. Not saying that's your problem but I think your mismatch of parts may be contributing. Would also speculate the cooler weather my be compounding issues.

What do you think I should do?

Cap the return line? Leave it open and disconnected? Vented cap? Or non vented cap?

[Lee H]

Originally Posted by 1972BlueC20

What do you think I should do?

Cap the return line? Leave it open and disconnected? Vented cap? Or non vented cap?

Trying to diagnose problems with mismatched parts is difficult at best.

I would get the correct fuel pump, hook up the lines, and run the correct gas cap for your application.

[1972BlueC20]

Originally Posted by Lee H

Trying to diagnose problems with mismatched parts is difficult at best.

I would get the correct fuel pump, hook up the lines, and run the correct gas cap for your application.

yeah i see where you're coming from and I don't disagree at all, but haven't many people ran edelbrock and holley fuel pumps without issue?

These aftermarket pumps all mostly have 1 in and 1 out.

Is there any solution short of replacing the pump?

[MARKDTN]

Originally Posted by 1972BlueC20

What do you think I should do?

Cap the return line? Leave it open and disconnected? Vented cap? Or non vented cap?

Use the cap you have and hook up the charcoal canister. Vented into the canister.

[1972BlueC20]

Originally Posted by MARKDTN

Use the cap you have and hook up the charcoal canister. Vented into the canister.

my truck doesn't have a charcoal canister, it's a C20

[Don Quixote]

I caught this post a little late and I'm sorry but I've run into a little bit of a similar issue that's been confounding me.

My truck is a 72 GMC C25. I have a non EEC in cab tank, the exact tank as what is being described here. I had to replace my gas cap last year and I know I need to use vented gas caps. This is where it gets crazy. I notice the gas cap is not venting. I bring it back thinking it is defective. I get a new one, same thing. I repeat the process three more times. Now everyone thinks I am crazy, including myself. Now every so often I have to burp the tank which I am not entirely comfortable with.

I am in the process of installing a second tank which has the provisions for venting. I was thinking of replacing the in cab tank with a vented one and running a vent line through a fuel filter into the PCV system via a check valve. Just an idea, any objections, input, suggestions...?

[trac209]

I used to have the same issue,sometimes the cap would blow right off after a slight twist. So I drilled a pin hole in it and no issue since

[joeydurango]

For anyone running into similar issues, now or in the future: As far as I know, modern "vented" caps are only truly vented in one direction - inward. This is so that as fuel leaves the tank a vacuum isn't created. There is (as someone alluded to above) a certain pressure above which the cap will vent outward as well - more of a safety feature - but 2 or 3 psi is still quite a lot of pressure when you open the cap and aren't expecting it. Modern vehicles don't need an outward-vented cap because they vent fumes due to heat expansion via other, cleaner means than simply into the open air. This is what the EEC system (charcoal canister) is for on, I believe, '71 and up 1/2 ton trucks.

On my truck, I'm running the stock pre-EEC tank (from a '70) with an LS engine. I did have an extra, upper bung brazed into the tank when I had one installed in the bottom for fuel supply - just in case I needed to create an EEC-type system myself for pressure issues - but it turns out my old gas cap is either truly vented or simply has a leaky gasket, and either way the truck runs great! If I needed to replace the cap with a modern one, however, I'd end up having to vent the tank.

[Don Quixote]

I had replaced my gas cap when it started leaking as anyone in their right mind would do. It worked normally. I then replaced my sending unit float and gasket. Who would have thought that gasket was junk as well? I've been smoking in the truck for 20 years. Oh well, if it wasn't safe, the ash tray wouldn't be so big. Now when I run long hauls, the tank makes all sorts of interesting noices and every 30 miles or so I have to burp the tank.

It is the original tank and I just bought one to go in the back as a secondary so I may buy a new cab tank with a vent line. I could run a canister or just a filter with a check valve going into the PCV port. The fumes can hang out with the fumes coming out of the float bowl.

[farmallboy15]

I had problems when I replaced the original vented cap with a replacement vented cap. I think most replacements aren't really vented even though they claim to be. I got a LMC one and no problems...

[Rich72C10]

So, my it seems pressure is building up and flooding the carburetor. This has happen twice to my brother after filling up the tank.

This is a C10 72 with the EEC tank/port on the Fuel Sending Unit off the original tank behind the seat.

What I am not understanding here is if the tank is suppose to only vent from this EEC line on the Fuel Sending Unit or by the Gas Cap?

Thanks!

[Sheepdip]

Originally Posted by trac209

I used to have the same issue,sometimes the cap would blow right off after a slight twist. So I drilled a pin hole in it and no issue since

^^^^^^^^^^^^^^This is what I did also^^^^^^^^^^^^^^^

[Rich72C10]

I saw that too and it could be a "Plan B" but I am looking this way: what it is really suppose to do? I would think people back in 72 where not drilling holes in their Caps (if the cap was not supposed to be vented)! LOL

[Sheepdip]

Originally Posted by Rich72C10

I saw that too and it could be a "Plan B" but I am looking this way: what it is really suppose to do? I would think people back in 72 where not drilling holes in their Caps (if the cap was not supposed to be vented)! LOL

I could be very well 180 out ...but I think todays replacement caps because of EPA regs have to hold some pressure to a prescribed relief point so they in a sense only vent one way which is inward to keep your tank vented as you consume the fuel, a truck equipped with a charcoal canister would see no ill effect.

A truck with a non canister tank was vented thru the cap both ways inward and outward.

If you do not have a OEM cap which almost no one does you can drill a 1/8" hole right in the center and it will vent both ways.
I have yet to spill any fuel down the cab corner and when the sun heats everything up the tank relieves the excess vapor pressure.
Do as you please but this worked for me.

[71CHEVYSHORTBED402]

Originally Posted by 1972BlueC20

I've discovered a new weird issue since getting my truck back from a new engine install. The shop that did the install is going to look at it for me and most likely fix it, but I'm hoping I can figure it out myself as I like tinkering with the truck, after all that's why I bought it.

Some of you might have been following my ongoing saga of issues over recent weeks, but the latest weird thing is puzzling me and I'm hoping you guys can help!

So I was having issues with dropping fuel pressure and air bubbles in the fuel filter after the truck was idling for a while. I noticed that my return fuel line was left open at the engine side but still hooked up to the tank. I also was running a non-vented fuel cap for the last 6 months with no felt problems (on old original engine).

I plugged the return inlet on the tank itself, and I installed a new vented gas cap.

The next time I started the truck it ran fine, but after I shut it off IT WOULD NOT RESTART! I tried a few times but nothing.

I decided to see if the gas cap was causing the issue and I went to take the cap off and there was TONS of pressure that released.

After taking the cap off the truck started right up!

I put the old cap back on, and so far it's starting and restarting fine.

Any ideas what's going on here??

I'm unfamiliar with your config., but right off the bat the return line isn't connected to the fuel pump? That aint right at all, wrong fuel pump? Just at a glance I would install the right fuel pump & reconnect the return line, that's assuming you have a two line fuel sender. Also check your vent line assembly.

What year is your truck, what block & transmission and is it an original type build or mod? Are you using the charcoal canister?

[Rich72C10]

well, I do have the canister, my brother just confirmed it. So either the vent line is plugged or the canister is. We also just replaced the QJet. For now the vent line is just disconnected at the canister.

As for the fuel pump, yes it's connect to the tank. My 72 has two lines off the Fuel Sending Unit (one for fuel and the other for vent).

[dmjlambert]

So with the vent line disconnected at the canister, does that take care of the problem with the pressure build up? If so, then perhaps the canister needs to be replaced. If you have an EEC system then you should not need a vented cap and don't need to drill a hole to make a vent. The canister should vent the tank and catch the gas vapors in the charcoal until the vapors are purged by sucking them out through the PCV hose going to the carburetor when the engine is running higher than idle speeds. This is assuming the canister is working to vent the tank and the lines aren't plugged.

[Rich72C10]

Yep, the Canister was confirmed plugged up tight yesterday afternoon. Unplugging it for now but that seemed to make a lot of other things happier.

Thanks guys