86 GMC vent check valve in fuel vent line?

[Carl King]

Hi All,

I've got an 86 GMC, originally with a carb 305. I replaced it several years ago with an LT1 MFI motor.

The issue I'm having lately is the electric fuel pump is not priming after its been running a while (clear fuel filter is empty on the inlet side of pump). It doesn't matter which tank I'm running on, it'll do the same thing either way. Tank switch seems to be OK, I can run on either tank.

I have one of the oddball mid 80s GMC where the 1/4" line is the return line and the 5/16" line is the vent. I've verified by checking at the dual tank valve and found the 5/16 line bypasses the tank switch and goes directly to each tank thru a T fitting. If I try to use the 1/4 for vent and 5/16 for return the fuel returns to the wrong tank. I've never done any work on the tank switch so it should be original.

I've found the fuel pump gets hot after a while and doesn't prime anymore until I either pour cold water on it to cool it or let the truck sit for a while. It did this with the existing pump so I replaced it. The new pump is doing the exact same thing.

Before I replaced the pump I disconnected vent line from the evap canister to eliminate that as an issue so the vent line is open now. I used a long fuel hose to ensure the end of it is well above the tank level. I've found that I can't blow thru the vent line into the tank but can easily suck air thru it. This would seem to prevent the tanks from getting air into the tank to replace spent fuel and cause a vacuum lock, making the pump work really hard until it couldn't overcome the resistance of the vaccum.

So that's the long way of asking the question, is there a one way check valve on the vent line that would/should prevent air from returning to the tank thru the vent line?

I've read about a check valve that's supposed to prevent fuel from going down the vent line to the canister and relieve tank pressure but it doesn't make sense to not allow air to enter the tank.

I'm going to try driving with the gas caps loosened and see what that gets me.

 

[highdesertrange]

what fuel pump are you running? where do you have it located? before or after the splitter? highdesertranger

 

[Carl King]

Its the Airtex E8248. Supposed to be good up to 120psi. Its mounted after the splitter, under the hood on the fender well. No more than 12 inches , if that, above the tank.

Strange thing is, the previous pump was an Airtex (as well) before this one and was mounted in the same place. It lasted 8 years before this problem cropped up. From what I've read about the life of an electric fuel pump, 8 years is an eternity.

If the pump is working too hard, do you think putting a second low pressure pump close to the tanks would help drive the fuel to the one under the hood?

 

[HotRodPC]

Electric fuel pumps, to the best of my knowledge should ALWAYS be closest to the tank as possible. It's easier for them to push the fuel than pull the fuel. I'd suggest that could be part of the problem, but since you mentioned it had one there already that lasted 8 years, then I guess that isn't the case this time.

 

[Carl King]

I was thinking I'd move it anyway, to at least eliminate the heat in the engine compartment.

I'm still not clear on why the vent line is only one way though. I'm going to disconnect it at the T and blow it out with compressed air to be sure its open. I'll see if its still blocked from the T to the tanks.