vapor lock issues again!!!

I'm sure this has been discussed several times in the past, however this one might be a little different. 1978 C10 Big Ten, so no evap system. Dual tanks, OEM. I built a 377 for it 2 years ago and fitted it with Edelbrock ProFlow 4 EFI system. I used the Edelbrock fuel sump kit so I could retain the factory dual tank switches and system. So the fuel delivery is all original style from the tanks to the mechanical fuel pump on the engine. No return and the tanks vent right off of the sending units. This year for the first time the engine died while driving on a day the ambient temp was 90+ degrees. I have a fuel pressure gage attached at the input of the low pressure side feed to the sump, between the mechanical pump and the sump inlet. It read 0 psi. I let it cool down about 20 minutes with the hood open and it restarted and pressure came up to 3 psi, normal was 5.5 psi. I figured it was a dead mechanical pump, one of the only 2 parts that came from the original engine. I bought a 110 GPH pump from Summit and changed it. When cold it runs 6.5 psi but after about 30 mins running it dropped to 3 psi. I bypassed the tank switch and went straight to the LH tank, no change. I filled the tank with clear premium, no ethanol (ouch) and no change. Cold pressure is fine, when running for longer than 45 mins with temps above 90 degrees it will drop to 0 psi, same as the old pump. This sure sounds like vapor lock, but over the last 2 years even in high heat it never did this. Is anyone experiencing similar issues? Could this be a RVP issue from winter blend fuel? I live in the Pacific NW and we are now getting California blends due to their loss of refineries and they are now getting gas from the Washington refineries and we are told they are now running the CA blends for everyone up here as well. I've attached a couple of photos of the configuration. Any ideas would be greatly appreciated!

I was fighting vapor lock for a while. 90 degrees was when it would start. In my case I ended up putting in 87 FI fuel tanks and using in tank pumps through a regulator to keep the mechanical pump fed. GM did a big study on vapor lock. The short version of it is vapor lock happens on the suction side of the fuel pump. Suction/low pressure causes fuel to boil, heat causes fuel to boil. There solutions were to shield fuel lines from heat sources like your exhaust, run larger fuel line between the tank and fuel pump or to to use an electric fuel pump mounted near the tank. Notice when we went to FI cars with fuel pumps in the tanks we no longer had to deal with vapor lock. A winter blend fuel RVP could make things worse.

Thank you for the rapid response. Much of this makes perfect sense. When you added the TBI sending units and pumps how did you wire them up? Did you need to change the tank switch at the dash or wire the relays through the existing switch?

Back in the day we used to clip wooden clothes pins to the metal fuel lines to act as heat sinks when temps started causing vapor locks. May have looked funny, was a cheap fix but it did work. Stopped using ethanol blended gas after my first experience.

My opinion is you need a return. Once the sump is filled all the fuel not being consumed sits in the lines and sump absorbing underhood heat.
Mechanical pump on the block plus the fuel lines run right by the header on the pssenger side doesn't help either.

Return is the sure fire way to fix it.

A return seems to be the most common response so far. I'm struggling a little on how to accomplish this as the vehicle didn't have a return from the start nor did it have an evap system. I did have to connect a vent from the sump to the fuel tank vents at the sending units. I can run a return line however wouldn't it require changing the tank switch valve to accomodate the return so it switches returns as well as supply. Has anyone done this and can let me know what is required? I would like to keep the 32 gallon capacity due to the new engine's increased appetite for dino-juice.

Well I do not know If this would work, but cheap and.easy enough to try. Liquid under vacuum boils at a lower temperature, liquid under pressure boils at a higher temp. These are the reason we Vacuum A/C it boils the water out and why we pressurize cooling systems, to raise the boiling point. Right now everything from the tank to the pump is under vacuum. I'd put a pump,like a posi flow faucet pump at 5psi immediately after the switching valve. I'd hope that inducing the 5 psi would raise the boiling point enough to fix the problem. Rough math on this looks like we could raise the boiling point~ 50°/60° maybe more I was using 3lbs vacuum you maybe pulling more than that, depending on your pump specifics and the condition of your fuel pump sock.

SS1019 said:

Thank you for the rapid response. Much of this makes perfect sense. When you added the TBI sending units and pumps how did you wire them up? Did you need to change the tank switch at the dash or wire the relays through the existing switch?

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The switch and wiring is different. It was a complete swap over ,dash switch,wiring and switch valve all had to be changed.

The fuel valve is easy. You can use a vavle from 86 or newer that has a return built in ( TBI has 6 ports vs. 3 for carb. ). The wiring wouldn't change. The valve gets power for tank 1 and loses power for tank 2. As for fuel pumps you could use a relay off your sending unit signal if both tanks read on same gauge. Or run a second switch. I'm currently working on a 73 crewcab dually going supercharged ls with dual in tank pumps. I'm using a two way relay from the valve control wire. Switch on = tank and pump 1. Switch off = tank and pump 2. The valve only needs 5 to 10 amps. My fuel pumps require 25 amps ( why I'm running a relay ). Hope this helps.

I was able to put the fuel filter with the return port in the system and plumbed the return back to the fuel tank filler to test the vapor lock fix. I had to put a #60 Holley jet in the hose to reduce the flow down to about 1 pint in 60 seconds of flow. The static pressue in the beginning was 6 psi. Over 15 mins of running it dropped to 3 psi at idle. I'm down to replacing all the rubber fuel lines. They were all replaced 3 years ago when I put the engine in and changed the sending units in both tanks. But they were jungle website chinesium and after looking up the specs it is not "ethenol resistant" so I'll be pretty pissed if the hoses are all crap now. I'll repost after replacing the hoses.

You can use a lawn mower fuel cut off valve as a return line restrictor. Open it more for more bypass less for less bypass more pressure. It's what I am using have been for at least a couple of years.Was going to use a ball valve from the hardware store @bucket thought of the lawn mower shutoff.

SS1019, I'm interested in your post also. I put a fuel guage on my truck after doing most everything you have done. When I installed the guage it was reading at about about 5.5 psi at the guage. Now it is reading zero psi on the guage but my truck is running great.