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Excess fuel tank pressure

K

Kaiserjeeps

Member
This is a three line fuel pump on a freshly rebuilt buick 350 from rock auto in a 1968 M715 restomod I just finished. I have 1 hour of run time on it. So the fuel pump has the return line. I was stunned at how much pressure was there when I opened the cap. Not just a small poof but a huge pressure relief. I have wondered what would cause this. Exhaust heat? The mufflers are 4 inches below the fuel tank. This is a two tank system with two Moeller brass three way selector valves controlling flow. Right now the rear tank is valved out so it is only the front tank I am dealing so far. This is odd. The OEM 3 inch filler neck has a vent tube on it. I pulled the vacuum cap off it and there is so much flow I am getting moisture (gas) from the vapor coming out. I am wondering if anyone has an idea on why the high tank pressure is happening.

Here is the truck.

 
Before I vented the tank on my J truck it would build an insane amount of pressure just moving from the cool garage to the relatively warm outside. I cant find a venting gas cap to fit so I just plumbed up a boat vent. It is not ideal since it lets the tank inhale and exhale all the time... real vents make it hold its breath until their pressure threshold is reached. Breathing is bad because it inhales water vapor which condenses and rusts the tank.
 
The caps are vented and if you turn them over there is a strap of metal that you can rotate from an open or vented position to a closed or non vented position. I think for deep water fording. They are both in the open position. But I think they breathe in only. Which makes sense for consuming fuel and vacuum relief. And I believe they were new Ebay M38A1 caps. I just brought the rear tank into service today by putting about 8 gallons in. I am going to try running off that tank and see if I get that much pressure build up. No mufflers under that one. Today I picked up the plastic 5 gallon gas can and it was ballooned out big time just from sitting in the sun despite pretty cool outside temps. It too had a stupid amount of pressure to bleed of slowly. I think I may just be dealing with muffler heat expanding the fumes. So I had better build a good vent system. The front tank is an El Camino tank so it's really wide. So lots of surface area for fumes to exist and expand. I guess I will keep an eye on it. I was just surprised by the amount of pressure. My CJ gives a little poof and its equalized. I cannot say how glad I am to have it running. I broke in the cam yesterday. It has been a 5 year build. This is my second M715. My first one was all original and museum quality. The parts availability have dried up so a restomod it is. I wish my late wife could see it done. We were jazzed about camping in it. Thanks for the replies and experiences.
 
Modern vented caps only allow air IN so they don't allow hydrocarbons (fuel vapors) out into the atmosphere. My 49 Willys 2WD truck built up crazy pressure just from sitting in the sun. I drilled a very small hole thru the cap so the tank could breathe. No more issues.
 
Kaiserjeeps said:
This is a three line fuel pump on a freshly rebuilt buick 350 from rock auto in a 1968 M715 restomod I just finished. I have 1 hour of run time on it. So the fuel pump has the return line. I was stunned at how much pressure was there when I opened the cap. Not just a small poof but a huge pressure relief. I have wondered what would cause this. Exhaust heat? The mufflers are 4 inches below the fuel tank. This is a two tank system with two Moeller brass three way selector valves controlling flow. Right now the rear tank is valved out so it is only the front tank I am dealing so far. This is odd. The OEM 3 inch filler neck has a vent tube on it. I pulled the vacuum cap off it and there is so much flow I am getting moisture (gas) from the vapor coming out. I am wondering if anyone has an idea on why the high tank pressure is happening.

Here is the truck.

Click to expand...
WOW!!!!!! That is a sick rig!
 
Thanks, It has been a haul. 5 years now. When I bought it it had a chev 502 the seller kept. The hood was at my forehead with 44 gumbo's and a spring over on it. I was a 2 wheel drive burn out rig. I corrected all that.
This is how it all started. I built this one the way I wanted. All real one ton stuff. The OEM axles while rated at 1 1/4 ton are actually very weak. Small shafts and it was just disposable military stuff. The front is a chev dana 60 and rear is a dodge dana 70.



I put a pressure gauge on the fuel line just before the filter and carb. I am getting 9-10 PSI from a stock engine mounted fuel pump. That seems high maybe. It's the Buick 350 so it will be higher that a 225 V-6. At least according to rock auto on their spec's for each pump. I was considering running vent lines up into a rear stake pocket. But need to find a roll over valve for each tank.
 
I have a Holley Red fuel pump with a low pressure Holley 1-4psi regulator and all -6AN fittings and Braided lines. The regulator has no return line capabilities. When I turn the jeep off after a goof run I have fuel what sounds like boiling back into the tank. But its just the excess pressure 7psi returning to the tank through the pump. I have a OLD SCHOOL STANT brand locking cap which is vented.
 
It appears the muffler heat is causing the pressure. I discovered the fuel level sender is not working in the forward tank so I started using the rear tank in preparations to lower the forward tank to check out the sending unit. I am not getting the pressure in the rear tank. So it appears that I have my answer. I still need to plumb in a vent line system that ties into the fording pressure tubes on each filler neck. Onward..
 
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