Results 1 to 6 of 6

Thread: Fuel tank building too much pressure.

  1. #1

    Default Fuel tank building too much pressure.

    So back story is that we installed the Holley Sniper EFI kit on a 1974 Chevrolet K30. This truck though has a Chevrolet 502 in it. The truck runs great and cranks up just fine. All the readings on the handheld are where they should be. The problem that I'm having is that the fuel tank, after running for five minutes, will start spitting gas out of the vent hose. We installed a bigger fuel tank vent on the fuel tank, and still after 7-8 minutes it will start coming out. It does have dual fuel tanks, and the fuel hoses are routed per the instructions to the selector valve. Only one of the fuel tanks is vented, but both have vented gas caps on them. Any help would be greatly appreciated.

  2. #2


    Are you running the Holley 534-38 dual tank kit with two pumps? Are these the instructions you're following? If you have the returns on this system routed to the wrong tanks it would overfill the tank that's not supplying the fuel. If you're using some other system what instructions are you referring to?

  3. #3


    No, I am not actually. More back story to this is, I work at a automotive shop and it's a customers truck and wanted to convert it to fuel injection. Of course one of the techs wanted to do it and I was opposed to it, due to not knowing what all we would be involved with. Our owner and the customer know each other real well and the customer has us for his fleet account which is why the owner said we can see what we can do. The technician wanted to order the Sniper 4150 kit. That is all he ordered. Of course this technician is a hot head and we've had multiple problems with him and let's just say he no longer works here anymore, but the truck is still here. I personally had to go out and fix a bunch of problems that I had found with his work and got everything going great on it, except the fuel issue. I have no experience with doing this and the customer knows that and is 100% OK. I always found to be completely honest with customers and you have less problems in the long run. So back to your question, no I do not have the Dual Fuel Tank kit. The truck literally only has 5 gallons worth of gas in both fuel tanks combined right now.

  4. #4


    If I was in your situation I'd be tempted to get the system running on only one tank - the bigger of the two - which hopefully is the one that's fully vented. It sounds like it clearly plumbed incorrectly and that the returns might not be directed to the bottom of the tank(s). You could then use the second tank with a small tank to re-fill the larger tank if necessary with a small transfer pump. Either than or spring for the Holley kit: In a proper return system, you pump a gallon into the Sniper and you get .999 gallons back. So, it should never over-fill the tank.

    Lastly, and then we'll let the experts chime in, hot all vented gas caps work in both directions - pressure and vacuum. Also, with time, unless they're new, they may clog up due to condensation and breakdown of the various elastomers. Pull both caps off temporarily and see how it works. You could tie-wrap a rubber glove over each inlet to see if you indeed have a pressure or vacuum issue. The KISS approach with a single tank and single pump may be your best option. If you do go that route, I'd install an in-tank pump that has the proper vent, outlet, and return ports and be done with it.

  5. #5


    Thank you so much for the help. I'll give that a try. I have reached out to Holley about the issue, but could not get a solution so thought I'd try on here. Which it's worth a try, running out of options so l'll update you on progress. Hopefully soon. We installed the Holley Sniper 4150, 550-510K.

  6. #6
    Join Date
    Feb 2020
    Wheeling, WV


    Remove the caps of both tanks and see if that solves the complaint. That will tell you if your building psi. Stock caps (after, I don't remember what year) only vented one way. Then eventually they were made to not vent at all, except under extreme psi or lack there of.

    What are you using for a vent?
    Are the pickup assemblies stock?
    Did the dual system work prior to the EFI?

    I'm guessing you have the plumbing incorrect? You can verify it by removing both returns and letting them dump into a catch can while you switch the valve and verify feed and return from one tank is flowing through the same tank.

    For longevity you may want to look into a complete EFI dual tank kit if keeping the dual tanks is a must. Pulling fuel through the stock switching valve at the very least will shorten the life of the pump.

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
About us
Holley has been the undisputed leader in fuel systems for over 100 years. Holley carburetors have powered every NASCAR® Sprint® Cup team and nearly every NHRA® Pro–Stock champion for four decades. Now, Holley EFI is dominating the performance world as well as our products for GM's LS engine. Holley's products also include performance fuel pumps, intake manifolds & engine dress–up products for street performance, race and marine applications. As a single solution, or partnered with products from other Holley companies - Hooker Headers, Flowtech Headers, NOS Nitrous, Weiand, Earl's Performance Plumbing, or Diablosport - Holley products can give you the edge you need over the competition.
Join us