Fuel pressure drop

My buddys cobra is having issues hoping someone can help us guide us on what might be the issue. The car has a lethal return style fuel system with Walbro 465s. Less than a year old. His fuel pressure drops to 20 psi but once he lets it cool off fuel pressure goes back to normal 42 psi. We are thinking it might need a Hobbs switch but we where hoping someone can shine in with some advice on where to start


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You set it to 40 psi, start the car and let it warm up. Then readjust the pressure to 40 again. It will drop from the original setting after it warms up. So set final pressure after it is hot.

Drops to 20psi while running?

Regardless, don’t see how the Hobbs switch would be the fix.

Could be a pump failing while hot.

Vapor lock (doubtful).

Bad regulator.


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MG0h3 said:

Drops to 20psi while running?

Regardless, don’t see how the Hobbs switch would be the fix.

Could be a pump failing while hot.

Vapor lock (doubtful).

Bad regulator.


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That’s what we where thinking a pump failing while getting hot, I did hear that if you don’t have a Hobbs switch they both run at 100% causing them to get hot that’s why I thought maybe a Hobbs switch would help. But on the vapor lock how would that be if you don’t mind explaining it to me. We did notice that he had some fuel leaking next to the fuel cap


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I’ve never had it happen on a fuel injected car but have on my old Jeep after it sat out in the 105* sun.

Basically the fuel vaporizes and the pumps can push the fuel. Typically going to vaporize in the fuel bowls (carb) or rails (FI).

Fuel leaking is prob the filler neck seal at the tank. Got damaged due to age and then moving it when you dropped the tank.

MalcolmV8 did extensive testing and the pumps don’t heat the fuel. It’s from recirculating through the hot rails.

I’d say it’s 50/50 on people running Hobbs or both pumps full time and no difference.

I doubt both pumps are failing though. Slim chance. One pump is more than enough to keep your pressure cruising around. My second pump doesn’t come on until 5psi and I’m on E.


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Make sure your in tank return line is above the fuel level and not aerating the fuel. Pumps need to be submerged for lube and cooling. Do not habitually run the tank low.

Did you measure the height of the pumps to the hat correctly? The fuel hose to pumps comes long from Lethal and you need to trim it down to get the pumps to insert all the way. Some times the pumps are hitting the floor of the tank and cannot suck fuel in. Heard this happening more often than not.

I run both 465 pumps all the time in my car without any issues.

03' White Snake said:

Did you measure the height of the pumps to the hat correctly? The fuel hose to pumps comes long from Lethal and you need to trim it down to get the pumps to insert all the way. Some times the pumps are hitting the floor of the tank and cannot suck fuel in. Heard this happening more often than not.

I run both 465 pumps all the time in my car without any issues.

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No they where just dropped in how the hose came from lethal, how much would you removed to cut off?


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Modestoe3 said:

No they where just dropped in how the hose came from lethal, how much would you removed to cut off?


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You need to trim off like 1/8"-1/4". I put the pump next to the mounting bracket and look how much needs to be trimmed. Cut the hose, then install the pumps.

Good luck getting the pumps out. I had to replace the hose on my friends setup because he just pushed the pump in without trimming and ended up tearing the hose trying to get it out. Hose needs to be submersible fuel hose, its about $20 a foot at O'Reillys.