Fuel pressure DANGEROUSLY high

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Mar 27, 2005 | 03:16 PM

Mr Man

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From: Gilbert, Arizona

Fuel pressure DANGEROUSLY high

I just finished swapping an EFI motor into my '87 F250. The entire fuel system from the selector valve to the fuel rails are from an '89 donor truck. I have an Autometer electric fuel pressure gauge mounted in the cab. While I was wiring up the EFI engine, the gauge always read 45psi with key-on, so I figured everything was OK.

Today my buddy and I fired up the engine for the first time. It was running extremely rich. I checked the fuel pressure gauge and it was pegged at 100psi. We shut the motor down and allowed it to cool off... Turned the key on again, and fuel was squirting all over the floor. I think I blew the fuel line at the pump, but haven't gotten down there to check yet. I'm just thankful that it happened when the headers were cool, otherwise my truck (and I) would be a smoldering pile.

What was the cause of 100+ psi fuel pressure? Is it common for the stock fuel pressure regulator to go bad and stick closed? The regulator is correctly hooked up to manifold vacuum. I'm almost certain that the fuel return lines aren't pinched anywhere.

Any help would be appreciated.
Scott

Mar 27, 2005 | 10:50 PM

LxMan1

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From: Louisville,Ky.

WOW, that's a lot of pressure. I guess a regulator could be bad, but it should show up without the engine running too. If you turn the key on and off several times, it should reach maximum pressure. If the pressure is getting that high only when running, I would check the voltage output of the alternator. It may be overcharging and causing the pump to spin way too fast.

Mar 29, 2005 | 08:26 PM

Mr Man

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From: Gilbert, Arizona

Thanks LxMan. I checked my regulator tonight and it is fine - it opens up at about 45psi. The fuel pressure problem occurs when the engine is off too, and I verified the pump is getting the proper voltage.

After checking the fuel lines more closely, the return line is the one that broke. I tried blowing compressed air thru the return line and into the selector valve. It was completely blocked off. I don't know exactly how the selector valve works, but it seems like something is stuck. Has this happened to anyone else? (This is the kind of selector valve that has no electrical connections.)

While I was installing the engine and fuel system, I probably did 40 or 50 Key on/off operations, and the fuel pumps would cycle each time. Would this screw up the selector valve somehow?

Thanks again,
Scott

Mar 29, 2005 | 09:37 PM

LxMan1

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From: Louisville,Ky.

Your switching solenoid doesn't have an electrical plug? My Dad's 89 does, that is what switches which supply fuel lines are used so that the right ones are open for the tank that you are using. Sounds like you may have a problem in that area. It could be in between the two positions.

Mar 29, 2005 | 10:11 PM

Mr Man

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From: Gilbert, Arizona

Nope, my valve doesn't have any electrical connections at all. (My '87 carb fuel system did, but the '89 does not.) It's a canister that switches from front to rear based on fuel pressure, I think. For example, when the rear pump is on, fuel from the rear tank fills the canister and switches the valve. Then when the front tank is selected, pressure from the front pump causes the valve to switch the other way. Somebody correct me if I'm way off please.

Maybe both the front and rear pumps are running and causing the valve to get stuck somewhere in the middle. I'll check tomorrow. Obviously the engine is getting plenty of fuel - it seems strange that just the return line would be blocked off and not the supply line too. Whatever is going on, I want to fully understand it and make sure it doesn't happen again. I don't need 100+ psi fuel pressure turning my truck into a fireball.

Mar 29, 2005 | 11:41 PM

EPNCSU2006

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From: Concord, NC

cycling the key on and off wouldn't mess up the selector valve. The valve is switched by pressure from the in-tank pumps (like this on all '87-'89 trucks with dual tanks, so I'm not sure what your dad's truck has on it, LxMan), and the pumps are powered through the selector switch in the dash. Unless there's a short in the power wire to one tank, only one in-tank pump should run at a time.

Mar 30, 2005 | 10:18 PM

Mr Man

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From: Gilbert, Arizona

I confirmed that my selector valve is bad. The fuel feed line switches correctly. But when switching from the rear to the front tank, the return line doesn't switch completely and gets blocked off. Thanks for the input, guys.

Scott