Key on engine off fuel pressure question
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Key on engine off fuel pressure question
93 f150 5.8 When the ignition is turned on, but not started, the fuel pump energizes briefly. Is this supposed to pressurize the fuel rail enough to show a pressure reading? If no pressure is present, could the injectors leak the fuel before it can be read on the pressure gauge? Would the check valve in the fuel pump be more likely?
I've eliminated the pressure regulator as the culprit
I've eliminated the pressure regulator as the culprit
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Nope. Luckily, I put 5 gallons in the empty front tank and for the first time ever (at least that was noticed) it transferred to the rear while running the front tank, leaving it bone dry. So now I know the rear pump is the problem. Kinda sucks that the front pump cannot be used when the rear one is faulty. I've always had a false sense of security knowing if one went down, there would be one left. Apparently that is not always the case.
Logistics Pro
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From: suburban atlanta
I'd say the tank selector valve is faulty. If the front tank is dry, the pump pumped it out. I guess you are saying it won't run on the rear tank, and if you put fuel in the front tank, it just pumps to the rear tank, no? The pumps can be replaced w/o replacing the sender in most cases. But you have to drop the tank. The selector would probably be a factory only part.
tom
tom
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From: Louisville,Ky.
A 93 doesn't have a tank selector. It has a check valve in each pump assembly (can't buy just the pump motor anymore) on the pressure side to keep the gas from pumping into the unused tank. I have seen this many times. The pump will pump fine, but when not in use will allow gas to be pumped back into the tank.
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From: suburban atlanta
So they switch the pumps on to select which tank?
Is it possible that he may have a defective switch then, not energizing the rear tank pump? I replaced a 'sending unit' on my Merc at about 150k, and it was about $200, and that was 8-9 years ago. I could have bought just the pump, but my wife wanted a fuel gauge that worked, so I had to buy the whole thing. A pump was like $40.
I would check aftermarket sources to see if they have come up with a replacement for just the pump, personally. Especially if the fuel gauges work correctly.
tom
Is it possible that he may have a defective switch then, not energizing the rear tank pump? I replaced a 'sending unit' on my Merc at about 150k, and it was about $200, and that was 8-9 years ago. I could have bought just the pump, but my wife wanted a fuel gauge that worked, so I had to buy the whole thing. A pump was like $40.
I would check aftermarket sources to see if they have come up with a replacement for just the pump, personally. Especially if the fuel gauges work correctly.
tom
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From: Louisville,Ky.
His rear pump is faulty due to the check valve being bad. The pump motor itself is probably bad too. Regardless, it will need to be replaced. I have never seen a switch fail, but anything is possible.
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Logistics Pro
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From: suburban atlanta
The check valve in the rear pump is faulty, and the pump in the rear tank is faulty. Could he not just put a plug in the fuel line leading from the rear tank, and use the front tank?
I realize that is a makeshift 'solution', but if the parts were no longer available... or finances were sort of short...
tom
I realize that is a makeshift 'solution', but if the parts were no longer available... or finances were sort of short...
tom
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As LxMan1 said, the 93 does not use a selector valve. The FDM consists of all the valves necessary. The switch on the dash energizes the pumps. The pump was sufficient enough to start, but since the check valve went out, it was not holding pressure and cross flowing. It's amazing how much can be learned when stuff starts to break.
Yes tomw, you're right about finances being short. That's seems to be a never ending cycle.
The pump was replaced and it does start up quicker now and seems to hold pressure.
Yes tomw, you're right about finances being short. That's seems to be a never ending cycle.
The pump was replaced and it does start up quicker now and seems to hold pressure.
Logistics Pro
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From: suburban atlanta
Uh, what does FDM stand for? Fuel Deliver Module? ... Fancy name for a pump, no?
Glad to hear it is functional. Now that I understand, it makes perfect sense to use the check valve as the 'switcher'. As long as it works...
Just like having two feed sources, and only energizing the one you want, the other is passive, but doesn't allow backflow.
Neat and simple.
tom
Glad to hear it is functional. Now that I understand, it makes perfect sense to use the check valve as the 'switcher'. As long as it works...
Just like having two feed sources, and only energizing the one you want, the other is passive, but doesn't allow backflow.
Neat and simple.
tom
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From: Louisville,Ky.
It is hard to block off the hard plastic lines, but in theory, you are correct.
They are all sold as a complete unit so individual components are not available.
This design eliminates the need for the selector valve and the pump on the frame. 2 high pressure pumps instead of 2 low pressure pumps, a selector valve, and a high pressure frame mounted tank.
Fuel Delivery Module sounds about right.
They are all sold as a complete unit so individual components are not available.
This design eliminates the need for the selector valve and the pump on the frame. 2 high pressure pumps instead of 2 low pressure pumps, a selector valve, and a high pressure frame mounted tank.
Fuel Delivery Module sounds about right.
Logistics Pro
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From: suburban atlanta
LX, you supposedly can get parts to repair the 'hard plastic' lines. In the shop manual I remember seeing a description of some of them, and how the plastic had to be heated in hot water, I think, to allow it to mold to the barbed fittings. Dunno if there is enough 'stuff' available to make up a plug, though.
tom
tom
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From: Lorain County, Ohio
LX, you supposedly can get parts to repair the 'hard plastic' lines. In the shop manual I remember seeing a description of some of them, and how the plastic had to be heated in hot water, I think, to allow it to mold to the barbed fittings. Dunno if there is enough 'stuff' available to make up a plug, though.
tom
tom
There is (was?) a Ford (OTC) tool for pushing those fittings into the lines after heating. It looked kind of like a pair of Channel-Locks. I know it exists because I personally used it at the dealership 3 times before it disappeared into somebodys home tool collection. It was as slick as black ice, it worked great. I have never seen another one, even on Ebay. OTC reps look at you like you have two heads if you ask them about it.
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