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Old 03-15-2009, 01:43 PM   #1
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Default Fuel Pump Pressure

1989 F150 5.0 2WD 5 spd. 195k miles original pumps and injectors

I decided to check my fuel pump pressure yesterday. I got the following readings.

Cold start: turned the key to the run position and the fuel pump would kick in and the pressure would jump to 35 psi (several attempts).....the pump would stay on for about 3-4 seconds...when the pump would shut off, the pressure would quickly fall to zero each time.

Hot idle: held constant at 35 psi. When I snapped the accelerator to about 2500 rpm and held it there, the pressure would drop to between 30 -33 psi (several attempts).

The truck runs great, but how much longer can 20 year-old fuel system parts continue to work ? Do the above readings sound normal to you ?

Thanks.

wormshoe
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Old 03-15-2009, 02:14 PM   #2
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The fuel system should stay pressurized after the fuel pump stops running with the truck not running. It can drop off some but not quickly.
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Old 03-15-2009, 02:19 PM   #3
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Your KOEO test pressure readings were on the low end of the specifications but the pressure should hold to keep the pump primed, not go to zero, that means the fuel supply check valve in the bottom of your fuel delivery module is bad.
Your KOER test pressure reading a idle is good, right in the middle of the specifications, but the pressure should increase as you accelerate. Its not dropping alot so you'll probably be all right.
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Old 03-15-2009, 02:27 PM   #4
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'89 doesn't have the FDM. That's the last year of the separate low pressure/ high pressure pumps, which has the single or dual function reservior. I'm not really sure what part of that system will cause the pressure drop. If it's running rich the injectors are faulty, but it would have to be running pretty rich for the pressure to drop that fast.
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Old 03-16-2009, 10:58 PM   #5
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Is the fuel module that you are talking about located in each gas tank ? How serious is it to have the fuel pressure fall back to zero as I first described my problem ? Can the check valves be replaced or does one have to buy an entire fuel module ? Expensive ?

Thank you for sharing your knowledge. This is my daily driver and even though it is older than a lot of trucks on the road, it is still the best vehicle that I have ever owned.

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Old 03-16-2009, 11:06 PM   #6
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I have consulted the book, and the most likely cause of your '89 (NO fuel module) trucks low fuel pressure would be the fuel pressure regulator.
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Old 03-17-2009, 09:23 AM   #7
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Thank you Sean.....have a good day sir.
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Old 03-17-2009, 09:23 AM
 
 
 
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