Fuel pump or fuel pressure regulator?

[Twenty8000]

Hi everyone,

94 Explorer Sport. 4.0 OHV. 169k miles.

I'm trying to track down a 556 KOEO code. The engine stumbles occasionally on moderate acceleration. I'm getting 12v to the connector at the fuel pump and the relays and PCM pin tests appear to be check out ok. I have not dropped the tank yet to check the connections at the pump, though. I decided to check the pressure first...here are the results:

--------------------------------------------------------
Begin test - system depressurized.

Key turned to ON position:
key ON #1 - to 5psi
key ON #2 - to 14psi
key ON #3 - to 20psi
key ON #4 - to 22psi

Fuel pressure then stays at 22psi until...


Start Engine
------------
KOER (idle) - 31psi
KOER (pulled vacuum line) - 34psi
--no gas leaking into vacuum line


Stop Engine
------------
Immediately at turning engine off, pressure rises from 31 to 38psi

at 5:00min - 36psi
at 6:00min - 34psi
at 8:00min - 33psi
at 10:00min - 32psi
at 14:00min - 30psi
at 18:00min - 29psi
at 22:00min - 25psi
at 25:00min - 23psi
at 30:00min - 22psi
----------------------------------------------------------

Weak pump? Fuel pressure regulator? (already changed fuel filter).

Thanks for any advice!

 

[matt0248]

The only for sure way to tell is to isolate the fuel tank and engine somehow with a rubber fuel line that can be pinched off with a set of pliers. At the shop we had a motorvac fuel injection cleaning system, it came with all kinds of different hoses and adapters to connect to fuel systems. We could then pinch off the pressure line (after the pump shut off) to see if the pressure dropped. IF it held it was a bad pump/check valve. If it still dropped the pressure loss/leak was at the engine. Could be leaking fuel injectors also. Recheck your fuel pressure regulator. Cut a thin strip of paper just small enough to fit down the vacuum port. let it sit in there for 10-15 minutes and pull it out. if is even the slightest bit wet or smells like gas replace the fuel pressure regulator.

 

[Twenty8000]

Thanks much, I'll give that a try...