Low fuel pressure?

[ratdude747]

My 1995 4.0 seems to have low fuel pressure. I was measuring around 20psi or so (no longer have fuel pressure gauge)

I have replaced:

• fuel pump
• fuel filter
• pump strainer
• fuel pump relay (from parts truck)
• fuel pressure regulator
• (unrelated?) alternator

I also checked the inertial cutoff switch. All looks good there. Fixed some battery issues, no change.

Getting exactly 12v at the fuel pump connector when the key is turned on (no load). Not 12.4v like I'd expect... Indicates a bad wire?

I also tried feeding power directly to the pump via the inertial switch connector. No change.

I'm running out of ideas... Suggestions?

 

[chewy012]

Leaky injectors? That's about the only thing I don't see on your list.

Try the clear flood test and see if she tried to fire. Warm engine, turn off, WOT and crank. No fire is what you want.

Sent from my XT1254 using Tapatalk

 

[RonD]

Alternator?

Battery voltage, engine off, should be 12.3 to 12.8volts
Battery voltage engine running should be 14.5 to 14.9volts just after start up, then drop to 13.5-13.7volts after 5 minutes or so of running

Yes, could be dirty connectors or bad wires are causing a voltage drop at fuel pump
Ground wire is as important as 12volt wire.
If a device needs 10amps to run and Ground wire is dirty or bad and can only pass 5amps then thats all the amps that can flow thru, DC power is like a hose from battery terminal to battery terminal, doesn't matter where you "kink" that hose flow is reduced for the whole thing.

 

[ratdude747]

Alternator?

Battery voltage, engine off, should be 12.3 to 12.8volts
Battery voltage engine running should be 14.5 to 14.9volts just after start up, then drop to 13.5-13.7volts after 5 minutes or so of running

Yes, could be dirty connectors or bad wires are causing a voltage drop at fuel pump
Ground wire is as important as 12volt wire.
If a device needs 10amps to run and Ground wire is dirty or bad and can only pass 5amps then thats all the amps that can flow thru, DC power is like a hose from battery terminal to battery terminal, doesn't matter where you "kink" that hose flow is reduced for the whole thing.

I checked most/all of that (not to brag, but FYI, I am an engineer with an electrical engineering technology degree, but sometimes even basic stuff knocks me on my ass once in a while).

---

Redid a pressure check, at cold start after a key cycle (no start) it read 40PSI, dropping to 35PSI after start attempt. So maybe fuel pressure wasn't the only issue?

I will do a clear flood test before I leave for a long trip (yay rental cars )...

I'll start a new thread with a better title.

 

[RonD]

Yes, that fuel pressure is normal(30-40psi), and if it held for a few minutes then injectors are OK as well.

Do 50/50 test if there is a no start
Spray gasoline or starting fluid(ether) onto intake and try to start
If it starts up and dies then yes fuel is the issue
If it doesn't start then spark is where to look
50/50