4.0 only runs for a couple seconds

[PNW Rob]

Took about an 80 mile trip and died while on the freeway. It had been running fine up until then. I found that I cannot hear the fuel pump when ignition is turned on and no fuel coming from the schrader valve. I removed the fuel pump relay, jumped the pins and heard the pump humming and got fuel to come out of the schrader. Replaced the relay and same issue, no fuel pump sound and nothing coming from the schrader.

I've already replaced the fuel pressure sensor, the TPS and the air idle control valve. The fuel pump (Omix brand) and fuel filter were replaced a couple years ago, probably only 5 or 6k miles on both. Any ideas?

 

[Chris]

What is the fuel pressure at the rail when it's running?

 

[Flyer58]

If you can jump the fuel pump relay and get the pump to run, then that may indicate the signal wire for that relay may not be working to energize the relay.

Will the engine run if you jump the relay and try starting it?

I believe the relay signal comes from the ECU. The ECU initially sends a 1-3 second signal to prime the injectors before starting. If all other parameters are met, the pump will start running again to keep the engine running. It has a safety function to shut off the pump if you're in an accident.

That programming is all built into the ECU. It doesn't necessarily mean the ECU is bad, You may have a bad wire or connection.

I don't know the wire color or pin number into the ECU but it can be found in a Factory Service Manual. See the Reference section.

 

[PNW Rob]

What is the fuel pressure at the rail when it's running?

Funny thing about pressure test. I rented a kit from O'Reilly's and continually got a zero pressure reading. I knew there was at least a little pressure because I could get it to squirt from the schrader. I was going nuts and then I wondered if the rented pressure gauge was working properly. I hooked it up to my Silverado and got "no pressure". I took it back and told them the damn thing was broken.

I have not rented another test kit at this point. I've actually had to step away from the Jeep saga, but I'm trying to tinker with it when I can.

 

[PNW Rob]

If you can jump the fuel pump relay and get the pump to run, then that may indicate the signal wire for that relay may not be working to energize the relay.

Will the engine run if you jump the relay and try starting it?

I believe the relay signal comes from the ECU. The ECU initially sends a 1-3 second signal to prime the injectors before starting. If all other parameters are met, the pump will start running again to keep the engine running. It has a safety function to shut off the pump if you're in an accident.

That programming is all built into the ECU. It doesn't necessarily mean the ECU is bad, You may have a bad wire or connection.

I don't know the wire color or pin number into the ECU but it can be found in a Factory Service Manual. See the Reference section.

I haven't fully inspected all the wiring, only the easy-to-get-to stuff so far. Same with the entire fuel lines. I'll check more thoroughly.

Yesterday, I got it to run a little longer by jumping the relay. I then plugged the relay in and also got it to run for a little longer than before (like 10-15 seconds both). It sounded fine at first, then idled down to almost die, then rev'd again. Did that a couple times and eventually died. Feathering the throttle got it to run a tiny bit longer, but too much throttle acted like it was flooding out. The other odd thing was as it was dropping RPM's like getting ready to die (or just die), the air intake had a real loud air suction sound (it is a K&N cold air intake, so louder than stock). That must mean something....I think!

FYI, the ECU is a re-man. unit somewhere around 7 or 8 years old. The top left corner is broken, so it is not tightly sealed. I was told the circuitry is encased in silicone so it is supposed to pretty watertight anyway. I don't do any heavy duty wheeling but that tiny gap always bothers me.

 

[Chris]

Just to rule out the ECU, you could send it to @Wranglerfix (wrangelrfix.com) and have him test it for you to rule out whether it's got an issue.

When you get back to the Jeep I'd be curious to know what the fuel pressure is at the rail. That's where I'd want to start just to rule out the most obvious.