1994 with fuel pressure problem?

[Plumbob1976]

My 94 Cherokee with the 2.5 4 banger has developed a fuel pressure proble, I tested the pressure at the fuel rail and it has no pressure then I removed the return line from the pressure regulator and plugged the line off and tested the pressure agian and it read 70+ psi. I also pulled the pressure regulator from my 95 cherokee and put it in the 94 and reconnected the return line to the regulator and it still will not run. I pulled the fuel filter off near the tank and tested the pressure there and it was over 70 psi. Am I missing something? Please help.

[bigjim350]

No pressure at the rail at all? What did the guage say at the rail after you swapped regulators? Was the reg from another 4 banger or from a 4.0? Have you checked for fuel volume?

[Plumbob1976]

The regulator is out of my 4.0 ho, my 4.0 ran great with the regulator out of the 2.5 so I know that the regulator is good, I tested the pressure at the rail with the good regulator out of my 4.0 and it still had no pressure until I disconnected the return line and plugged it off, once the line was plugged of it had 70 psi and the motor would start and run but it ran rough.

[Plumbob1976]

Thanks for taking the time to try and help me, I would be awesome if you could give me a call and walk me through a few things.
Thanks Robert
503-860-8719

[4.3LXJ]

I assume you are getting good spark at the plugs? As Jim said, you might not be getting good volume. How is the fuel filter? Also, if all that starting has made the plugs wet, you will need to pull then and dry then out.

[Plumbob1976]

Checked the filter and it is good, I am stumped.

[Plumbob1976]

I checked the spark and it's good, the motor will start when I plug off the return line from the regulator but the rail has 70 psi so it runs like crap but when I reconnect the return line to the rail it won't run. I swapped the regulator out of my other cherokee and it still will not run unless I plug off the return line. Correct me if I am wrong, the pump must be good if I am getting 70 psi to the rail (with the return line plugged), and the regulator must be good if it works fine in my other Cherokee. I have checked the fuse for the pump relay and it is good. HELP.

[bigjim350]

Guess it could still be low pressure or volume from the pump. When you plug the return line it allows the pump to build up more pressure. A un-regulated xj fuel pump should be running close to 100psi

[bigjim350]

From Novak adapters V8 swap guide

Fuel System
It is not only possible, but advantageous to use the factory Jeep fuel injection pump, should a future replacement ever be required.
The Jeep fuel pump in 1987-2001 Cherokees is capable of natively producing ~100 PSI. This fuel supply is, however
regulated down to 40 PSI up through 45 PSI for the 2.5L & 4.0L engines.

[4.3LXJ]

I still did not read what the pressure was with the other regulator on. Turn it on and wait for the fuel pump to shut down and read the pressure.

[jeepxj95]

sounds like dieing pump. not enough volume. cycle the key a few times then see if it will start.

[Plumbob1976]

I started thinking like a plumber and decided to throttle down the return line right before the tank, I used some vise grips and pinched the line until the pressure at the rail was reduced to 31 psi and it started right up, this can only mean that the regulator is bad so I replaced it and the jeep runs like a champ. I am a little baffled because in the beginning I took the regulator out of my 4.0 and put it in the 2.5 and it still would not run so then I put the regulator from the 2.5 into the 4.0 and the 4.0 ran great. So long story short the 2.5 will not run with a pressure regulator from a 4.0 but a 4.0 will run with a pressure regulator from a 2.5. The specs say that both the 2.5 and the 4.0 have the same pressure at the pump and the same pressure after the regulator so what am I missing?

[Mudderoy]

I started thinking like a plumber and decided to throttle down the return line right before the tank, I used some vise grips and pinched the line until the pressure at the rail was reduced to 31 psi and it started right up, this can only mean that the regulator is bad so I replaced it and the jeep runs like a champ. I am a little baffled because in the beginning I took the regulator out of my 4.0 and put it in the 2.5 and it still would not run so then I put the regulator from the 2.5 into the 4.0 and the 4.0 ran great. So long story short the 2.5 will not run with a pressure regulator from a 4.0 but a 4.0 will run with a pressure regulator from a 2.5. The specs say that both the 2.5 and the 4.0 have the same pressure at the pump and the same pressure after the regulator so what am I missing?

Smart! Yeah something doesn't make sense there. :thumbsup:

[4.3LXJ]

Just because it fits doesn't mean it is right. If there are different part numbers there between the two, then that would explain it. Personally I would be surprised if they both did work right on both engines.

[xj4life2]

Ok a word from the parts guy both vehicles take the same regulator , so they are interchangeable. Here are the spec on them:
MPI Fuel System Pressure


(with vacuum applied to regulator) 21 kPa (31 psi)

MPI Fuel System Pressure


(without vacuum to pressure regulator) 269-276 kPa (39-41 psi)

MPI Fuel System Pressure Drop


(fuel pump not engaged) Up to 138 kPa (20 psi)
Now add to insult to injury if you took the one off the 4.0 and it did the same thing you have a bad regulator on it also. When you plug the line and get 70psi its a sure sign that the regulator is open. The 4.0 is way more forgiving than the 2.5 and will run that way however I would guess the fuel milage is terrable on it. Its certainly worth another look at the 4.0.

[Plumbob1976]

I tested the pressure on the 4.0 and it is 31 psi with the vacuum line attached and 40 when it is disconnected, same regulator and same specs for both motors and the reg in my 4.0 test out good. I don't get it.both of my cherokees are running great so I angling to move on to working on my 1st gen Yota, I am working on transforming it into a prerunner. Thanks to all that helped me through this strange ordeal.