OK, I have a 99 Cherokee 4.0 with 148,000 miles. The Jeep was fine until one day it started to bog down at start up, loss of power and hard to start when warm. No codes thrown, got worse so I changed the upstream 02 and tps (both seemed high on the list and looked like they were quite old). No change but threw a p0122 code, low voltage tps. I changed the tps again to another brand and no change. I tested the fuel pressure and it is way low at 20 psi. The fuel pump was replaced 2 years ago. I am not sure if this is a bad pump or a voltage issue that is affecting the pump as well? The Jeep has new plugs, wires, cap and rotor as well as a new head and lifters. This all came about at once. Any ideas here? Thank you all for any input you may have.

 

For starters:

Did you clear the CEL (P0122) after replacing the TPS again?

What brand fuel pump module was installed 2 years ago? Airtex?

 

I cleared the code, not sure what fuel pump was used as I had it done at a shop. I jumped the relay and still only get 20 psi. The Jeep is clearly not getting enough fuel but am not sure if the pump is bad or no getting enough power.

 

As a note: Airtex fuel pumps are notorious for premature failure. If that was an Airtex pump you had installed it may be the root cause of your problem. In any case, you should do a voltage drop test.

Here's a video that shows how to check for voltage drop in the fuel pump circuit. When you open this link scroll down to "How to Test for Fuel Pump Voltage Drop".

http://www.fuelpumpu.com/airtex-fuel-tv.aspx

Here's an image of the fuel pump module connector that includes a pinout chart of the fuel pump body harness connector, which is the mirror image of the module connector. The pinout chart shows the wire colors in the body harness, not the pump module wires.



If you want to follow the procedures in the video back probe the connnector using the body harness wire colors as a guide to power and ground. Pins 1 and 6 respectively. I use safety Pins as back probes.

Here's an image that shows the location of the fuel pump ground (G302). Behind the spare tire mount.



I made a jumper switch setup for checking my fuel pump Amp draw using a 10 Amp fuse in place of the normal 15 Amp fuse (FSM check). The switch is an simple on-off switch rated at 15 amps. The connectors for insertion in the fuel pump relay pin sockets are .250" wide blade type crimp connectors. As I recall the wires on the switch are 18 gauge. Jump from pin socket 30 to 87 as shown in the pic. Have the switch in the OFF position before connecting it to the pin sockets. When jumped, the pump will be running directly off the battery, no need to run the engine or keep cycling the ignition switch. It will keep running until the battery dies so make sure your battery is fully charged before running the pump.

 

Great info thank you, just what I was looking for. I will try to sort this out over the weekend. I picked up a fuel pump from a pull a part from a 99 Cherokee that was hit in the drivers door and totaled, at least it was running, so I can check if a different pump will flow properly. I will post what I find out.