Peter Wallen

Hey everyone, so the other day I lost all fuel pressure at the rail and my 98 xj wouldn't start. Thinking it was the fuel pump I dropped the tank thinking I would need to replace it. Just in case, I tried going directly from the battery to the pump with some cables and got full pressure, but when I took a voltage reading at the connector the pump plugs into it would go from 11.8 volts to nothing in about a second. I'm thinking now there's something stopping power to the pump immediately after turning the jeep on and not that there's an issue with the pump. Does anyone have any idea why this may be happening? I already have a new CPS on the way so hopefully that solves it but any other ideas?

GreaseMonkey17

gat

Does it crank, start, then die? If so then is there pressure built when the key is put in the on position for a couple of seconds prior to turning the key to start?

I believe that on your model the fuel pump relay delivers +12v from the battery on start and the computer deliver +5v when the engine is running. These are two different wires, if I am correct. I believe that you have one +12v priming/starting wire +5v running wire, fuel gauge wire for the dash, and a ground wire.

If you have access to wiring diagrams, Haynes manual, Chilton manual, Cruiser54.com factory service manual, or a local library, then confirm these wires and test them in the proper conditions.

If losing pressure is the issue, then you could have leaky injectors or a bad valve (which should be a part of the fuel pump assembly).

Let us know what more help that you need and how these tests went.

Peter Wallen

Hey, appreciate the responses. It will crank but does not start and if I leave it in the on position for a few seconds then depress the schrader valve there is some pressure built. I also tried priming it a few times before attempting to start it with no luck. I'm tempted to say it's not an issue with the pump though because if I hook it directly to the battery it will pump fuel. Am I wrong with that assumption?

gat

You can jump the voltage across where the fuel pump relay is to start the vehicle. That would be a better test for fuel pressure than just does the rail leak. It still isn't a good substitute for a freely rentable fuel pressure gauge.

Try bypassing the fuel pump really with the jump to test that relay or swap that relay with another one like the automatic shutdown. There are pin numbers for the relay. You can post what voltages that you have at that relay spot.

​​​​​​​We want to ensure that the electronics are fine to the pump before we replace the pump.