Originally Posted by jdjonesdr

I have once again gone through the problem of lack of power and then improved acceleration above 3000 rpms, accompanied by "pops" through the throttle body.

The first thing that entered my mind was a fuel problem.

The next morning, the beast wouldn't start. I couldn't hear the fuel pump charging up either.

I got underneath and checked voltage to the pump, and discovered it was very low.

I ran a wire directly from the battery to the pump, at which point it functioned normally, and I was able to return home.

What the pump didn't do was shut off once it made pressure after a few seconds, which it normally does.

By that I mean that usually every time I turn on the key, the pump runs until it builds pressure, then shuts off. It doesn't keep running if the engine isn't running.

Is there some sort of pressure sensor that cycles it on and off as required?

Does the pump voltage have to pass through the relay near the coil? How is it even possible that I get low voltage out of a relay? I though voltage out of a relay was all or nada.

I was thinking about buying a fuel pressure tester, but instead bought a pressure gauge to install on the fuel line. Shouldn't they both give me the same information?

Originally Posted by cruiser54

ECU controlled. Either gauge should read properly as long as they're for liquid.
How's your fuel pump ballast resistor? Fuel pump relay?

Originally Posted by cruiser54

ECU controlled. Either gauge should read properly as long as they're for liquid.
How's your fuel pump ballast resistor? Fuel pump relay?

Originally Posted by C H E R O K E E 98XJ

I've had this weird starting issue. Jeep will start exceptionally well when warm every time and run and drive beautifully. But, the problem is when the engine is completely cold, as if sitting over night. (ambient temp 75*) When cold, it will turn over for a split second, sputter a tad and then either barely start or just kill. Here's the kicker... I wait 30 seconds after it doesn't start and it starts like there's no problem at all.
Thanks

Originally Posted by salad

When cold, try turning the key to ON and back to OFF a few times before going to START. This lets the fuel pump build up pressure before the engine tries to fire.

Usually the symptoms you report are indicative of a bad check valve in the fuel pump module inside the tank. When that valve is bad, after sitting for a while, pressure releases back into the tank.

Originally Posted by jdjonesdr

And, of course, that was the problem.

Stuck a jumper on the two wires, fired right up.

You're going to have to fly down here for a vacation for all the beers I owe you.

Originally Posted by C H E R O K E E 98XJ

Thanks for replying salad. I have indeed checked that already and measured fuel pressure with key turned on (48 psi) and then running (48 psi) and then waited 5 min after shutting down and the pressure did not drop at all. I also just had my battery load tested and it passed at 855 cca when rated at 800.
Could the ignition coil be intermittent? Is there any sensors that come to mind could be causing this?

Originally Posted by salad

Could have a wacky temperature sensor. Both the IAT and ECT are consulted, but that should in theory cause issues while running, too, not just starting.

If you have access to a scanner, it should be pretty simple to just let the XJ cool off overnight. In the morning the IAT and ECT should be the same number.

So when you do the "poor man's prime" it doesn't help any?

Originally Posted by C H E R O K E E 98XJ

Poor mans prime does not help at all. (I don't know if it was a coincidence but when I did that it seemed to start worse?)
I do have a simple OBDII scanner but I wouldn't know how to check those sensors. I'm going to go research these in my FSM...

Originally Posted by C H E R O K E E 98XJ

What does the Coolant Temp Sensor and Intake Air Temp Sensor actually do when the Jeep is running? Say I unplugged the thermostat coolant temp sensor, will the Jeep run fine still? How do they affect the drivability/runability?

Originally Posted by NewKindOfClown

Unplugging the CTS will cause the PCM to run in Open Loop, using preset parameters instead of sensor input. The test for a bad CTS is to unplug it and see if the problems go away. I suspect disconnecting the IAT would result in the same operating procedure.

Originally Posted by C H E R O K E E 98XJ

Thanks clown. I have not tried that although it runs perfectly when warm/operating temp. I did check resistance to both the CTS and IAT and they were 8100 ohms and 9000 ohms respectively. With ambient temps here at 75-80*F that seems about right according to the chart I was looking at in my FSM

edit: this is the chart...