zman6988

Ok guys gotta 92 XJ 4.0 with 252XXX. Gotta couple issues that i think I have figured out just need confirmation. It does not like to idle right in the cold like it is starving for fuel. When it warms up driving down the road it will loose power and start back firing through the intake. After a couple of times of me flooring the throttle it will stop and never do it again until the truck completely cools back down. I have changed the fuel pressure regulator, idle air control valve, fuel filter, and CPS. I also tried the wiggling harness trick to no success. It was recently tuned up new cap,rotor, plugs, wires, and coil however this problem was present even before I tuned it up. Today I performed a fuel pressure test. At idle it has 28 PSI with the vacuum line on the reg. 35 PSI without the vacuum line on the reg. I changed the fuel reg out with no change in PSI. If I rev the engine the pressure will increase to 40 PSI, but fall back down to 28 PSI. When I turn the Jeep off it does not hold pressure it will fall back to 0 PSI Also I can't get it to idle below 1500 RPM no matter what I do (timing or idle screw on throttle body). So after all this explaining I would think that it is a failing fuel pump unless I have missed something. Please let me know what you guys think. Thanks for taking the time to help guys.

Turbo X_J

"start back firing through the intake."

Ignition or valve timing issue.

zman6988

Timing was set 3 degrees advance after reading your post I went out and reset it to 0 degrees but it still pops back through the intake but like I said it only does it when it is cold?!?

captainwoot

Running lean? vac leak causing the lean condition and high idle? How about flow testing the pump (although it sounds bad already)

zman6988

I also have done the Bosch III (4 hole ) injector swap and I reset the computer. I forgot to mention that. Captain woot how would I flow test the pump?

zman6988

Also do my fuel pressure readings indicate a bad fuel pump? From what I've gathered from this site as well as others the most common pressure should be between 38-42 PSI right?

fwak1234

With you saying it won't hold pressure makes me think something more in depth than the fuel pump because the rail/injectors should hold pressure after the jeep shuts off. I would try to purge air from the shrader on the fuel rail. Start it and let it run for a minute. Shut the jeep off then bleed the shrader again. Then start it back up and redo the pressure test. I would think if the pump was going it would be more consistent rather than do it then stomp the gas and goes away. But I've seen weird things happen before with fuel systems.

Just an idea for you to try. May help with diagnosing. Hope it helps.

freegdr

Theres no timing or idle adjustment screw every things controlled by ecm .

freegdr

What codes have you got when you read them ?

zman6988

So the idle screw on the throttle body does nothing?!?!? funny because thats how I idled it up when I fixed the A/C the RPMs would drop so low it almost stalled so I turned the idle screw in to idle the engine up. And my distributer does adjust I can advance and retard the timing I achieved this earlier today. Also there are no codes present in the ECM. To fwak1234 are you suggesting that the injectors maybe leaking the rail pressure down when the engine is shut off? Ok tried your advice fwak1234 this is what i got plenty of fuel sprayed out of the engine easch time i bled the scrader valve. Now when I shut the engine off the pressure will fall down to 5-8 PSI then after about thirty seconds it will fall to 0. Thanks again for all the help you guys maybe we are on to something here.

zman6988

All vacuum hoses checked and have no cracks I also pulled the vacuum ports out and cleaned them but no success. I am begining to wonder if I have a faulty TPS. Also I need to ask a stupid question I think I read a post stating even if you unplug the coolant temp. sensor it will not help with diagnosis is this also true if I unplug the o2 sensor or will it show something else. I don't have much money so throwing money on sensors that are hard isolate is not an option for me if you know what I mean. If this helps any the engine will go into this problem after I've driven about 3-5 miles, this is when it starts acting up. Sometimes I can stop turn the engine off and start it back up and problem goes away. Thanks again guys for the help.

sweeper

I know you tuned it up but pull all your plugs and check them again to see how they look. I had a coil pack go bad on my 2000 that had the same symptoms as you describe. I pulled random plugs and they looked good so started changing sensors and come to find the coil pack later.