Jbam1979

Hello all, i am new to posting but not new to jeeps. I bought a 4.0 auto GC non-running. I replaced CPS,pick up coil, cap, rotor, wires, plugs. I cleaned TB and replced IAC and got it running. Head gasket was blown and the radiator blew out. I replaced head gasket cleaned cooling system and replaced water pump, t-stat, temp sender, temp sensor, radiator, clutch fan, TPS, O2, and MAP sensor. I have good compression and fuel pressure is good. It runs good except for a low idle about 500- 600 rpms when driving it accelerates fine. When coming to a stop it will drop to about 100-200 rpms not all the time but most times. I have checked for vac leaks and found none. I also deleted the catalytic converter. I only have an up-stream o2 sensor so shouldn't have any adverse effects. I think it might be my ECU but i do not have alot of experience with them. Any help would be greatly appreciated. This forum has taught me alot over the years and assisted me in many years of jeepjoyment.

jon-d

perhaps your injectors are plugged? also, consider changing to a 4-hole injector from the stock single hole injector. that might settle down the idle.

Jbam1979

Thank you i am going to check fuel pressure again tonight. I just want to make sure it hasn't changed. Then i might pull injectors and test them.

Jbam1979

Okay fuel pressure is good swapped injectors and replaced distributer cap and rotor. Both speed sensors changed oil. Compression test all with in spec. I have a dead cylinder at idle #2 pulled injector plug no change pulled plug wire no change. Starts up slow and idles great for a few minutes then idle goes down to 500 rpms and starts to drop at stops from speed drops to about 200 but doesnt stall. Any ideas please help could a wrak fuel pump cause this. Or should i just swap a new engine in.

dave1123

Except for a possible loss of power to injector #2's plug, you obviously have a mechanical problem in that cylinder. Check the plug for 12v supply, the other side is grounded thru a driver in the PCM. All the power in the world is not going to make that injector function if it has an open ground circuit. The PCM only grounds the injector when it needs fuel from it.

There is a thing called a "noid" light that you can clip on the injector plug that'll tell you if it's working or not.

I find it hard to believe there's a fault with the internals of cylinder #2 if there's no noise, smoke, or other indications of it. It sounds to me like it's just not getting fuel. Check compression in that cylinder to be sure.

This is for everybody! When doing a compression test, pull the ASD relay to eliminate fuel and spark that might cause a problem.

Jbam1979

Thank you for the reply. I pulled the fuel rail and unhooked the coil wire had my wife bump the ignitiin and i got fuel spray from all injectors. I completely agree on the no smoke or noise. I have a hard time believing a mechanacle problem with cylinder. If i pull the coil wire while doing a compression test do i still need to pull the ASD relay. I will look in to getting a noid light and check for power yo injector aswell. Again thank you

dave1123

The ASD relay controls both the injectors and the ignition. It's the easiest way to eliminate both at the same time. If you're getting fuel spray from all injectors, you must be getting power and ground on #2.

Jbam1979

Thats what i thought aswell and next compression check i will pull asd relay instead of coil wire. It is just a gremlin due to the fact when at higher speed it doesnt feel like there is a miss. Only at idle and when stopping from speed. I can brake from low speed and no change in idle but can feel a miss. Sometimes i have noticed when making a hard right turn the engine sputter like not getting enough fuel but cleans up as everything levels out. So will probably replace fuel pump aswell when i can afford it. I have never had a miss from a weak fuel pump though. Thanks for all the help any other ideas? I am pretty good with 4.0s but this one is proving a little tricky.