Out of Idea's Bad running 4.0l!!

hey there ive hit a wall trying to fix my jeep. ive been a jeep mecanic for 7 years and ive never seen or have had this problem. lets start with> it a 98 cherokee 203,000 miles. it has one of the worst idle ive ever heard is sounds like it has a monster cam in it. driveablity is fine. it is a intermitent problem. when it starts running like crap idle drops to about 300-400 rpm and u can crack the gas and it sound fine and it will just go back to crap idle. driving when its in a lovely mood anything below 1/2 throttle it pops back though the intake and mis fire, like some thing internal has broke, stab the gas and it runs fine like i said this is intermitent problem it got better for a little bit and then got worse and than gone away. i just took it on a 200 mile trip down the highway and it run perfect! I got about 22-24 mpg. got off the highway started run like s*** again.
ive checked, wiggled, replaced,fix.
no CEL's or stored codes
3 different crank sensor ,one new 2 used from running jeeps
3 different Tps 2 new ones.
had a new TB sitting around with a new MAP, and IAC
new plug's wires cap rotor.
brand new O2s front and back.
fuel pressure 49psi stay at it when shut off to
checked and cleaned every connetor on the car
i 'm thinking the computer might be going but i hope not>
ive been messing with it for about 2 months now and just about ready to thow the towel in!
i got a video of it doing it sorry about the intake sound. when it's running like crap that intake is freking loud
any insight would great!
~thank you~http://www.youtube.com/watch?v=Mol7o0vDRQs&feature=youtu.be

It's got OBD2, use it. I would datalog short and long term fuel trims, rpm, tp, iac, map, etc.

What is the vacuum at idle?
Wet/dry compression?
Plug readings?

Just a guess: A restriction in the exhaust sytem.., such as a broken baffle in the cat, or muffler, that will move about, or around, under power in either housing will cause intermittant, yet similar issues when in the wrong, or correct place. This guess could be a long shot, but I've had restricted exhaust in the past causing backfire through the carb on a 258 cu. in. I would reckon that the XJ's ecu could indicate all kinds of different fault codes, or symptoms, due to such an intermittant restrictive exhaust flow.

I dont think the exhust is the problem i got long tube header 2 1/2 witha 3" flowblocker on it and super flow cat. After work today i will do a vacuum test if i can get it to run like crap. today its been good and i'll pull the plugs. When im not busy at work i will start a data log the only thing i saw when it was running like crap. And plugged in timming was 5 degree and would jump to 18 degrees would the cam sensor mess with it that much? Also it seems to do it more when warm - hot. Maybe coil? Im thinkin its a sensor or computer just because of it being intermittent
Thank guy
~craig~

Compression test done 1-152psi 2-160psi 3-159psi 4-166psi 5-158psi 6- 160 psi plugs look perfect have it idle now for about a hour and runs good. Ran like crap 4 hours ago jeeps have a mind of there own!

You certainly have addressed the usual suspects. How about swapping out your camshaft position sensor in the distributor?

And I certainly like JJacobs idea of using a scan tool to check out EVERYTHING.

classicherokee98 said:

I dont think the exhust is the problem i got long tube header 2 1/2 witha 3" flowblocker on it and super flow cat. After work today i will do a vacuum test if i can get it to run like crap. today its been good and i'll pull the plugs. When im not busy at work i will start a data log the only thing i saw when it was running like crap. And plugged in timming was 5 degree and would jump to 18 degrees would the cam sensor mess with it that much? Also it seems to do it more when warm - hot. Maybe coil? Im thinkin its a sensor or computer just because of it being intermittent
Thank guy
~craig~

Click to expand...

Timing is set by the Crankshaft Position Sensor, determining the stroke is determined by the Camshaft Position Sensor. IIRC, once it starts, the cam position sensor is no longer read, or at least that is the way it is on the Renix.

Heat soak? That's what it is sounding like. Wrap the fuel rail and injectors with some plug wire heat wrap. Just a hunch but with the exhaust work you have done it my just be too darn hot under the hood and even the cool fuel from the tank isn't cooling it enough.

There was a recall about the heat soak issue if I remember right as well.

classicherokee98 said:

I dont think the exhust is the problem i got long tube header 2 1/2 witha 3" flowblocker on it and super flow cat. After work today i will do a vacuum test if i can get it to run like crap. today its been good and i'll pull the plugs. When im not busy at work i will start a data log the only thing i saw when it was running like crap. And plugged in timming was 5 degree and would jump to 18 degrees would the cam sensor mess with it that much? Also it seems to do it more when warm - hot. Maybe coil? Im thinkin its a sensor or computer just because of it being intermittent
Thank guy
~craig~

Click to expand...

The Crankshaft Position Sensor adjusts the timing through the ignition coil. 5 degrees is way too low. The 18 degrees looks about normal. At idle you should see the timing being adjusted by the CPS, +/- around the 18 degree mark. The Camshaft Position Sensor is active in all modes of operation, from idle to WOT and back. It adjusts the injector pulse width (rich/lean) in response to the O2S inputs and inconjuction with the CPS timing adjustments.

What's that loud a$$ed air cleaner noise in the vid? It sounds like the TB is wide open. Are you sure you don't have an intermittant vacuum leak and the CPS is trying to keep up with it.

Sidewinder CC said:

The Crankshaft Position Sensor adjusts the timing through the ignition coil. 5 degrees is way too low. The 18 degrees looks about normal. At idle you should see the timing being adjusted by the CPS, +/- around the 18 degree mark. The Camshaft Position Sensor is active in all modes of operation, from idle to WOT and back. It adjusts the injector pulse width (rich/lean) in response to the O2S inputs and inconjuction with the CPS timing adjustments.

What's that loud a$$ed air cleaner noise in the vid? It sounds like the TB is wide open. Are you sure you don't have an intermittant vacuum leak and the CPS is trying to keep up with it.

Click to expand...

What?

All the Cam sensor does is tell the ECU where the #1 cylinder is so it knows when to inject the fuel in a sequential manner. The crankshaft position is the one deals with the timing of the ignition.

But a proper scan tool would help you figure this one out.

I wonder if its your coolant temp sensor. Mine sounds like that when its cold outside and the engine has not run all night.
If its doing that because it thinks its cold but it is actually at operating temperature it may be trying to flood itself.

I have found that the IAC is what allows that sucking noise.

Just my thoughts...

Edit: A proper scan tool should tell you what the temp sensors are telling the ECU.

sidewaysstarion said:

What?

All the Cam sensor does is tell the ECU where the #1 cylinder is so it knows when to inject the fuel in a sequential manner.

Click to expand...

Yes, then #5, then #3, then #6, etc.

This is from the '99 FSM:

CAMSHAFT POSITION SENSOR
The camshaft position sensor is located in the distributor on all engines.
The sensor contains a hall effect device called a sync signal generator to generate a fuel sync signal. This sync signal generator detects a rotating pulse ring (shutter) on the distributor shaft. The pulse ring rotates 180 degrees through the sync signal generator. Its signal is used in conjunction with the crankshaft position sensor to differentiate between fuel injection and spark events. It is also used to synchronize the fuel injectors with their respective cylinders. When the leading edge of the pulse ring (shutter) enters the sync signal generator, the following occurs: The interruption of magnetic field causes the voltage to switch high resulting in a sync signal of approximately 5 volts. When the trailing edge of the pulse ring (shutter) leaves the sync signal generator, the following occurs: The change of the magnetic field causes the sync signal
voltage to switch low to 0 volts.

CAMSHAFT POSITION SENSOR—PCM INPUT
A sync signal is provide by the camshaft position sensor located in the distributor. The sync signal from this sensor works in conjunction with the crankshaft position sensor to provide the Powertrain Control Module (PCM) with inputs. This is done to establish and maintain correct injector firing order.

I had a VW that acted like that for nearly a year! I even bought a replacement ECU for it!
I was screwing around under the hood one day where the 5th injector was located (used for cold starts) Near it was a 3 or 4 ground wires connected to the intake manifold. One was really limp. Guess what, broken wire. Sometimes it would make contact and the thing would run great. Other times it ran like crap. How in the world do you diagnose that?
Hate the electrical problems cause they are so hard to diagnose.

Thats kind of the direction I was gonna go with this... almost sounds electrical, with it being intermittent and all.... bad grounds maybe? I say grounds plural, because you may have multiple issues going on here. Maybe just a bad ground at the block, maybe the computer is not seeing a good (enough) ground, or there's something going on with the voltage in general.... bad voltage regulator? I'm just throwing stuff out there, i'm sure much of this has prob crossed your mind already. I would look into getting a schematic for the main ECU harness and start testing voltages and ground continuities. Good luck man, you got one heck of a battle there!

thanks guys for the input! ive been working on cherokee's since i was a kid, i know something not right if it takes me years to find it oh well i drive the car about 100 miles a day it anything above 1/2 throttle it fine, doesn't seem to be running rich or lean i order a new coil for it somethings point to that being bad so with 203.000 on it what the hell. i'm going to pull the dist. out and look at the gear it might be worn? ive been going though the wirring harness slowly. cleaning and checking each wire (Not a fun job) IS THERE anything in the trans that would cause this? seems to do it more when in gear or it could just be when it has a load? the resson i ask is i just had my trans out and built the crap out of it to support plans for the jeep. it seems to have started do it 1-2 week after putting the trans in?
Thanks everone i'm going keep on pushing on
~Craig~

oh yea vaccum is 15-18 lbs when runnig bad. no vaccum leaks ive check plenty of times that intake is scavening for air.

classicherokee98 said:

i just had my trans out and built the crap out of

Click to expand...

What did you have done to the trans?

All internals soft and hard raybestos red clutches custom built valve body, a340t internals custom cooling system and a hand built TC ITS built to take hopefully 700hp