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I'm gonna shoot the damn thing!

Frank Z

Frank Z

NAXJA Forum User
Location
Colorado Springs
1997 XJ, Sport, AW4
The motor is 4.0 (4.6L Stroker) with approximately 12k since it went in.

I’ve fighting this damn problem for a couple months and I’m completely stumped.

The problem seems rather common, stumbling, hesitation, buck/lurch at speed or from a standing start.

From a standing start and accelerating normally the jeep wants to die, it stumbles really bad and has died on occasion. Restart takes a bit of cranking with the pedal mashed to the floor.

At about 50 mph it will buck and lurch until I mash the gas and get the RPM’s up to about 3k. Generally this starts at about 2k to 2.5K RPM’s.

So here’s a list of the parts that have been replaced after searching here and elsewhere for a solution to this problem. Note that all of these are new within the last 3-6 months.

Fuel pump assy. (today)
Fuel pump (only)
TPS
CPS (crank)
CPS (cam)
IAC valve
Distributor
Cap and rotor

A few of the measurements, if it helps:
5VDC into the Crank sensor
5VDC into the Cam sensor
13.6VDC at the fuel pump connector

Fuel pressure before new pump assy. was 39 PSI, after the new pump went in it went to an amazing 40.5 PSI. I thought that the 97-01 fuel systems were supposed to be at 49 PSI?

The idle is not steady. It fluctuates between approximately 650-850 RPM.

I’ve even gone as far as replacing the transmission. Yeah I know it sounds nuts but some of the symptoms pointed to faulty shift solenoids and I got the tranny for a great price (Thanks Yellaheep!!).

OBDII reports a “43” Cylinder 3 Misfire, but today is the first time it’s ever come up.

So what am I missing?
 
Yea, bigger injectors and an adjustable map should fix it.
 
Even in the absence of a code, I don't think you can rule out the MAP sensor. The very poor running and difficult starting are typical symptoms of a bad MAP sensor. Check the voltage from the middle (output) wire with the ignition on/engine off and when the engine's idling, and post the results.
 
I didn't see fuel filter in your list.. If you did change it, take it off an see if it is clear..Also check all gaslines that are rubber, Could be sucking air
 
Not exactly what Ive seen a quirky upstreat O2 sensor do, but would be worth checking at this point.
 
So far I've thoroughly cleaned the throttlebody.
Pulled and cleaned the IAC
Tested the TPS and it checks good
Checked the voltage at the MAP while engine was running (3.6 with the key on and 1.1-1.2volts at idle)
Confirmed proper resistance value of the Intake Manifold Air Temp Sensor and the Coolant Temp Sensor
Replaced both O2 sensors
Checked for bad grounds (none found)
Checked for loose connections or faulty splices (none found)
Verified proper fuel pressure at the fuel rail (49.5psi)

Ran motor at idle with IAC connector unplugged, no change in variable idle rpm (approx 750 to 500 rpm)
Ran motor at idle with Manifold Air Temp. connector unplugged, no change.
Ran motor at idle and unplugged MAP sensor, no change.
Ran motor at idle and unplugged TPS, engine shuts down.
 
Check your ign system for proper install. could be the distributor is not timed properly, cap and or rotor could be bad, check plugs and plug wire for proper fit and sequence. Check the injector clips and make sure they are in the right order.
Got younger brother who scored a very nice clean older Toy pick up cheap cause some idiot had the injector and the SP fireing order all messed up.
 
thought of something else. check the flywheel/flexplate bolts. I know the engine was just installed but hear me out. Mine came loose on me once on my 93 4.0 and casued some of the same mysterious symtoms. Mine had actually cracked then broke allowing the flywheel to free float on the end of the crank. CPS would missread the fylwheel at random. Caused all kinds of wierd stuff to happen.

Are you sure your flywheel is compatable with the stroker kit and everything is mechanically timed correctly?

One of my Little brothers has a 96xj and he chased these same grimlins down to a bad step motor thats mounted on the throttle body. It dose what the old EGR valves use to do only better. If memory serves their is a ohms test that can be preformed on this step motor. Give that a shot......Good luck.
 
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