I have a 1990 xj 4.0 with AW4 trans. Just recently its been having a few problems. First is that it does not have a constant idle. Jumps up and down and causes the jeep to stumble but doesnt stall. Also when I give it full throttle while driving it seems like it not going anywhere. It kinda pops and bogs down and does not speed up. It has a 4.6 so it has tons of power. I was able to spin my 33's before so this is not normal. I have replaced the CPS since it would stall randomly. Also just changed the fuel pump and filter. O2 sensor as well. Full tune up so ignition is solid. Grinded down all ground wires on the jeep including battery and upgraded engine ground so all are solid. Checked resistance to most sensors with ohm meter and they passed. TPS is a little off but doesnt seem like it would be the culprit. Any help would be greatly appreciated as im stuck and do not know what to do next. Thanks

 

Change the tps and go from there..have you changed your spark plugs and distributor

 

Yes. Plugs, wires, cap, and rotor were all changed recently. TPS is within range and giving the correct voltage

 

I would try cleaning throttle body

 

Also maybe your cat converter..could be a lot of things start with cleaning your throttle body then start narrowing it down..I had the same problem it was between my cat converter and tps changed both now im good to go..

 

Well I didnt mention ALL the parts ive replaced but I did thoroughly clean the throttle body/butterfly and ports for IAC, TPS, and MAP sensor vacuum line when I changed all my engine seals and head gasket about 2 months back. Has a brand new magnaflow cat and flowmaster exhaust with new piping all the way back..

 

I am only running the 4.0, but am having this same problem, I have put new cap and rotor on, new plugs, wires were changed not to long ago, cleaned the tb, and checked everything I could find to check,no cat, clear exhaust, just seem to bog down for a bit then it does take off but only after popping and stuttering.

 

Get some actual fuel pressure readings with a gauge. Gotta rule this in or out early in the troubleshooting process.

 

FYI, my tps was in range but the spindle was just slightly off by a hair. It was causing similar issues. Had a mechanic a little stumped. So on a hunch I just ran to the parts store and replaced the TPS and all is well.

 

Have you messed with the distributor? You may have the base timing retarded.

 

base timing is non-adjustable on XJs

 

at WOT, the input from the TPS tells the PCM to run in open loop mode, during which the a/f mixture is controlled by the MAP sensor, Coolant Temperature Sensor, and Crankshaft Position Sensor (aka engine speed sensor). If your TPS and fuel pressure check out ok at WOT, i would look at possibly the MAP sensor malfunctioning at the WOT range.

 

ok i got some fuel pressure readings. at idle the pressure is about 29-30 psi and with the vacuum line disconnected it jumps up to 39 psi. shut the jeep off and it remains at 38 psi

 

sounds good

test the map sensor


MAP Sensor Test Procedure:


Inspect vacuum lines at MAP sensor and at Throttle Body. Repair as needed.

WHEN TESTING THE MAP SENSOR, ENSURE THAT THE HARNESS WIRES ARE NOT DAMAGED BY THE TEST METER PROBES.

Test the MAP Sensor output voltage at the MAP Sensor connector terminal B (as marked on the sensor body). This is done with the ignition switch ON and the engine OFF. Output voltage should be 4-5 volts.

The voltage should drop to 1.5-2.1 volts with a hot, neutral idle speed condition.
Test engine controller (terminal 5) for the same voltage as described above to verify the wire harness condition. Repair as necessary.

Test MAP Sensor supply voltage at sensor connector (terminal C) with the ignition ON. The voltage should be approximately 5V (+/- 0.5V). 5V (+/- 0.5V) should also be at terminal 6 of the engine controller wire harness connector.

Test the MAP Sensor ground circuit at the sensor connector (terminal A) and the engine controller connector (terminal 4).

Test the MAP Sensor ground circuit at the engine controller connector between terminal 4 and terminal 11 with an ohmmeter. If the ohmmeter indicates an open circuit, inspect for a defective sensor ground connection. This connection is located on the right side of the cylinder block, at the oil dipstick tube mounting stud. If the ground connection is good, replace the engine controller. If terminal 4 has a short circuit to 12V, correct this condition before replacing the engine controller.

 

o2's...