[brandontrek]

What happens while the 4.0 is "warming up" from a cold start that doesn't happen when it is running? trying to trace down why my jeep likes to lurch and buck until the temp hits ~190.

[CJ7-Tim]

During warm up, the engine computer (ECU) ignores all the engine sensors and runs on preprogrammed data tables. When the proper temperature is reached the ECU reads data from the sensors.

Fully describe what happens and when, exactly, it occurs. You probably have a faulty sensor.

[brandontrek]

so here it goes step by step.

1. get in jeep
2. turn the key
3. VRROOOOMMM 4.0 comes to life
4. Idles fine sitting still
5. put into "D" and proceed to move forward
6. jeep begins to feel like it is loosing power, if you press the pedal it bucks
7. let off of pedal the engine sets to idle
8. push pedal anywhere past a feathers weight it bucks
9. come to a stop the engine wants to die, idle drops to 200 or less rpm
10. jeep realizes dying at an intersection isn't a good idea and revs back to normal idle
11. steps 6-10 continue until engine is warm


side note: when engine looses power i can slam the throttle and it responds like normal.

Replaced items include but not limited to:
plugs
wires
cap
rotor
TPS sensor
IAC sensor
fuel check valve
battery terminals
seafoamed engine


Possible problems:
getting a large evap leak CEL(can't find any signs of a leak, doubt it would cause bucking)
battery might be weak???
coolant temp sensor?

[CJ7-Tim]

Originally Posted by brandontrek

Possible problems:
getting a large evap leak CEL(can't find any signs of a leak, doubt it would cause bucking)
battery might be weak???
coolant temp sensor?

Evap leaks are usually the gas cap, buy a new one. The battery can be load tested at a repair shop or auto parts store.

Clean the throttle body and the Idle Air Controller.

From personal experience, I would say the coolant temp sensor on the thermostat housing is going bad and confusing the ECU. I would test or replace the coolant sensor.

[Carves]

Originally Posted by brandontrek

What happens while the 4.0 is "warming up" from a cold start that doesn't happen when it is running? trying to trace down why my jeep likes to lurch and buck until the temp hits ~190.

Battery voltage and O2 sensors have been causes of that "lurching" on a few vehicles over here - give the ASD Relay a rattle and make sure the connections are clean/contacting as well.

You mentioned 190'F ... If your temp gauge is reading normally and accurately ... its not likely to be a faulty sensor causing the issue ... altho like CJ-7 ... I would be suspicious of it as well. Performing the sensor test in the FSM would be worthwhile, unless its easier just to throw a new one in ...

Originally Posted by CJ7-Tim

During warm up, the engine computer (ECU) ignores all the engine sensors and runs on preprogrammed data tables. When the proper temperature is reached the ECU reads data from the sensors.

Fully describe what happens and when, exactly, it occurs. You probably have a faulty sensor.

According the 97+ manual I have ...

Engine startup / Warmup mode ... (Open Loop) ... the PCM is getting feeds from lots of sensors and applies preset data according to the inputs.

- Battery voltage
- Engine coolant temperature sensor
- Crankshaft position sensor
- Intake manifold air temperature sensor
- Manifold absolute pressure (MAP) sensor
- Throttle position sensor (TPS)
- Camshaft position sensor signal ... and others.

That "proper temperature" according to scangauge monitoring of my vehicles is approx 20'C ... which is when the system goes closed loop and starts taking notice of the O2 sensors as well.

[brandontrek]

the temp gauge seems to be working ok. keep in mind the vehicle was sitting for some time, the battery looks to be old, etc...

the engine only has 72,xxx miles

tested the crankshaft position sensor. i am going to try(one at a time) the coolant temp sensor, then the battery(waiting to get a red top), then major wiring(alternator to battery, etc)

need to get the other car on the road so i can start "project XJ restore"

[brandontrek]

PROBLEM FIXED!!!


upstream O2 sensor