Jeep Enthusiast Forums banner

Won't Accelerate!

19K views 34 replies 9 participants last post by  ZeeJay1997 
#1 ·
I have a ZJ 4.0L that won't accelerate past 30mph. It WILL shift fine but once it gets close to 30mph it starts to bog down and sometimes it will stall. Then once i throw it in neutral it will start right back up and then do the same thing near 30mph. rpms seem normal until it starts bogging down then the rpms will stay the same or will drop a bit. No codes. New upstream o2. No Catalytic converter. Newer CPS/CKS. I take good care of it. clean icv. newly rebuilt engine. Please help!

My transmission pan seems extremely hot after driving it untouchably hot. Could an overheating transmission be causing this? (my fluid levels are fine and its a remaned tranny not a year old)
 
#2 ·
Check for a bad Throttle Position Sensor or a stuck throttle.

Also, read this post from Oldfrog that we were discussing yesterday, and see if his symptoms match close to yours. His was a defective TPS.

==========

As for the transmission, if the problem is happening when you first start out driving (when the tranny is cold), then it isn't related to an overheated transmission.
 
#5 ·
Tested it and then replaced it. the problem still persists. i also tested the cks and it says that the resistance is 5.6 Mega ohms which is 5600000 ohms.
 
#6 ·
The TPS was bad? If so, try resetting your PCM after replacement so it can re-learn stuff (reset by disconnecting your battery for about 10 min).

On restart, I've heard that you should not touch the gas pedal and let the Jeep idle up to operating temp so the PCM can learn idle through the new TPS.
 
#13 ·
Crankshaft Position Sensor, often abbreviated as CPS or CKS (I use CKPS, and I don't like to use "CPS" because that acronym can get confused with the Camshaft Position Sensor).

The CKPS is on the top of the transmission's bell housing.
 
#9 · (Edited)
I reset the PCM still having the problem. Sebian i thought it should have infinite resistance when your ohm meter is set to 1000 to 10000 ohms? So in that case wouldn't it be infinite resistance? Also i just tested the CKS by watching the voltage as i rotate the crankshaft and the results were normal.

To expand on this issue a bit more. it is now happening at any speed. whenever i touch the gas peddle to accelerate it starts to hesitate and sputter
but as soon as i left off the gas or just let it idle it runs fine. the rpms will flutctuate a little but not more than 100. also when it dies it always has NO problem starting right back up. When it is in neutral or park i can rev the engine and it seems fine, it only tries to die in drive
 
#15 ·
IIRC your OHM meter should be set to 20k ohm if available. And during the check, it should read "0" ohms, indicating that there is no resistance in the circuit. Double check your multimeter specs to make sure it will read open resistance like that.
 
#11 ·
Could the overheating transmission and the stalling both be symptoms of a problem with the transmission?

I am not a transmission guy, but if something is causing the transmission to bind up it could bog down the engine. And after driving it for a while, the trans would get hot from the added friction. Once its in neutral, the resistance isn't there and it idles fine.
 
#12 ·
Trev, I agree with ZeeJay as far as your testing process. I believe that the common non-transmission causes are TPS, clogged cat, air supply and fuel supply. Though it may seem less likely, you should test your fuel pressure to eliminate your fuel system as the cause (more fuel is delivered under load than just reving) ... I don't think you have a vacuum leak or other air supply problem because your TPS and idle seem good.

Second, yes, your testing of the CKPS makes me believe that it is good ... plus, a failing CKPS would likely have problems in neutral and while reving, and not merely under load.

Finally, if you paid a shop to install your remanufactured transmission in the last year, then call the shop that installed it and ask them if your symptoms are consistent with transmission problems.
 
#14 ·
thanks for your help so far guys but still stumped and loosing my cool. here is an update. At idle it is acting a little funny like it wants to start to die but in a split second it jumps back to a normal idle. Never dies from an idle though. Checked grounds and vacuum lines, all good. recleaned icv. checked cks and tps sensors. still bogs down when its given gas but only while in drive. both o2 sensors are unplugged. fuel pressure is at 50psi. I'm stumped guys.
 
#22 ·
If you want to check the Crank sensor...here is the correct procedure:
But if it isnt a genuine Mopar sensor, you're pizzing in the wind, cuz Jeeps hate aftermarket sensors.


I'm not sure that's the problem though. Have you pulled the codes ?...wait...nevermind, I re-read your post. No codes.

Open the distributor cap and check everything out, visually.
 
#26 ·
crank sensor is mopar tests fine. The tps is not the issue i have tested it 3 times, bought a new one, tested again, and drove with it unplugged and nothing changed
 
#27 ·
Looks like it was the map sensor. the first few times i checked it it was out of range but only about a couple tenths of a volt, so i just wrote it off thinking it was fine. but it wasn't problem solved thanks for all your help guys
 
#29 ·
Wasn't it, i lied. replaced it with a new map sensor and still the same thing. but yesterday when i unplugged the sensor completely it would run crappy but better and i could accelerate. could this mean pcm?
 
#30 ·
Here is an update. Problem still persists. Dies and hesitates randomly but only while it is in drive. shifts fine. Sometimes it dies from putting it into drive and sometimes is dies when accelerating. It is always hesitating and won't accelerate past 30mph or so. New cks, map, tps, iac, and not duracrap. all electronics test fine and i have tested them 3+ times each. grounds are good. fuel pressure is good. vacuum lines are good. TB clean. I really need help on this one guys im stuck. I rebuilt the engine a couple weeks ago but thats not the problem because it was doing this before the rebuild. Please help me out.
 
#32 ·
Have you done the reference voltage checks out of the PCM?
How bout the coil?

It may be worth your time to do a stall check.

FSM procedure:

STALL TEST PROCEDURE

(1) Connect tachometer to engine. Position tachometer
so it can be viewed from driver's seat.
(2) Drive vehicle to bring transmission fluid up to
normal operating temperature. Vehicle can be driven
on road or on chassis dynamometer, if available.
(3) Check transmission fluid level. Add fluid if necessary.
(4) Block front wheels.
(5) Fully apply service and parking brakes.
(FSM doesnt tell you this... put it in drive)
(6) Open throttle completely and record maximum
engine speed registered on tachometer. It takes 4-10
seconds to reach max rpm. Once max rpm has
been achieved, do not hold wide open throttle
for more than 4-5 seconds.

CAUTION: Stalling the converter causes a rapid
increase in fluid temperature. To avoid fluid overheating,
hold the engine at maximum rpm for no
more than 5 seconds. If engine exceeds 2500 rpm
during the test, release the accelerator pedal immediately;
transmission clutch slippage is occurring.

(7) If a second stall test is required, cool down
fluid before proceeding. Shift into NEUTRAL and run
engine at 1000 rpm for 20-30 seconds to cool fluid.

STALL TEST ANALYSIS

Stall Speed Too High
If the stall speed exceeds 2500 rpm, transmission
clutch slippage is indicated.
Stall Speed Low
Low stall speed with a properly tuned engine indicate
a torque converter overrunning clutch problem.
The condition should be confirmed by road testing. A
stall speed 250-350 rpm below normal indicates the
converter overrunning clutch is slipping. The vehicle
also exhibits poor acceleration but operates normally
once highway cruise speeds are reached. Torque converter
replacement will be necessary.
Stall Speed Normal But Acceleration Poor
If stall speeds are normal (1800-2300 rpm) but
abnormal throttle opening is required for acceleration,
or to maintain cruise speed, the converter overrunning
clutch is seized. The torque converter will
have to be replaced.
Converter Noise During Test
A whining noise caused by fluid flow is normal during
a stall test. However, loud metallic noises indicate
a damaged converter. To confirm that the noise
is originating from the converter, operate the vehicle
at light throttle in DRIVE and NEUTRAL on a hoist
and listen for noise coming from the converter housing.
 
#33 ·
I have seen a couple where the distributor was loose. You could check fuel pressure while driving. Had a diesel truck come in once that would only stall out when you were pulling a heavy load up a steep hill but drive fine around town unloaded and idle just fine. Happened to be a Towel in the gas tank. No idea why. But when you were going up that hill it would suck that towel up and block off the flow to the pump. You might just have a dirty fuel filter.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top