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xj stall/stumble when trying to accelerate

S

simonm

NAXJA Forum User
Location
montreal
hi
i have a 1994 country 4.0l manual with np231 stock diff

the truck keep trying to stall/stumble when at speed trying to accelerate between 2-3000rpm
when i reach 3500 rpm it usually stop (not tried a lot)
it usually get worst until i park the truck for the night to let it cool down
the truck only does that when hot and it also stop sometime at red light.

part changed by me or the old owner

CKS
Fuel pump
high flow water pump
IACV
Map sensor
Radiator
Clutch, flywheel, flexplate, throw out bearings
master cyl
slave cyl
spark plugs and wires
cap and rotor

i can't figure it out
is there an easy way to test the coil without getting electrocuted
what about the ecu

p.s. the truck had water inside at one point.

thank you
 
Check the CKP (also called a CPS) signal while running using AC volts scale, back probe the quick connector. Try a hair dryer, blow on the CKP (and later the HV coil) to see if heat trips it off. Also wash it off if it has clutch/iron dust gathered on it, as it weakens the CKP magnetic field. Check the peak fuel flow rate, may be a plugged fuel filter, or bad FPR, not delivering enough fuel for high RPM demand. Too little fuel, or loss of spark at high rpm sounds like the issue.
 
i've ruled out fuel issue because when i can get it over 3500rpm it doesn'T have the problem anymore.

when cold(this morning it was -1C) i was good for 3ominute with out any problem the truck ran like a champ after that it stumbled it even stalled at a light and after the problem got worst
i'll try to check for the ckp if i have time this week end

thank you

p.s. sorry for my bad grammar english isn'T my first language
 
In that case, I would take a very close look at the O2 sensor and its wiring. Check for 12-14 volts on the large gauge wire to the O2 sensor, and check the O2 sensor voltage swing while running. A slow O2 sensor, or one with a bad internal heater, or missing voltage to that heater in the O2 sensor can cause your problem. The pre Cat O2 sensor if you have two of them. I think at above 3000 RPM and WOT the O2 sensor is ignored. But it is critical to response time during acceleration. So is the MAP sensor and TPS!!!! All three can be back probed and tested live while the engine is running.Also since it is heat related you can take a hair dryer and blow heat on the TPS, the CPS and other parts to see which one makes it die when it gets hot. Also check for an exhaust leak near the O2 sensor.
 
Last edited:
I second Economike's pointing to the TPS (Throttle Position Sensor). When it goes, it
usually does this gradually and begins by making your car will spit and stall during
acceleration. As it gets worse, your car will spit and conk out randomly at idle.
It's easy to change as long as the screws that hold it to the throttle body do not strip out.
 
which wire should i back probe on the tps and what should i look for ( volt or ohm)

i'll try everything tomorrow

thank you all for your answer

for the o2 sensor i don'T think it is the culprit because the truck doesn'T struggle to accelerate it stall then start back on compression. i'll test it just to make sure since it's free to test

p.s. i'va changed the coil since my uncle (who's a mechanic) tought it could be it from the symptom but it didn't solve the problem(he hasn'T seen the car we talked over the phone). the truck now feel more powerful when cold but it still stall when hot
 
Last edited:
If the O2 sensor is not getting that 12-14 volts, the ECU will ignor it until it is hot enough to work, but in that case it will get too cold at idle, and stop working, thus confusing the ECU until the RPM is back uop enough to reheat the O2 sensor so it starts working again, thus the delay on initial acceleration. The ECU ignores the O2 until it is at full operating temp, about 600 F IIRC.

Check out some of my old posts such as The RenX files here:

http://www.naxja.org/forum/showthread.php?t=905849

For everything you need to know about Renix TPS issues and O2 sensor issues.

Also see:

http://www.naxja.org/forum/showthread.php?t=1012701&page=7

Also see:

http://www.naxja.org/forum/showthread.php?t=1072534
 
so today i was testing sensor etc. everything checked out ok(from the fsm i don'T have a diagnostic machine) so i go out and it still stallbut now it flash the check engine light. i got the following code
12-14-55 12 is battery disconnected and 55 the end of code sequence.

14 is map sensor voltage too high or too low
the input voltage with the ignition on engine off is 5v(4.97)(pin a-c) and pin a-b also give me 5 volt(4.87) wich is in the spec so from what i understand the pcm is good and not the problem
the sensor would be bad but it is less then 1000km old(i have the invoice from the old owner) is thete a way to make sure it is the sensor that is bad and not the wiring/ground or the pcm that would give too high or too low voltage to the sensor
 
What is the MAP output voltage at idle with the engine running? The PCM may not be seeing the MAP output voltage, thus a return wire from MAP to PCM issue. Also recheck the MAP input voltage with engine running. And check the sensor ground to the battery, with power off. If it is over 1 ohm, you have a problem there too.


simonm said:
so today i was testing sensor etc. everything checked out ok(from the fsm i don'T have a diagnostic machine) so i go out and it still stallbut now it flash the check engine light. i got the following code
12-14-55 12 is battery disconnected and 55 the end of code sequence.

14 is map sensor voltage too high or too low
the input voltage with the ignition on engine off is 5v(4.97)(pin a-c) and pin a-b also give me 5 volt(4.87) wich is in the spec so from what i understand the pcm is good and not the problem
the sensor would be bad but it is less then 1000km old(i have the invoice from the old owner) is thete a way to make sure it is the sensor that is bad and not the wiring/ground or the pcm that would give too high or too low voltage to the sensor
Click to expand...
 
Last edited:
i tested continuity from pin A to pin 4 of the pcm and resistance from pin 4 to pin 11( got 0.03ohm)

the sensor ground to battery you are talking the pin A on the sensor or pin 4 on the pcm wiring

the sensor input in the pcm is pin1? if so i'll check to make sure i have continuity between the 2 pin ( pin b and pin 1)

i can't back probe the plug because i can't find anything small enough to not destroy the wire/connector

thank you for your help
 
i've checked resistance from pin a of the map sensor and the negative battery terminal and got around 7 ohm so my problem could be/is bad ground?
 
Disconnect the connector from the MAP sensor. Test from the battery negative post, to the ground pin "A" in the loose connector (which is attached to the wiring harness.



simonm said:
i tested continuity from pin A to pin 4 of the pcm and resistance from pin 4 to pin 11( got 0.03ohm)

the sensor ground to battery you are talking the pin A on the sensor or pin 4 on the pcm wiring

the sensor input in the pcm is pin1? if so i'll check to make sure i have continuity between the 2 pin ( pin b and pin 1)

i can't back probe the plug because i can't find anything small enough to not destroy the wire/connector

thank you for your help
Click to expand...
 
i have 7ohm

i have back probed pin 1 of the ecu and pin B of the sensor
i have 0 volt between the two when cold the truck is heating as i write this

at idle i have 1.5 volt output and 4.75 volt with the engine off
 
well thing have gone from bad to worst

the truck won'T start anymore when hot

it crank start ( rev jump up to around 1500rpm) than it dies
 
Interesting. It is usually 87-90 XJ's that have ground problems, and 7 ohms is a real problem on the ground. I do not know for sure how the ground wiring is on your year, but All the sensors need a ground of less than 1 ohm, otherwise the ECU/PCM gets bad data from the sensor.

IAC, CPS, and IAT could be causing you problems with starting. When it refuses to start, hold the gas peddle to the floor while cranking. That will shut of the fuel injectors, and let it start if it is flooded, and if it has spark. Then see if it will idle with a little peddle applied, and then if it dies when you stop giving it gas. That would indicate a sticking IAC valve, or a bad B+ relay that powers the IAC at shut down so it set for the next start cycle.

Check and CLEAN and recheck all the sensor, firewall, block, and battery cable ground connections and get all the sensor to battery negative post ground readings down under 1 ohm! Then we can go from there.
 
I would also check the PCM ground when you get a chance!!!
 
i'll make a list of all ground tomorow and i'll go back to the truck wednesday
since the truck is 2 hour away at my uncle house

to clean them should i remove all the paint under the ground?or just sand the head of the bolt or shoudl i change teh bolt where i can for bigger boltand bigger washer

thank you for your help
 
Just get both sides of the ground connections down to bare bright metal and reconnect, and if any crimp connections are internally corroded, you may need to go under the crimp, clean the wire, and re-crimp or re clamp.
 
CTS, IAT and O2 sensor could be causing you problems with warmed up engine (hot) starting. When it refuses to start, hold the gas peddle to the floor while cranking. That will shut of the fuel injectors, and let it start if it is flooded, and if it has spark. Then see if it will idle with a little peddle applied, and then if it dies when you stop giving it gas.

Check all three hot and compare the sensor temperature and resistance to the FSM for the CTS and IAT. Also have you checked the O2 sensor running live data yet?

If that MAP sensor code comes back (did you clear it?, did you disconnect the battery to clear the PCM memory yet? Fixing the poor ground, but not clearing old PCM sensor data may be your current problem too), it may be a shorted wire in the harness or a PCM problem. Please post sensor data, temp, ohms, voltages....

The TPS is also still a possible issue. Have you verified the TPS voltage at idle, and check for a smooth change in voltage as you go from idle to WOT using an analog volt meter yet?
 
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