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Engine running rough and RPM's jumping possible PCM ECM

J

JeepLovinMonkey

NAXJA Forum User
Location
East Coast
1999 Jeep Cherokee XJ 4WD Automatic 4.0L I6

Likely Problem: PCM/ECM

Symptoms: Engine lagging and loping (jerking forward, rpm’s shoot up from 2-3K for a second) overall rough idle. Paid for diagnostics test and to find out what was wrong at dealer.

Cleaned Throttle Body and IAC along with IAC housing. Ran great for a few hours then back to lagging and loping (jerking forward, rpm’s shoot up from 2-3K for a second)

Replaced Cam Position Sensor ran great for a day then back to poor performance

Replaced TPS ran great for a day and then back to same issues.

Each thing I do, involving resetting the computer, works fine for a bit then back to the same problems.

The dealer diagnostics test, advance auto's diagnostics and I think it is the PCM ECM and am going to replace this or fix it next. My dad and I put a meter to it per one of the tests I saw online and there is voltage where it shouldn't be (on the ground) so definitely seems like PCM problem.

I originally wanted to refurbish it myself but the case is super difficult to open and circuit board is on both sides of inside case. Are there any good DIY for this type of PCM ECM? I have a low emissions No Security PCM P56041 493AD.

Any suggestions? Any good places that are trustworthy to send your PCM ECM to get fixed or buy refurbished one?
 
Wondering how/why you diagnosed the symptoms as a faulty PCM, and not an OBD-II engine sensor, or a simple wiring fault, which would be more likely, and more logical ?

Is the Check Engine Light ON ? What trouble codes are present ? What exactly was the diagnostic report from the Dealership ? You do know that most Dealerships are really bad with diagnostics, right ?

Where exactly is this voltage ? What testing procedure was performed to find the voltage ?

Did you install genuine Jeep sensors, or some crappy Chinese copies ?

Always begin any diagnostics with the basics, check fuses/test wires/test OBD sensors/clean grounds. XJ Cherokee PCM failure is very uncommon, and should only be considered after a thorough and logical diagnostics and testing procedure of the more common XJ Cherokee issues. Usually the PCM either works, or it does not. Most people who install a used or rebuilt PCM did not have any PCM failure.
 
Check for a vacuum leak, ie. throttle body or intake gasket.
 
Actually re-reading the symptoms and thinking about it. I had the same thing happen to my 99 when the TPS went south, which if it was replaced with an el-cheapo-zone sensor could cause the same issues again.
 
You didn't say if it is on a hot motor or a cold motor. Open loop to closed loop kind of changes in stages on the OBD 2. First the O2 sensor kicks in, usually within a few minutes of start up, as soon as the O2 heater has it heated up. Many sensors and trim kick in after the motor starts to warm up about the time it idles down, usually when the temp. gauge is around a quarter or halfway to operating temp.

On my 96 my O2 sensor never threw a code until the O2 senor was completely dead. It was causing grief for months before it completely died. Symptoms where, it would want to stutter/stall when cold and would occasionally run really rough after the motor warmed up. A computer reboot seemed to help for awhile, after the computer relearned some it would get worse. The computer in your Jeep is like most other computers, garbage in/garbage out. The computer is programmed to compensate for many sensor flaws, but not all. Really hard for the programmers to anticipate every possible sensor combination and/or conflict.

It may be a PCM issue but IMO much more likely to be a sensor (or wiring) issue. The sensors are bundled on a common power supply, not all sensors, but say around half on one power supply and the other half on another power supply. A partial short on one sensor can lower the input voltage to multiple sensors. If it gets bad enough, the computer will shut down to protect itself and you get a no bus condition.
 
Tim_MN said:
Wondering how/why you diagnosed the symptoms as a faulty PCM, and not an OBD-II engine sensor, or a simple wiring fault, which would be more likely, and more logical ?

Is the Check Engine Light ON ? What trouble codes are present ? What exactly was the diagnostic report from the Dealership ? You do know that most Dealerships are really bad with diagnostics, right ?

Where exactly is this voltage ? What testing procedure was performed to find the voltage ?

Did you install genuine Jeep sensors, or some crappy Chinese copies ?

Always begin any diagnostics with the basics, check fuses/test wires/test OBD sensors/clean grounds. XJ Cherokee PCM failure is very uncommon, and should only be considered after a thorough and logical diagnostics and testing procedure of the more common XJ Cherokee issues. Usually the PCM either works, or it does not. Most people who install a used or rebuilt PCM did not have any PCM failure.
Click to expand...

____________________________________________________________

Thanks again so much to this awesome Jeep community for your time and help. I would have answered sooner but my Jeep is an hour away and this is my first opportunity to respond.

I will respond to each of your questions as follows:

Wondering how/why you diagnosed the symptoms as a faulty PCM, and not an OBD-II engine sensor, or a simple wiring fault, which would be more likely, and more logical ?

I don't know how to test the OBD-II sensor, should we replace it to see what happens?

Is the Check Engine Light ON ? Check engine light was disabled somehow by previous owner. The dealer acted like it would be a pain to reconnect, said it was disconnected in two places.

What trouble codes are present ? Trouble codes are as follows:
P0201 Injector circuit/ open-cyclinder 1
P0340 No Camshaft Position Sensor Circuit Failure
P0203 Injector circuit/ open-cyclinder 3
P0202 Injector circuit/ open-cyclinder 2
P0204 Injector circuit/ open-cyclinder 4
P0205 Injector circuit/ open-cyclinder 5
P0206 Injector circuit/ open-cyclinder 6
P1391 CKP or CMP Sensor Signal Intermittent

What exactly was the diagnostic report from the Dealership ? Dealer did not provide me with codes but I have codes from autozone which also concluded PCM as likely cause.

You do know that most Dealerships are really bad with diagnostics, right ?
Yes I agree in general with this statement

Where exactly is this voltage ? Voltage was on the ground pins of the PCM, 5 and 11 if memory serves correct.

What testing procedure was performed to find the voltage ? We unplugged the negative cable off and put the black lead of the meter on the ground of the battery lead and the red lead of the meter on the 5, 11 and 15 grounds of the PCM pins. It was supposed to read 0 OHMS if the PCM is good and it registered 244 OHMS on pins 5 and 11. I think 15 read zero ohms.

Did you install genuine Jeep sensors, or some crappy Chinese copies ? Installed Mopar Cam Position Sensor (which was made in China) and installed aftermarket TPS from a reliable independent parts place that me and my Dad have gone to since I was a kid. TPS was not Mopar.

We checked fuses, dealer supposedly checked wires and other things. The mechanic was a little upset because it took him over 2 hours to diagnose my Jeep. They wanted to make up the money by charging me $400 to install the CPS but I said no thanks.
Always begin any diagnostics with the basics, check fuses/test wires/test OBD sensors/clean grounds. XJ Cherokee PCM failure is very uncommon, and should only be considered after a thorough and logical diagnostics and testing procedure of the more common XJ Cherokee issues. Usually the PCM either works, or it does not. Most people who install a used or rebuilt PCM did not have any PCM failure.
 
Simple test you can do, check the voltage at the Orange wire going to the Crank position sensor or the Cam position sensor. Should be around 5 volts, if it is low, disconnect the crank position sensor and see what happens. Still low, disconnect the cam position sensor and see what happens.
 
Installing random new parts with the hope of accidentally fixing an undiagnosed problem seldom works, and gets expensive fast. Do not let the Dealership do this, and do not do this in your driveway. Test and diagnose any problem, especially if you have Check Engine Light trouble codes, and begin your repairs with that knowledge. The AutoZone people are nice, and try to be helpful, but most know nothing about Jeeps.

I very strongly recommend that you spend $7.95 at www.PacificCoastManuals.com for a genuine Jeep Service Manual. Inside you will find test procedures and expected test results for everything.

Most auto parts stores sell cheap crappy Chinese "Lifetime Warranty" parts that are often out of specification, or even failed, right out of the box. Many times they have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep sensors. Numerous threads detail long and frustrating searches for a "problem" that ended up being cured with genuine Jeep repair parts.

Cheap parts are cheap for a reason.



Follow the CEL trouble codes. Start with the P1391 code. Test the OBD sensors, and find the fuel injector circuit wiring failure.


P1391 - Intermittent Loss of CMP or CKP Signal. Loss of the Cam Position Sensor or Crank Position sensor signal has occurred.

P1391 – Possible symptoms:
(1) random misfire/stumble/stalling, often over bumps or right turns and when going into reverse.
(2) shorted out the Fuel pump/ECU power wire.

Possible causes
- Faulty Camshaft Position (CMP) Sensor
- Camshaft Position (CMP) Sensor harness is open or shorted
- Camshaft Position (CMP) Sensor circuit poor electrical connection.

Google Scan Tools, O-Scopes and Toothpicks, and read the article.


P1391-INTERMITTENT LOSS OF CMP OR CKP

CMP SENSOR OUT OF SYNC
-DAMAGED TONE WHEEL/FLEX PLATE (CRANKSHAFT)
-DAMAGED TONE WHEEL/PULSE RING (CAMSHAFT)
-IRREGULAR LAB SCOPE PATTERN OF CMP SIGNAL

INTERMITTENT CKP SIGNAL LOSS WHEN WIRING IS WIGGLED
-IRREGULAR LAB SCOPE PATTERN OF CKP SIGNAL
-WIRING HARNESS INTERMITTENT

INTERMITTENT CMP SIGNAL LOSS WHEN WIRING IS WIGGLED
-SENSOR CONNECTOR/WIRING
-CAMSHAFT POSITION SENSOR
-CKP SENSOR CONNECTOR/WIRING
-CRANKSHAFT POSITION SENSOR

Potential Fixes:
Fix #1:
Inspect your wire harnesses for damaged wires, chafed insulation, or failed wire splices. Use a volts/ohms multi-meter to check the resistance of the ground wire circuits from each sensor. Replace corroded ground wires/connectors and clean ground wire connection points.
Fix #2:
Timing may off. Re-index the distributor/cam sensor per the FSM instructions, or replace faulty distributor/cam sensor pickup module.

A mis-indexed distributor/cam sensor may exhibit surging, light bucking or intermittent engine misfiring. This will most likely occur when the vehicle is at operating temperature and under a light load at approximately 2,000 rpm. The factory procedure must be used as outlined in the service manual.
 
Last edited:
Don't know if you solved this yet, but my 1991 had been running poorly for months when I finally fixed it. I removed and cleaned the IAT (?) sensor, it's right next to the throttle body. Just unhook sensor wire and unscrew it. It was covered in gunk/oil (its original), 30mins with a spray bottle of throttle body cleaner and a toothbrush, and my bucket has never run better. Worth a try-
 
truckeejeeper said:
.. my 1991 had been running poorly for months when I finally fixed it. I removed and cleaned the IAT (?) sensor, it's right next to the throttle body.
Click to expand...

This might be helpful if the symptoms described or Trouble Codes downloaded were related to the IAT.
 
truckeejeeper said:
Don't know if you solved this yet, but my 1991 had been running poorly for months when I finally fixed it. I removed and cleaned the IAT (?) sensor, it's right next to the throttle body. Just unhook sensor wire and unscrew it. It was covered in gunk/oil (its original), 30mins with a spray bottle of throttle body cleaner and a toothbrush, and my bucket has never run better. Worth a try-
Click to expand...


Thanks. I did that already!
 
Tim_MN said:
Installing random new parts with the hope of accidentally fixing an undiagnosed problem seldom works, and gets expensive fast. Do not let the Dealership do this, and do not do this in your driveway. Test and diagnose any problem, especially if you have Check Engine Light trouble codes, and begin your repairs with that knowledge. The AutoZone people are nice, and try to be helpful, but most know nothing about Jeeps.

I very strongly recommend that you spend $7.95 at www.PacificCoastManuals.com for a genuine Jeep Service Manual. Inside you will find test procedures and expected test results for everything.

Most auto parts stores sell cheap crappy Chinese "Lifetime Warranty" parts that are often out of specification, or even failed, right out of the box. Many times they have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep sensors. Numerous threads detail long and frustrating searches for a "problem" that ended up being cured with genuine Jeep repair parts.

Cheap parts are cheap for a reason.



Follow the CEL trouble codes. Start with the P1391 code. Test the OBD sensors, and find the fuel injector circuit wiring failure.


P1391 - Intermittent Loss of CMP or CKP Signal. Loss of the Cam Position Sensor or Crank Position sensor signal has occurred.

P1391 – Possible symptoms:
(1) random misfire/stumble/stalling, often over bumps or right turns and when going into reverse.
(2) shorted out the Fuel pump/ECU power wire.

Possible causes
- Faulty Camshaft Position (CMP) Sensor
- Camshaft Position (CMP) Sensor harness is open or shorted
- Camshaft Position (CMP) Sensor circuit poor electrical connection.

Google Scan Tools, O-Scopes and Toothpicks, and read the article.


P1391-INTERMITTENT LOSS OF CMP OR CKP

CMP SENSOR OUT OF SYNC
-DAMAGED TONE WHEEL/FLEX PLATE (CRANKSHAFT)
-DAMAGED TONE WHEEL/PULSE RING (CAMSHAFT)
-IRREGULAR LAB SCOPE PATTERN OF CMP SIGNAL

INTERMITTENT CKP SIGNAL LOSS WHEN WIRING IS WIGGLED
-IRREGULAR LAB SCOPE PATTERN OF CKP SIGNAL
-WIRING HARNESS INTERMITTENT

INTERMITTENT CMP SIGNAL LOSS WHEN WIRING IS WIGGLED
-SENSOR CONNECTOR/WIRING
-CAMSHAFT POSITION SENSOR
-CKP SENSOR CONNECTOR/WIRING
-CRANKSHAFT POSITION SENSOR

Potential Fixes:
Fix #1:
Inspect your wire harnesses for damaged wires, chafed insulation, or failed wire splices. Use a volts/ohms multi-meter to check the resistance of the ground wire circuits from each sensor. Replace corroded ground wires/connectors and clean ground wire connection points.
Fix #2:
Timing may off. Re-index the distributor/cam sensor per the FSM instructions, or replace faulty distributor/cam sensor pickup module.

A mis-indexed distributor/cam sensor may exhibit surging, light bucking or intermittent engine misfiring. This will most likely occur when the vehicle is at operating temperature and under a light load at approximately 2,000 rpm. The factory procedure must be used as outlined in the service manual.
Click to expand...


Thanks a ton! I complained to the Jeep dealer about the poor diagnostics job and they took it back in to let an experienced tech analyze it for me. They found the following 3 problems:

1) Spark plugs were platinum bosch. I switched them out to copper champions.

2) PCM is definitely bad

3) PDC or Power distribution center has some circuitry that is corroded. They said since they don't make this I either buy the kit to fix it or get one used.

I think the PCM is next logical step. Even if PDC is going bad it still might run great for a while if I get a refurbed PCM.

I will report back once all this is done. Apologies for the delay as I have been working like crazy, family got sick, then I got sick and had to work some more. Merry Christmas fellow Jeep Lovers!
 
Is still strongly recommend that you follow the OBD-II trouble codes. Technicians with poor diagnostic skills are far too eager to blame the PCM when they can't find their backside with both hands, oops I mean diagnose a fault. If you find a used PCM for $30 or less, you could try swapping that in, but I am NOT saying that you should install random parts until you pin down the cause of the trouble codes.
 
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