• Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

Computer or CPS

J

*Jonathan

NAXJA Forum User
Location
North Jersey
So the jeep had a check engine light for the Camshaft position sensor and a misfire code. Drove fine and the light went out by itself.

Flash forward 2 weeks, I'm driving along and the jeep starts to stutter going up hill about 35mph. Then it drives fine for the remaining 2 min ride home.
Decided to take it to the shop and it wont start. I wait 2 hours and it starts right up.

I'm reading alot on the forums that the Crankshaft position sensor (on bell housing) and Camshaft position sensor can cause these symptoms. I brought it to my mechanic today and he says he thinks that its the computer. (says he checked everythign else)

At this point.. it will start up but you have to wait for it to cool down after you restart it.. and it will sometimes miss while driving.

Is there any real way for him to rule out those sensors as the culprit? It doesn't help that I have a 97 XJ and they only made my computer for 1 year. I'd really rather it not be the computer. Where is it even located?
 
Hot restart problem suspect is a thermal failure in the CPS/CKP crank sensor. You will have no spark.

The synch sensor in the distributor can cause a no start condition in the Chryco XJ, you will have spark but no injection, however I have not seen a thermal failure of the synch sensor.

The PCM (computer) is on the fenderwell, driver's side.

I would get it into a no start condition and test the crank sensor.
 
joe_peters said:
Hot restart problem suspect is a thermal failure in the CPS/CKP crank sensor. You will have no spark.

The synch sensor in the distributor can cause a no start condition in the Chryco XJ, you will have spark but no injection, however I have not seen a thermal failure of the synch sensor.

The PCM (computer) is on the fenderwell, driver's side.

I would get it into a no start condition and test the crank sensor.
Click to expand...

Yea thats a good idea.. how do I test the CPS? I assume an ohm meter?
what values?
 
*Jonathan said:
So the jeep had a check engine light for the Camshaft position sensor and a misfire code. Drove fine and the light went out by itself.......At this point.. it will start up but you have to wait for it to cool down after you restart it.. and it will sometimes miss while driving.
Click to expand...

So, you had CEL that you ignored. Now you have drive-ability issues. Wonder if maybe they are related ? Search the NAXJA Forum or GOOGLE for the CranKshaft Position Sensor (CKP) and Camshaft Position Sensor testing procedures. Both are relatively simple to test or replace and do not require a mechanic to do so.

Sensor failure is much more likely than Computer failure. The CranKshaft Position Sensor is a very common failure.
 
*Jonathan said:
Yea thats a good idea.. how do I test the CPS? I assume an ohm meter?
what values?
Click to expand...

You can test it with an ohmmeter - I don't recall the HO spec.

To isolate a thermal failure, get a hair dryer and your DMM. Plug in the DMM and test the sensor, then start heating it up with the hair dryer on LOW (HIGH if you can't get within six inches of the thing.) If the reading starts to get fluky, you have a heat-related failure of the sensor.

I'll agree with Joe - I've not heard of a thermal failure of the CMP. It's "too exposed" for that, and the aluminum distributor housing works at a heat sink. The CKP is too close to the exhaust for my comfort, but I've yet to devise a convenient heath shield approach.

A hair dryer is something you should keep in your toolbox anyhow - or a low-powered heat gun (150-175*C rated temperature. Don't let's get stupid - you're not blistering and stripping paint here! 150-175*C is plenty for isolating a heat-related failure by heating a single component of the system.)
 
Alright thanks for all the help guys. The mech says he is 100% sure that the ECU is to blame. He said that he tested all the sensors and looked at it for 2 hours and somehow seems to think that the ECU is wonky.

I dunno if I believe him. I'm going to run some controlled tests (hairdryer) and get some readings of my own. One thing that I just cant get straight is the difference between the CAMSHAFT position sensor and the CRANKSHAFT position sensor. The Crankshaft PS is on the bell housing right? The other is on the Distributor? Or do I have that backwards?

Before I go out and get a new ECU I may even just change the CPS for good measure. The thing isn't going to last forever and I have 155K on my XJ.
 
dont even worry about testing...

this issue is as common as the shits after taco bell.
time to replace it. ive done about 10 of these on jeeps in the last year
 
*Jonathan said:
Alright thanks for all the help guys. The mech says he is 100% sure that the ECU is to blame. He said that he tested all the sensors and looked at it for 2 hours and somehow seems to think that the ECU is wonky.

I dunno if I believe him. I'm going to run some controlled tests (hairdryer) and get some readings of my own. One thing that I just cant get straight is the difference between the CAMSHAFT position sensor and the CRANKSHAFT position sensor. The Crankshaft PS is on the bell housing right? The other is on the Distributor? Or do I have that backwards?
Click to expand...

You got it right. But anyone who says they are 100% sure of anything is kidding themselves. Has he checked the inputs and outputs at the ECU and found something wrong? Not likely. :passgas:
 
winterbeater said:
You got it right. But anyone who says they are 100% sure of anything is kidding themselves. Has he checked the inputs and outputs at the ECU and found something wrong? Not likely. :passgas:
Click to expand...

I agree with Winterbeater. Put it in fault mode and then check the crank sensor.
 
I'd bet against the ECU, they don't fail that often (it does happen occasionally.) Mechanics LOVE to blame the computer though...
 
*Jonathan said:
Alright thanks for all the help guys. The mech says he is 100% sure that the ECU is to blame. He said that he tested all the sensors and looked at it for 2 hours and somehow seems to think that the ECU is wonky.

I dunno if I believe him. I'm going to run some controlled tests (hairdryer) and get some readings of my own. One thing that I just cant get straight is the difference between the CAMSHAFT position sensor and the CRANKSHAFT position sensor. The Crankshaft PS is on the bell housing right? The other is on the Distributor? Or do I have that backwards?

Before I go out and get a new ECU I may even just change the CPS for good measure. The thing isn't going to last forever and I have 155K on my XJ.
Click to expand...

Crankshaft Position Sensor - looking forward, it's at about 10:30 on the bellhousing (you can see if if you look past the end of the intake down toward the crankshaft. Easier to access the screws from underneath - using about 30-36" of extension and an 11m/m "universal" socket.) Can be done in 20-30 minutes, with practise.

Camshaft Position Sensor - located inside the distributor, below the rotor. Requires removal and disassembly of the distributor to replace, and takes about an hour and a half. Takes some care so that the distributor is timed and indexed properly when it goes back in.

Failure of the CKP is relatively common - primarily due to its location (which is relatively close to the #6 exhaust primary tube.) Even the ChryCo computers are fairly durable - while the typical service life of the CKP seems to run 150-200Kmiles...
 
Good advice here. The computer doesn't generally fail because of heat.

The crankshaft position sensor is the most common sensor failure on the Jeep 4.0 and it is very common to have thermal fail with it.

I have had mixed (poor) luck with aftermarket crank sensors as have others. It is one of the few things that I purchase directly from Jeep for best reliability and longevity. Some of the aftermarket ones work for a while, BUT crap out early and can leave you on the side of the road or trail.
 
Well, I finally had time to go and order a Crankshaft Sensor from the dealer. They didn't have any in stock. The sensor tested out OK but since it is such an intermittent problem, I couldn't test it while the jeep was missing, as it does on acceleration, especially up inclines.

I figure if it doesn't cure my problem, at least I'll know I have a fresh sensor in there.

When the jeep doesn't want to start after its hot I checked the following-

CPS- Resistance is within spec
AIR- Filter and hoses are clear
Fuel- Pressure is fine
Spark- No spark
Coil- Tests fine
NSS- Tests fine

Recap on Symptoms, Starts fine cold always.. drives fine but cuts out for 1/2 sec. when you give it gas on an incline almost rhymically, hasn't stalled out on me yet.. sometimes it wont restart until after you let it sit for a while.. I have a CEL for misfire and Camshaft position sensor.. which i hear can sometimes also mean crankshaft even though it says camshaft.
 
Usually up inclines eh...

Have you replaced the motor mounts recently? Make sure the CPS wiring is not hanging against the exhaust manifold / something else extremely hot.
 
What is the exact code it's throwing for the cam sensor?

I'd honestly probably start with replacing that. I've seen them go bad before on XJ's...even had one that came bad on a reman distributor I was installing on an XJ once.
 
UNCC_99XJ said:
What is the exact code it's throwing for the cam sensor?

I'd honestly probably start with replacing that. I've seen them go bad before on XJ's...even had one that came bad on a reman distributor I was installing on an XJ once.
Click to expand...

The Code was P0340.
 
What do you know??? It was the PCM after all!!

I replaced the CKP and CPS with no results. Even though they tested ok I decided to change them for preventative measures and so that I could rule them out.

Next I found a re-manufacturer of PCMs and had them ship me one. Only $150 and it has a lifetime warranty. I when i got it in the mail.. it looked like a new unit, aside from the remanf sticker on it..

Once installed the jeep started right up.. faster than it ever did since I've owned it. No check engine light, no stumbles, smooth idle, and superfast and easy restarts.

Did I jump the gun on grabbing a PCM? Yea probably, but the mechanic said he tested everything, I know I tested everything.

I finally get to drive the jeep since the I installed my 4.5" lift.
 
$150 is a good price for a tested computer.

As pointing to a faulty computer is a "diagnosis of exclusion", you did okay. It does fail less than most other components, but yes.... it CAN fail!

Troubleshooting is a process of elimination.

Diagnosing a problem means going through the process finding out what it is NOT before you can hone in on what it IS.
 
Last edited:
You must log in or register to reply here.

Similar threads

J
2001 4.0L issues
Replies
15
Views
260
Saudade
Saudade
B
Jeep xj surging
Replies
11
Views
432
Bmfinney
B
Xtreme XJ
Crank, no start even with new CPS...
Replies
13
Views
2K
Xtreme XJ
Xtreme XJ
D
1996 4.0 No Crank, Starter Works When Jumped
Replies
17
Views
242
Anak
Anak
D
T-case sensor- same for 231 or 242?
Replies
8
Views
2K
DPG
D
Back
Top