1997 XJ 4.0 - Ignition timing angle question

[vaco]

Hi!
While solving issue that turned to be caused by upstream O2 sensor I probably replaced CPS that was ok with factory-broken one, I suspect that it causes another issue now. I have 1997 4.0 XJ, what is happening is: randomly engine lowers (this happens more often, in idle it goes down to like 300 RPMs) or raises (rarely, in idle to about 1500-2000 RPMs) RPMs for fraction of second. These changes happen on idle and while driving. It happens only when the engine is hot. I can hear the RPMs change and also see it on gauge. To make sure that crappy CPS is the source I need some knowledge, maybe someone of you can help me. With simple OBD interface (ELM327) I can see that ignition timing angle changes while the problem occurs, when engine works ok then the angle is about 12 degrees, while the issue shows up then the angle goes as low as 4 degrees or up to 20 degrees, the direction of timing angle change is correlated with RPMs change direction, sorry, I do not remember now which directions are "paired", if it matters I will check it.
My questions are:
1) Am I right that in my case (the engine has old school ignition rotor) ignition timing angle cannot be changed by ECU or anything else and if mechanically everything is ok then these changes are caused by broken CPS or cam shaft position sensor? To be clear: I know that angle can be adjusted with the cap, what I mean is that engine/ECU itself cannot modify the angle in real-time. In other words: in my case the ignition angle should be more or less constant regardless of RPMs, load or anything else, right?
2) Am I right that timing angle read through OBD is calculated as difference between CPS and camshaft position sensor readings and my symptoms could be caused by failure of one of these sensors?
3) If 1. and 2. answers are yes the is there a method to check which sensor is broken?
4) My theory is that broken CPS or camshaft sensor randomly causes fuel injection at wrong time and that causes RPMs change. Does it make any sense?

Thank you for any help!

 

[RCP Phx]

Your reaching for things now, but add to the list a worn distributor! Any vertical play in the shaft can cause timing fluctuations along with a worn timing chain! Also a worn timing chain snubber.

 

[Tim_MN]

You can't have a logical theory for the root cause of the symptoms without any diagnostics.

- Test the CPS. The test is pass/fail.
- As already suggested check the distributor for excess wear, a damaged or contaminated cam sensor magnet, corroded, bent or pushed back wire plugs pins, and failed wire insulation.

When my 1998 suffered those physical rpm symptoms, the Idle Air Controller (IAC) was defective.



Crank Position Sensors can have intermittent “thermal failure”. This means that the CPS/CKP fails when the engine gets hot, but works again when it cools back down.

CPS Testing
TESTING PROCEDURE 1991 –2001 4.0L H.O. engines
1. Near the rear of fuel rail, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C or 2 and 3. Ohmmeter should be set to the 10K-or-2OK scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.

If you buy a new CPS, get a genuine Jeep CPS. Most auto parts stores sell cheap crappy Chinese "Lifetime Warranty" parts that are poorly manufactured from inferior materials and are often out of specification, or even failed, right out of the box. The ones that aren’t faulty often 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 simply with genuine Jeep repair parts. Buy good quality repair parts and genuine Jeep sensors for best results.

 

[vaco]

Sorry, I did not write that: distributor cap and rotor, IAC, MAP, TPS were replaced few days before the CPS and it had no effects neither positive or negative. RPMs jumping started a day after CPS replacement. I would simply replace CPS again but it is difficult so I would like to be sure it needs to be replaced before doing this again. I will make this test, I assume I need to perform that while engine is hot, right?

 

[vaco]

Still the most important information for me is whether I am right that ignition timing angle should be constant, should it?

 

[Tim_MN]

New parts can be defective.

Base timing is set by finding TDC on Cylinder One and installing the distributor. No further timing adjustments can be made to base timing by the mechanic. The PCM does make real time timing adjustments based upon the sensor inputs. I have never watched the timing change on a diagnostic instrument, but I have owned and maintained 7 different 1997-2001 XJ Cherokee over the last 22 years, and 4.0L idle has never been Japanese or German engine smooth.

The idle should be 750 rpm, plus or minus 50 rpm, any variations to 300 or +1200 rpm would suggests Idle Air Controller malfunction.

 

[vaco]

The PCM does make real time timing adjustments based upon the sensor inputs.

I can understand that fuel injection is controlled by PCM but I do not understand how does it control ignition if it is done by distributor.
I thought that it works this way: there is always current on the input of distributor and ignition happens at the moment the rotor and one of output contacts meet. Isn't it that simple?

 

[RCP Phx]

No, it's not that simple! Even the old school ignitions had vacuum/centrifugal advance!

 

[vaco]

No, it's not that simple! Even the old school ignitions had vacuum/centrifugal advance!

Yes, you are right, my knowledge about engines is small and mostly theoretical, educational engine models I've seen did not have ignition advance or I missed it.

No further timing adjustments can be made to base timing by the mechanic.

Yes, now I know it, he told me that he "adjusted ignition advance", I will have to ask him about that.


I read FSM few times and tested everything I think could be related to my issue and I could test with tools I have, also checked the distributor alignment and wear, checked all grounds and verified grounds in sensor connectors, everything seems to be ok.
I also marked water pump wheel with regular lines to be able to catch rotation speed changes and I am 99% sure that rapid RPM gauge spikes do not show real RPMs rises. From informations I found it seems that RPM gauge gets informations from CPS, that could mean that the new CPS does not work properly when it is hot.
As I already spent a lot of money on replacing good sensors I want to verify it before buying new one, especially that I want to buy Mopar sensor this time, it costs about $250 in my country. My idea is to take an oscilloscope, connect it to CPS and camshaft sensor and check whether both signals are in sync, that should tell me whether CPS (or maybe camshaft sensor) is ok, am I right?

 

[Tim_MN]

This webpage may be helpful. It has images of proper cam and cranks sensor signals.

https://web.archive.org/web/20200820085756/http://jeep.blackonyx.net/pdfs/jcss.pdf

On YouTube.com ScannerDanner is very informative.

 

[RCP Phx]

There is still an acceptable tolerance due all the timing components, see if your fsm covers that spec.

 

[vaco]

I solved the issue, the cause was brand new CPS made by Blueprint (budget brand of Bilstein). On 2-channel oscilloscope I checked waveforms from CPS and camshaft sensor, some pulses of CPS were much longer that others and in about 50% of times when longer pulses showed up engine was stalling. I replaced it with NTK sensor and everything works perfectly.
A tip: if you cannot lift your XJ (and even if you can) then disconnect shaft at front differential, you will get much easier access to the sensor. I've read a stories that people spent hours on CPS replacement because of poor access, if you remove the shaft you can do it in 20 minutes.

 

[Saudade]

Glad you fixed it.

NTK for the win (again).