View Full Version : ECM Not Reading CPS

March 26th, 2015, 08:58
Keep Getting : P1391- Intermittent Loss of CMP or CKP.
Tried 3 Different Crank Pos. Sensors. Get Same Reading.
Engine Runs ROUGH, MisFires & Stalls at Idle.
Also Shifts Erratically & won't Hold OD.
Tried Clearing Codes & Resetting ECM Memory.
Still getting Same Code.

To Try & Cure Misfire & Rough Idle, I Have Replaced:
1) New Distributor w/ New Cam Pos. Sensor plate, Cap & Rotor.
2) New Copper Spark Plugs & 7mm Plug Wires.
3) New CPS, MAP & TPS Sensors. New Idle Control Unit.
4) New Coolant & Manifold Temp. Sensors & New T-Stat.
5) All New 4 Hole Injectors w/ New O-Rings.
6) New Neutral Safety Switch (NSS)
7) New Trans Shift Solenoids, Gasket, Filter & Fluid.
8) New Fuel Pump, Sock & Filter.
9) Antifreeze, Oil & Filter Changed Too.

After All This, Looks Like the Computer (ECM) is Failing.
Any Clues as to, How to Check or FIX the ECM?
Anything I'm Missing Here? Or, Should I just Replace the ECM?
HELP!!! This thing is Costing me a Fortune & Lots of Time.

March 26th, 2015, 10:02
You probably need to re-index the distributor. Search for the tooth-pick method.

Sidewinder CC
March 26th, 2015, 10:28
You probably need to re-index the distributor. Search for the tooth-pick method.

I think he has a '96 XJ. A tooth pick won't help him much.

March 26th, 2015, 10:30
Before I spend big money I always check the connectors and wiring. Less likely on the Chrysler years XJ, but the CPS signal is tiny (same with the TPS and other sensors) and it really doesn't take much to degrade it.

I've found the tiny pins in a connector pushed out the back of a connector. Sometimes you need a good light or even a magnifying glass. I've found where oil or coolant has seeped into a connector, the connectors may be moisture resistant, but they sure aren't moisture proof.

Lastly the 5 or 7 volt supply voltage for the CPS (depends on the year, usually 5 volts) can partially short through one sensor and affect another sensor on the same branch. Multiple sensors share the same supply voltage source (again it depends on the year, which sensors are on the same 5 volt branch).

March 26th, 2015, 10:48
I have a 96 and the tooth-pick method applies to it as well. You put it in the hole beside the trailing edge of the half-moon under the cam sensor and then in the hole in the base of the dizzy. Make sure engine is set to 0*tdc at end of compression stroke via spinning crank bolt clockwise. Make sure slot in oil pump is oriented at 11/5 o'clock from passenger side perspective. Put rotor on and face it to 1 o'clock then drop it in and the whole dizzy will spin to 5 o'clock when finished and the trailing edge of the rotor will be just past the #1 terminal. Remove rotor, toothpick, put in cam sensor, rotor, cap, wires. . .

P.S. You can use a nail or small, round driver to lock the two holes--it doesn't have to be a toothpick.

Sidewinder CC
March 26th, 2015, 12:13
The normal procedure is to use a 3/16" pin punch or drill bit (see pic).



March 27th, 2015, 07:21
Re-Indexed the Distributor (as per info found Here)
Runs a Lot Better Now & No Codes.
Still Not Perfect. Idles Low & Still Misfires a Little on acceleration.
At least now it's Drivable. Will Play with it more this Weekend.
Gonna Pull the Valve Cover & Verify #1 TDC on Compression Stroke.
Then I'll Re-Set the Distributor & See Where it Runs Best.
Might Need to be 1 Tooth Off Advanced (Might Not)

March 27th, 2015, 12:44
You can verify that you are 0* tdc at the end of the compression stroke by taking out #1 sparkplug and have someone spin the crank to 0* while your finger is on the hole. Air will push past your finger if it's the end of the compression stroke.

March 28th, 2015, 15:44
Sidewinder CC,
Thanx for the Pics. Very Descriptive & Helpful.
Especially Like the Cut-Off Cap.
Makes it Easier to Understand the Correct Positioning of the Rotor.
In Process of Doing it Now.
Just Now Looked at your Pics for Reference.
Thanx Again,

March 28th, 2015, 18:07
Got it DONE. Runs Much Better (So Far) :clap:
Only took One Try (Thanks to Y'all) :worship:

Will Know for Sure when I go into Town tomorrow.
Need to get on the Paved Road & Up to Highway Speed to be Sure.
Also Need to Fix a Small Exhaust Leak in the AM.
Time for a Drink of Sweet Potato Wine & a Movie. :party:

April 2nd, 2015, 12:26
Re-Indexed Distributor & Had to Set it Lined Up Directly In Line w/ #1 Wire on Cap, to get Code to Go Away. Seems to be Running a little Rich.

Now a New Problem has Popped Up: A Major Exhaust Manifold Leak.
Black Soot is Plainly Visible on the Intake Manifold adjacent to the # 4 & 5 Cylinder Exhaust Flanges.
Misfiring & Stumbling is Getting BAD. I'm thinking that if the Exhaust is Leaking, then the Intake Is Also Leaking.

Tried Re-Torquing the Manifold Bolts (3 Were a Little Loose).
Didn't Help at all. Now I'm worried that the Manifold is Warped.
From Where the Carbon Deposits Are At, it Looks Like the Welds may have Failed at the Exhaust Flange & Pipe joint.
May have to get it Re-Welded & Straightened too. Hopefully it's just a Blown Gasket.

Just Bought a New Gasket & will Disassemble & Find Out this weekend.

Anyone Wanna Buy a 2 Door 4WD '96 Jeep Cherokee?

Like the Saying Goes: JustEmptyEveryPocket!


April 5th, 2015, 09:55
Got the Header Re-Welded & Installed a New Manifold Gasket.
Also Cleaned the Throttle Body & Intake Manifold.
Runs a Lot Better & Quieter, But......
Now I Keep Getting a P0171 & P0172 Code Together???
Any Clues as to What would Cause This?
I'm Thinking One or Both O2 Sensors gone Bad?
Any Help Greatly Appreciated.

April 5th, 2015, 19:12
Check the tiny fuses in the pdc(underhood fuse box). You probably blew the o2 heater circuit.