transient firing abnormality . . .

[Bruski]

After driving my '87 4.0 4WD comanche for a period of time

(usually about 20-30 mintues) it begins skipping a beat (hiccups quickly,

but momentarily)

but always stays running. This transient abnormality will occur maybe every couple

of minutes or so. No consistent pattern except that it runs fine for awhile and then

develops the 'miss'. I checked plug wire connections, etc . . . the usually.

It more like an ignition miss (on all cylinders) rather than just one,

as opposed to dirty fuel filter. I suspect heat has some influence, mean after the

vehicle runs warm for a while. Everything else normal in the way operating guage

ranges. Anyone have similar experience or my have potential fixs?

Bruski

[jimoshel]

Almost has to be heat related. Take a hair dryer, or any heat producing source, NO OPEN FLAME, and concentrate it on all ignition related parts, ECU, Ign Coil, etc. Or just start swapping parts. Darn intermittent has to be the most difficult problem to solve. Good Luck.

[airspeed]

When the engine is warm and running take a spray bottle of water and spray the plug wires. Cross fire on the plug wires makes it seem like the whole engine is dying.

[Eagle]

How old is the CPS (crankshaft position sensor)?

[cruiser54]

Test your CPS per the following instructions. Even test it when cold and post up your results. Better to test hot though.

 

Renix CPS Testing and Adjusting

 

 

 

Renix CPSs have to put out a strong enough signal to the ECU so that it will provide spark.

 

Most tests for the CPS suggest checking it for an ohms value. This is unreliable and can cause some wasted time and aggravation in your diagnosis of a no-start issue as the CPS will test good when in fact it is bad.

 

The problem with the ohms test is you can have the correct amount of resistance through the CPS but it isn’t generating enough voltage to trigger the ECU to provide spark.

 

Unplug the harness connector from the CPS. Using your voltmeter set on AC volts and probing both wires in the connector going to the CPS, crank the engine over. It won’t start with the CPS disconnected.

 

You should get a reading of .5 AC volts.

 

If you are down in the .35 AC volts range or lower on your meter reading, you can have intermittent crank/no-start conditions from your Renix Jeep. Some NEW CPSs (from the big box parts stores) have registered only .2 AC volts while reading the proper resistance!! That’s a definite no-start condition. Best to buy your CPS from Napa or the dealer.

 

Sometimes on a manual transmission equipped Renix Jeep there is an accumulation of debris on the tip of the CPS. It’s worn off clutch material and since the CPS is a magnet, the metal sticks to the tip of the CPS causing a reduced voltage signal. You MAY get by with cleaning the tip of the CPS off.

 

A little trick for increasing the output of your CPS is to drill out it’s mounting holes with the first drill bit that just won’t fit through the original holes. Then, when mounting it, hold the CPS down as close to the flywheel as you can while tightening the bolts.

 

 

Revised 11-29-2011