Need help - intermittent spark out of coil..

Greetings Everyone,

I've just finished swapping another 4.0 into my '91 XJ. My engine ran fine before but developed crank-walk.

Currently new engine is not starting. Sometimes it coughs.

Checked spark. It was there sometimes, sometimes not.

I put a spark plug in the coil wire. The spark is weak and white in color. The results were "spark-spark...............(nothing)................spark-spark............(nothing)............ Essentially two quick sparks with gaps of nothing in between.

New items include Crank Position Sensor & flex-plate.

Crank Position Sensor was from NAPA.

We compared the new flex-plate with the old one when they were both out. 4 sets of slots seemed to line up when we stacked them.

Kind of at a loss here. Can anyone make sense of this?

Thanks for your help.

I take it this is not a coil-on-plug set up.

Anything but a blue spark indicates weak coil output. This could be due to shorted windings inside the coil, or dirty connections, or a bad ignition module.

I would start with the connections.

That's correct Hypoid. Regular old-school coil to distributor to plugs.

I will go through the connections tomorrow and try another coil if I can get my hands on one. Maybe throw my old CPS in as well.

Since it ran fine before, I would think the ignition would be fine still.

Thanks for your thoughts.

It's easy enough to just check the coil flash with test light. If the coil trigger is there but the spark is erratic you could assume the coil is bad. But if the trigger is erratic then it's something else. When my coil went bad I parked the Jeep and then it never started again.

Dark green/orange is power into the coil, measure it while cranking. Voltage falls below around 9 volts while cranking and things get really iffy, should be above 10, above 11 is good.. May be as simple as slow crank-low battery.

As soon as the ASD relay closes the coil has constant power, it triggers on the ground side through the PCM.

If your CPS is missing a beat it usually makes for an erratic spark, not a weak spark.

Like mentioned it is easy to oil soak a connector, crush a wire or bend a pin in a connector when reinstalling a motor. The pins can come loose and when pushing the connector together a pin can back out and make iffy contact.

I always spray clean every connector before reconnecting and do a good visual with a flashlight. It can save trouble later. I use a quality contact cleaner and forget about dielectric grease, the contact cleaner has an anti corrosion agent already mixed in.

Hallelujah.

It runs.

Thanks everyone.

Turns out the brand new CPS from NAPA was garbage straight out of the box. It was the cause of the symptoms I described. I recommend avoiding their CPS and get one from Jeep if they still sell them. I will be purchasing an extra to keep handy.

Just to throw my two cents in ,i called Jeep to replace my CPS and they said that they do not manufacture them anymore. so I was stuck buying from Napa.... the engines running but still throws a light on but no codes? Except 12 and 55

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pezman65 said:

Just to throw my two cents in ,i called Jeep to replace my CPS and they said that they do not manufacture them anymore. so I was stuck buying from Napa.... the engines running but still throws a light on but no codes? Except 12 and 55

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Click to expand...

Well that's not cool.

Confirming Jeep/Chrysler no longer make the CPS.

Discontinued March of 2017.

Guess it's time to stock up at the junk yard. :/