1989 Jeep Comanche hesitation backfires rpm between 2500-3000

I have a 1989 Jeep Comanche 4.0l 5sp. One day it hesitated for a second and turned off. I couldn't turn it back on, I towed it home. The next day it started drove around the block and it did the same thing.

I changed the distributor and it would not run. I changed the CPS and now it starts right away and idles normally.

The problem is when rpm's are ~2500-3000 it backfires, hesitates and won't go past 3000 rpm.

I've had the computer checked for codes, no codes were found, the MAP, has been replaced, and the voltage across the TPS is correct.

Does anyone know what this might be? Or have any links information for this.

Renix (87-90) HAVE NO CODES--strictly real time data, you need a DRB dealer tool or a Snap-On MT2500 "brick" with the correct Jeep cable and cartridge to get that real-time data. That is the bad news.

The good news: most Renix testing can be done with two multimeters--one digital and one analog. You need the analog meter to test the TPS through its full range of travel and also to test the synch sensor in the distributor. Every other sensor test can be done with a digital meter.

Ok. Pull your distributor cap and check the shaft. A little up-down movement is Ok and normal. Any sideways movement/wobble of the shaft and you need to replace the distributor.

Now, backfiring means there is too much o2 in the exhaust and unburned HC are being ignited in the exhaust. This could be an issue with the synch sensor in the distributor grounding against the wobbly distributor shaft.

Good luck.

Is that distributor indexed correctly? Or could it be off a tooth or two?

joe_peters said:

you need a DRB dealer tool or a Snap-On MT2500 "brick" with the correct Jeep cable and cartridge to get that real-time data.

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Are the DRB dealer tool still a common item for most dealerships to still have?

Yes, they are still using the DRB, up to version III or IV now.

I would try re-indexing the distributor, you could be just one tooth off in either direction, your symptoms and the work you did points in that direction.

joe_peters said:

Yes, they are still using the DRB, up to version III or IV now.

Click to expand...

Interesting - I'm thinking of taking my MJ and seeing if they'll scan it and give me a report.

Bingo!!!! That was one of the failures in my old distributor, I spent 2 weeks debugging. It was wobbling and grounding!!!! But the sheer pin finally sheered that holds the helical gear in place. New distributor and the back fire at 3000 rpm is back, but the other problems are gone. The synch sensor was loose!! Shaft had no wobble.

joe_peters said:

You need the analog meter to test the TPS through its full range of travel and also to test the synch sensor in the distributor.

Ok. Pull your distributor cap and check the shaft. A little up-down movement is Ok and normal. Any sideways movement/wobble of the shaft and you need to replace the distributor.

Now, backfiring means there is too much o2 in the exhaust and unburned HC are being ignited in the exhaust. This could be an issue with the synch sensor in the distributor grounding against the wobbly distributor shaft.

Good luck.

Click to expand...