JSJJ388

I put a new engine in my 94 ZJ out of a 98 XJ. Jeep ran fine until it exploded a piston. Pulled old motor, swapped sensors and dizzy, used the old manifolds and fuel rail. Jeep cranks right up every time, but, most of the time it skips and bogs down/backfires when given throttle. If I keep cranking and killing it, it eventually cranks and runs fine. It might crank right the first time or it might take 20 times. No apparent rhyme or reason.

When it cranks right it runs good until 2800-3000 RPMs. At that point it starts to skip and bog down. It also seems to bog down if Im on the interstate for ~15 minutes, but I can drive the backroads for hours with no issues.

Anyone have an issue similar to this? Any ideas on where to go from here?

JSJJ388

Also, I have replaced the CPS (got hung up and broke in the swap), TPS, IAC, and MAF sensors in an effort to solve the issue. One friend thinks I need a new computer.

4.3L XJ

Surely if it dies it will have at least one code

JSJJ388

negative ghost rider. It doesn't die, just runs like crap.

4.3L XJ

So how long since the O2 sensor was changed?

JSJJ388

put one on over the summer with hopes that was it. No dice.

4.3L XJ

Fuel pressure OK?

JSJJ388

This was the idle pressure. Didnt change when running good or bad. From what Ive read, thats high, but its definately got fuel pressure.

xjcountry97

Since all this started after the engine swap, and you have covered many sensors and checked the fuel pressure, I would double check all the grounds to make sure they are all there and tight. There should be several that attach to a stud where the oil dip stick bolts to the block. I believe the ECU and TCM ground here. There should be one from the firewall to the back of the head at the last head bolt. There should be one run from the battery negative to the passenger side of the engine block somewhere. I think this one bolts to the same stud at the dip stick. Then there should be a smaller one run from the battery negative to the passenger inner fender. There may be one im missing, but I think that is it. Good Luck!

JSJJ388

I havent checked those........ Hadnt thought of it. Thanks

jernigan

I had a similar problem with my motor swap. I put a new crank sensor in that came with my jeep. Mine did give codes though, I think the codes were #1 injector fault, cam sensor, and another I cant remember. Anyway I started searching online found some info on cheap crank sensors, lots of people had the same problem. I put my old one in my new motor and that was it ran fine. All the info I found said to use oem quality crank sensors,yours is not giving codes but mine ran about the same way idled rough and as the rpm went up it ran worse.

dave1123

A little science here, CAUTION< SCIENCE CONTENT! The crank sensor is a Hall-Effect switch that uses a magnetic field to sense motion in it's proximity. This one reads the windows in the flexplate to detect the position of the engine crankshaft. It produces a MILLIVOLT signal that lets the PCM fire the spark plugs on time. The PCM controls spark advance by measuring the time between pulses. All this relies on that magnetic field. MOPAR sensors use a neodymium magnet whereas aftermarket ones generally don't.

Enough said!

4.3L XJ

Aha, Thanks Dave. So did you use the 98 flywheel/flexplate or the 94. To make it run properly you will need the 94 flexplate on the 94 bell housing, and make sure you use the 94 distributor too, timed properly

dave1123

Just had a brain fart! Is the 94 a Renix and are there any differences between them and a late model XJ? I'm thinking like spark advance or something like that.

JSJJ388

I had to replace the CPS once before and used an oreilly part, with good success. Wouldnt have hadto replace it again, but it was a casualty of the swap.


no, its not renix. All ZJs are after the renix years. And I used all of my sensors and my dizzy as wel as the fuel rail.