That Jeep Guy XJMJ

So ive spent the last couple weeks with the 4.0 out of my 98 XJ (slow progress). Originally the torque converter seal went bad so while the engine was out i replaced a few things. All new gaskets and seals throughout the motor and new torque converter. The flex plate bolt holes were a little egg-d out so i replaced that as well.

Now i have everything back together and went to crank but no fire... Alass-crank sensor was bad. Replaced it with new part from advance auto and now the jeep runs.... but very poorly! spark knocking when i apply throttle, RPMs are bouncing around - its weird. Its in no way drive-able the way it runs, I idled the thing out of my garage for the time being.

Now with the scanner on the jeep it reads an issue with cps but what do you guys think... I'm curious to know how often these flexplate's are faulty from factory (window timing off or etc). Im going to likely try another crank sensor (or 2) before separating the engine and trans again but other than that gimme some options.

OldTires

I don't think the flexplate would give you those symptoms.

I sounds more like an ignition issue. I would recheck all the plugs and wires, the cam sensor.

Check fuel pressure too, including the injectors.

Also make sure your grounds are clean and making good contact.

After that, I would go back to the CPS. Maybe try a MOPAR from the dealer.

97grand4.0

New CPS from AA is your problem. MOPAR ONLY on these. Well documented on this forum. And I speak from personal experience doing exactly the same thing. Magnets are not as strong on the aftermarket stuff.
Same thing with the cam sensor.
ASSUMING of course that the new flex plate is EXACTLY the right one, because if not, ..well...the location of the steel pieces on the flex plate determine when the CPS will pick up a signal. So much the same thing might happen.

Turbo X_J

Replaced it with what?

That Jeep Guy XJMJ

sucked it up and pulled the engine back out and installed the old flex plate back on the motor. Re-installed the engine and it fired right up!!

the way the windows were stamped on the new flex plate kinda Dimpled them in. On top of that the windows themselves were layed lower than ones on the original flexplate. Both of these issues were causing a gap issue for the sensor giving me the problems with running rough...

Clayzer

Im dealing with this same issue. When you say stamped lower, which direction? Lower towards the ring gear or away. I wonder if I shimming my CPS will fix the issue.

That Jeep Guy XJMJ

the windows are stamped from outside-in. so the windows were dipping in towards the flywheel bolts. if i remember right (i really should have measured this part) i think the overall outside diameter of the new flex plate was smaller than the original - that would give the cps sensor a longer gap to try and read.

Jeep is still running great as is with the original flex plate fwiw

cruiser54

I'd hunt down a good used flexplate before using a "new" one.

Clayzer

Update:
Flexplate is good. Pulled the tranny again and double checked it with the OE one. Everything is the same.
CPS is good. Terminal 1 shows 5V and 0V when I rotate my flexplate. This means it is reading the windows in the flexplate. Terminal 2 has 12volts, and Terminal 3 has the 5V reference coming from the PCM
I got the jeep to run for a bit. Shut it all off to button up drive shafts and a few other things. Then it wouldn't start. Went back out a few hours later and it would start again however this time I got 2 codes. P0122 (TPS low voltage) and P1694 (fault/failure/short circuit of any 5 volts sensor). I have a hunch my TPS is bad. I have a few tests to run on it when I get back from work.