Post Engine Replacement Problem Goose Hunt

This is my last effort before taking my xj to the shop and spending a bunch of money on diagnostic tests.

2001 xj 4.0 died so i swapped in a 98 4.0. put all new gaskets in the 98, ditched the distributor for the 01's coil pack, new plugs, new rebuilt injectors, adapted 01 o2 wiring for 98 exhaust manifold.

Now the jeep loses power randomly. rpm fluctuates very slightly, but power loss is very noticeable. It doesnt matter how fast im going. I have chased a bunch of vacuum lines and replaced some hoses and connectors and it still bogs down. No thrown codes. I replaced the TPS, cleaned throttle body and IAC with no luck. jeep runs good if i disconnect and reconnect the battery for about 50ish miles then starts to bog down randomly under load.

i am at a complete loss what it could be. It has to be a sensor right? maybe cps was installed wrong? bad injectors or plugs? computer thinking its not getting enough air or gas? or maybe too much? It doesnt smoke. no backfires.

engine does idle a little rough after engine gets hot.

today i started noticing a random thud when it is bogging hard. only heard it like 5 times on my 45min drive home. maybe thats motor mount or trans mount starting to go bad?

Any thoughts or ideas would be greatly appreciated!

Plugged cat?

I don't think its a plugged cat as there aren't any codes and the donor jeep ran great before the engine replacement.

I'm wondering if CPS indexing could be slightly off? But would that cause the random power loss to come and go randomly?

What is fuel pressure at the time it bogs?

Man, that sounds like a tough one! good luck!

Long shot, but speedometer is reading good? At one point my speedometer sensor was failing. Would run decent most of the time, but under load the speedo needle would bounce around and the engine would loose power. Turned out that the sensor was sending garbage speed data to the truck, which was kicking on the speed governor.

I had an issue with the giant connector near the trans dipstick having a loose connection. Cleaned, straightened (bent out to make more solid contact) the pins, added a zip-tie around the connectors to hold them together better and secured in a spot where it wasn't tight as a guitar string (thanks Chrysler) and problem solved.

https://www.youtube.com/shorts/2iglERDOvHU

^^^ Those are the ignition/fuel pump/ECU relays clicking away as I move it.

Speedo seems fine.

Today i decided to hook up obd2 while on my way to work. No check engine light but it was throwing p0300, p0301, p0303, & P0456. I will check the evap vacuum lines after work and see if that helps. I guess a P0456 can lead to misfires?

No, the 300 codes say there is a problem with the ignition system!

P0456 most likely cause is a leaky fuel cap. Second most likely is a faulty purge valve. As was stated, shouldn't be connected with the other issues.

The primary problem is always the first code that appears, which are the misfire codes...?

So turns out my cousins xj also has the same exact problem after he rebuilt his head... His is a 98 with a distributor vs my 01 with coil pack. both new spark plugs. we both replaced TPS and crankshaft sensor. he tried a new map sensor, we both cleaned idle air and throttle body. he replaced entire fuel pump. at this point i think it must be just a wiring issue or a short...

what were your symptoms of this?

what do you mean your 01 "died"
was it catastrophic mechanical engine failure?
or
Did it just quit running?

on a different subject ...how did you bolt down the 01 coil packs on a 98 head?

I know a guy .. one day his car would not start so he removed the engine.
I asked him
"now that the engine is out, how are you going to figure out
why it would not start??

Jude Cochran said:

I don't think its a plugged cat as there aren't any codes and the donor jeep ran great before the engine replacement.

I'm wondering if CPS indexing could be slightly off? But would that cause the random power loss to come and go randomly?

Click to expand...

I’ve had a plugged cat with no codes

Feel the floor behind the passenger seat and see how hot it gets

You might want to look/test the engine wiring harness that's where it all started with my 2k!

Head Cracked, leaked coolant into engine, didn't know it, kept driving it, engine seized, engine started back up, piston 3 go BOOM. Metal EVERYWHERE!

Coil back bolted on with custom brackets made out of a metal ruler lol.

I dont think its a clogged cat. whole exhaust is maybe 3 years old and my cousin cut his cat off and didnt notice any change either.