Unanswered 4.0 swap questions…

Jeffncs

Jeffncs

Well-Known Member
Joined
Apr 4, 2017
Location
Wake Forest
I’ve been reading and reading about swapping a 4.0 from a ‘98 Cherokee into a 82 CJ7. Planning to use the 4.0 engine harness, PCM, and EFI.

The CJ7 has a stock 4.2 and a new, aftermarket wiring harness from Centech.

I can’t seem to find answers to the following questions.

1. Can I get away with using the minimum required wires from my harness to run the EFI / CPU / engine? I’m trying to avoid splicing and cutting the XJ harness if I can avoid using it.

2. The earlier 4.0 engines require a fuel return. however, the later engines supposedly don’t. Am I correct in saying that the 98 does NOT require a fuel return line? That seems to be what I’ve read.

Opinion question - 4.0 has ~200k on it and leaking oil. Should I just reseal it and drop it in? Or, should I just wait until the winter and get it rebuilt?


I’ll have more questions as i
 
I put a 1998 4.0 and AX15 in my 258 equipped 81 CJ. First, read all you can on jeepforum.com.

Second, you don't need a return line, there is a specific fuel filter/regulator used.

Third, I'll have to find it, but there is a good 3 part YouTube vid explaining the wiring and the 5 wires to fire. Take the entire XJ harness and find 5 wires, splice them into the specific CJ wires, and you're good. Helped me greatly.
 
The optional question... I'd just do seals and gaskets and run it. If it knocks or smokes it's not a horrible job to yoink it out in the off season. Yank an engine from a CJ one time and the next time you can do it in a few hours.
 
Do you have the engine? I've got a 7120 head and wiring harness from an early 90s XJ I'm not doing anything with. I even have a box of water soluble packing peanuts to do the head work.
 
@orange150

Thanks. Actually - The engine is sitting on a stand in my garage as I type this. I got the full long block, complete harness, intake/ exhaust and EFI parts.

Aside from a CPS kit from Hesco and the fuel pump and filter, I’m ready to go. Only remaining question is whether I pull a running 4.2 with 190k to install a running 4.0 with 200k.
 
Jeffncs said:
@orange150

Thanks. Actually - The engine is sitting on a stand in my garage as I type this. I got the full long block, complete harness, intake/ exhaust and EFI parts.

Aside from a CPS kit from Hesco and the fuel pump and filter, I’m ready to go. Only remaining question is whether I pull a running 4.2 with 190k to install a running 4.0 with 200k.
Click to expand...

skyhighZJ said:
The only real question is, do you pull the running 4.2 and mash it up with the 4.0 to make a F.I. Stroker?!!!
Click to expand...

Or put the 4.0 in pull the 4.2 find a 4.0 core build stroker then swap it in....
 
If/when far into the future my ‘04 4pointslow dies, it will be resurrected as a stroker with diesel like torque.
 
I, honestly, think that my 4.2 is maxing out at 75hp…likely less. Even a stock 4.0 would feel like a rocket ship!
 
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Jeffncs said:
I, honestly, think that my 4.2 is maxing out at 75hp…likely less
Click to expand...
These later 4.0s are fairly anemic too. The older 4.0 and especially HO were much stronger IMO. Not sure what the differences are internally or if tuning would wake em up.
 
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