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Easiest way to a 4.0l swap?

4

420manche

NAXJA Forum User
Location
West pittston, pa
Ok all feel free to chime In with facts and opinions, here's the question? I have an 87 short bed with a tbi 2.5. Can I just swap the engine bay harness out of a 4.0 mj without having to do anything in cab behind without removing the dash
 
There's definitely enough room for a 4.0L, should be able to swap in engine harness.
 
sandsgg said:
Doesn't The 4.0 Have A Different Firewall
Click to expand...

Pre-87 XJ/MJ were not provisioned for the I6, so their firewall/bulkhead was not shaped correctly to fit it. Some assertiveness with a BFH solves that tho. 87+ XJ/MJ have the "larger" engine compartment with correct firewall/bulkhead.
 
the cab harness is the same so long as you stay in the same engine management range (Renix). I swapped an 87 4.0 into my 87 2.5 MJ using an 89 MJ underhood harness. The only thing I had to change was the HVAC blower motor because the connector on the 89 harness was different than the 87 motor (could have spliced the old connector into the harness if I wanted to). The pinouts on the C100 connector (opposite the fuse panel) are identical between 2.5 and 4.0


And, by using an 89 harness, I eliminated the C101 connector.
Jeff
 
IIRC the connector for the ECU under the dash is on a branch of the engine bay harness, it goes through the firewall at the big rubber boot near the back of the intake manifold.

And yeah, you're going to need to swap the radiator and some other stuff plus the crossmember and driveshafts iirc but you were probably going to do some of that anyways.
 
I wanna say you'll need to change out the fuel pump and sending unit too..... including the wiring. IIRC, the fuel gauge sending unit for the 4 cyl is a 2-wire unit, the 4.0 is a 3-wire unit.
 
Depends on the donor for that. And yeah, a TBI 2.5 from the 80s is around 14-15psi and a 4.0 is going to want high 30s through high 40s or so depending on year (I always forget which is which) so you'll need to switch that.

carbed sender/pump units are 2 wire (no fuel pump feed, just ground+signal), TBI and MPFI ones are 3 wire. 91-92 senders have a resistance curve that runs opposite 90-down ones, so your gauge will go backwards, and it's not solvable by simply switching wires or anything. Some people have swapped just the fuel gauge in the cluster with a little custom wiring and gotten it working.
 
I'm running an HO pump in mine....gauge reads backwards and about 1/4 tank off. Not a daily driver for me, so it's no big deal, just have to remember when it reads "F" I'm just about out of gas. LOL.

Jeff
 
Yeah I have a RENIX sender in my HO... I have to remember it reads from Excellent! to FAAAAAAAAAAAWK! instead of Empty to Full. LOL

And yeah, it's not exactly linear in mine either. 1/2 actual tank still reads E, 1/4 tank is around 1/2 tank on the gauge, and by the time I hit 1/8 to running on fumes, it's somewhere near 7/8 of a tank on the gauge.
 
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