YJ 2.5L to 4.0 Build thread

I intended to put something together about this project but really have just been struggling to make time for everything I have going on. I will keep it brief and just provide some details of what we are doing to Josh's jeep, what parts were used, and any complications.

Josh's jeep is a 1991 YJ with the 2.5L and AX-5 transmission. He had been planning the swap and started having transmission issues so we tore into it. The drivetrain was removed leaving the factory wiring harness in the vehicle. The donor 4.0 was removed from a 1996 Jeep Grand Cherokee. The donor motor was in good shape with no leaks from the rear main, valve cover, or oil pan. An AX-15 from a 1997 TJ was purchased already rebuilt from East Coast Offroad and coupled to the 4.0 with a external slave bellhousing. Josh's 231 T-case already had a SYE installed but I had to tear it down to swap input shafts. I was able to salvage the input from a T-case repair I had done on a 2001 Dodge Ram 1500 a few months ago.

The first issue I had was needing the transmission mounting adapter for the AX-15. I sourced a broken one (common issue) from Neil and repaired it. I then duplicated it out of some 1/4" steel plate. This worked out perfectly.

The original motor mount provisions were cut from the frame and things were cleaned up. I slid the entire drivetrain in and mounted it to the skid plate. This allowed me to locate and tack weld the provisions to the frame. I then pulled the drivetrain back out and fully welded in the motor mounts.

We had to source another fuel rail as the YJ uses a return system and the Grand Cherokee that the donor came from was a returnless setup.

I will add some picture back in shortly. They are on my phone.

Quick update. The drive train is back in, just need to get my hands on an AX-15 shifter assembly. I was able to pin the wires for the two injectors straight into the 4.0 computer using the original 2.5 wire harness. I had to grab some pins at a local yard as the 96 ZJ harness used a different size pin. I was also able to straighten out the fuel lines to make them fit. Its really tight though, so once we verify everything is running these may need to be sourced from a yard/ebay as well. I have the exhaust mostly mocked up just need to make one cut and weld. I used the 4.0 ZJ crossover pipe, ZJ cat, into the 2.5 YJ muffler and tailpipe. All of the parts were in pretty good shape so it will save Josh a few bucks to not need to buy new parts. I will hopefully finish the exhaust up tonight. I need to trim more off of the 2.5L tan shroud before I can permanently mount the grille shell and radiator. Then its just fluids and start it.

The exhaust is finished. Both shifters are installed, not sure why but the t-case shifter in 2wd just barely touches the tub so I will have to trim a little. It should still fit under the factory trim ring though. The front clip is back on with radiator hoses it. I had to extend the O2 sensor wires on the drivers side. The T-case, trans, and engine have all gotten fresh fluids. All it needs is a new power steering pressure line, heater hoses, and some coolant and it should fire up.

Quote from BBB_80

nice build. aside from the exhaust, have you finished installing the new power steering pressure line, coolant and other stuff needed for your jeep?

Yes. Everything is wrapped up and running/driving.

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