Nuts and Bolts: 4.2L MPI Swap

    Nuts & Bolts Tech Advice

    I have a CJ-7 with a 258ci engine but recently got everything from a YJ to do a fuel-injection swap. I’ve researched extensively what it takes to swap a 4.0L head onto the 4.2L block, so I’m good there. My question is what else am I going to need? I have the head, intake, ECM, and the entire underhood wiring harness. Are there any more sensors I need in order for the MPI system to work correctly?

    Ian H.

    A couple of years ago I did a complete 4.0L swap into a CJ-7 for our sister publication Jp Magazine ( Despite covering an entire engine swap, there’s quite a bit of information in this article that you’ll be able to use for your head swap.

    Aside from what you’ve already grabbed, you’ll need to add a high-pressure fuel pump and regulator, 4.0L temp sensor, crank position sensor, all of the sensors that were on the intake manifold, the throttle body, O2 sensor, and a speed sensor. All but the speed sensor can be sourced from the donor vehicle. The fuel-injection system needs to know vehicle speed, but we imagine you also want to maintain the mechanical speedometer. Therefore you need to scavenge a speed sensor from a Renix-injected YJ, or an XJ. You can also find them in early fuel-injected Dodge Dakotas. The sensor is located on the transfer case and is what the speedometer cable hooks to, so it’s easy to find. Grab the electrical plug for the sensor at the same time. As far as we know these sensors are no longer available new, so a junkyard is your only option.

    Only a portion of the entire underhood harness will be used. For a clean installation you should pare the harness down to only what the EFI system needs. Get your hands on a factory service manual (we have found inexpensive scans of many manuals online) and start studying, then carefully start cutting what you don’t need out of the harness. This stage is intimidating to some people and takes time, but it’s not hard as long as you pay attention. As long as everything came from the same vehicle, all of the sensors should plug right back into your pared-down harness with the exception of the speed sensor plug, which you will need to splice into the harness (the color codes should match). The computer does not need to see oil pressure in order to run but does need to see water temperature in order to enter closed loop operation. It will also run without an O2 sensor, but we would recommend adding one to your exhaust system along with a better cat than what was originally on your CJ.

    One last critical piece of the puzzle is a crank position sensor. On factory MPI applications this sensor is mounted on the transmission and reads crank position via notches in the flywheel. Your CJ transmission and flywheel will not have this stuff, and while it’s possible to retrofit a later flywheel and add the mounting provisions for the sensor to older transmissions, many people have had mixed results. Therefore the easiest thing to do in your situation is to use a kit that moves the sensor to the harmonic balancer. Hesco ( makes a kit that includes everything you need. The entire conversion is quite a bit of work, but like many things, it’s totally worth it.