Last month we listed the parts required for a skilled enthusiast to swap a Gen III Hemi V-8 into a JK. This time we show highlights of the swap and discuss what to do in case the engine doesn’t fire or run.
4WS (Missouri 4Wheelers Supply) continues where we left off: All parts are in hand and the new powertrain (6.1L Hemi, 545RFE automatic, NVG231 transfer case) was assembled and prepped with a Hotwire Auto harness. 4WS has done dozens of these swaps, dating back to shortly after the JK was introduced in 2007. Company owner Kevin Wyas says that pulling the old powertrain and getting the new one in position is actually the easy part. Details such as exhaust routing and double-checking all electrical and plumbing connections are the time-consumers.
Motor mounts: There are two schools of thought on motor mounts. Some shops prefer to mount the Hemi low and rearward for improved center of gravity. This involves dimpling the firewall for engine clearance. In contrast, 4WS test-fits the powertrain so that the engine clears the firewall and the fan motor (by about half an inch); the transfer case also needs space at the gas tank. These fixed points determine motor mount location. The motor mounts are tacked into place and final-welded after clearances are verified.
Exhaust: 4WS developed its own tuned Hemi swap headers (PN JK-EH5761), which clear the OE suspension mounts. Another option is using 6.1L SRT8 headers and modifying their flanges to clear the UCA mounts. Patching in the stock exhaust is possible but ill advised; 4WS recommends a custom 3-inch exhaust to maximize the Hemi’s power.
Transmission mounting: Ideally, obtain the crossmember mount with the transmission (the 545RFE has different output housings depending on OE application). 4WS welds a piece of 1⁄4-inch plate to the JK’s crossmember and then drills the plate to align with the transmission mount.
Steering shaft relocation: The intermediate shaft is removed when the stock engine is pulled. It needs to be repositioned 1⁄4 to 3⁄8 inch outboard by grinding out its hole in the firewall and modifying/reclocking the mounting bracket.
Battery tray: The tray needs to be trimmed to clear the Hemi. 4WS begins by cutting along the line through the webbing on the tray’s bottom. With the Hemi in place, the tray is test-fit and fine-tuned.
Heater hoses: Trim the OE JK hoses to length to reach the Hemi’s ports.
Transmission cooler lines: Reroute, cut, and reflare as necessary.
A/C compressor: The OE JK unit can be modified to fit the Hemi. 4WS prefers to use one from a Grand Cherokee/Commander, which bolts up as-is on non-VVT Hemis.
Driveshafts: OE shafts can be retained on four-door JKs. Still, 4WS recommends upgrading to CV-/U-joint style shafts for additional clearance and because most Hemi-converted JKs also have suspension lifts.
Occasionally the swapped-in Hemi fires up on the first try. Often it doesn’t. Here is 4WS’s troubleshooting checklist.
• Get factory Hemi specs from a service manual or diagnostic software. You’ll need to know proper voltages and fluid pressures.
• Big-buck diagnostic equipment isn’t normally required, but cheap parts-store scanners don’t display enough data. Chrysler’s professional-market StarScan is ideal but not cost-effective for a single job. 4WS uses Auto Enginuity, a scan tool/software package that currently costs about $500. It adapts an existing laptop to the Jeep’s OBD-II port.
• Verify good wiring connections/pin contacts: (1) ground straps on both the driver and passenger sides; (2) the C100 firewall connector, where the Hotwire Auto harness plugs into the OE harness; (3) all plugs in the TIPM. Also, secure the main alternator wire as far away from the exhaust as possible.
• Won’t start: The starter needs both constant and switched power. If it starts by jumping across the two relays, the PCM programming is incorrect.
• ABS and other systems must pass internal checks before the PCM will allow the vehicle to start. A diagnostic tool will reveal errors that prevent start-up.
• A fuel-pressure check will isolate possible fuel-pump problems.
• The throttle might need to be calibrated. The factory service manual outlines the procedure.
• 4WS rarely sees PCM problems (other than the fact that trouble codes need to be checked and cleared). TIPM failures aren’t uncommon. 4WS removes this module and stores it in a dust-free environment during the swap. Even then, the OE TIPM might not recognize the Hemi and will need to be replaced by a new module.
• The PCM flash must maintain at least 200 psi of line pressure to the 545RFE automatic. Otherwise the Reverse clutches can fail from insufficient fluid pressure. Low fluid level can also cause transmission failure.
• Other common transmission issues can be traced to check-valve problems or loops in the cooler lines, which trap air.
• Not knowing the Hemi and transmission’s histories can create issues. For example, failure to fire could be caused by plugged injectors. Transmissions stored outdoors can easily become contaminated.
• 4WS spends about a week test-driving each new Hemi swap. Several start/stop cycles are required to throw certain trouble codes.
The Bottom Line
This is a broad overview of the process. The bottom line is that JK owners who have more time than money can save thousands by doing their own swaps and sourcing the necessary parts a la carte using the list in the Mar. ’13 issue. Those who don’t like scavenger hunts can buy packaged swap kits from 4WS, AEV, Burnsville, and others.
For enthusiasts who want all-new everything, Mopar will offer ’07-’11 JK Hemi swap kits beginning in summer 2013. A 375hp 5.7L package will have an MSRP of $15,599; the 470hp 6.4L/W5A580 five-speed automatic kit will carry an $18,599 suggested price.