*Warning, this is a long post sharing my experience with my swap while highlighting things I wish I knew before getting into this. There are not many DIY posts on this, so I thought I would share some random insights in hopes that I would help someone else down the line.

Original Vehicle is a 2007 Jeep JK 2 door 6 speed. Engine knock started around 120k miles, I decided to replace it with a V8 instead of buying a new one. I have never done a swap before, but have done most of the work on my vehicle.

I wanted to have a shop do it but pricing was getting close to $30k so I decided to do it myself. I picked RPM Extreme for my kit because they are closer to me and in the same time zone for support. Their kit is supposed to be simpler than most also.

I decided on a Gen V 5.3 for better performance, direct injection, and the GM 8 speed, which gets some flak on the forums but they drive very well when dialed in. I got in over my head with the Gen V programming, see the programming section.

Powertrain Removal

I did the whole swap with the body on. If you can get access to a lift your life will be much easier, but it is doable with the body on. Engine removal is relatively straight forward. I had a hard time with the master cylinder and slave cylinder, so I just started cutting lines and yanking. There is little room on the firewall to get to this.

Most instructions will recommend dismounting the brake master cylinder and ABS module, but you do not need to if you are keeping the body on. This is common sense to some, but I didn’t catch it until I broke a couple connectors at the brake fluid reservoir.

I pulled the transfer case separate from the rest of the drivetrain. Clearance would have been a problem with the body on. Top bolts suck, but there are plenty of threads on TC removal.

Drop the crossmember so you can get more pitch with the drive train when pulling it.

Remove front radiator support and label all the electronic connectors.

Watch brake lines and power steering lines that cross over just above the tube of the front Swaybar, it’s easy to snag them as you pull the drive train.

Bag all bolts and label, don’t discard any.

I did most of this myself, I had a helper when actually pulling the drivetrain.
The New Powertrain

I bought my powertrain online from a company that modifies and resells stock LS engines, most of them very low mileage. I got the wrong engine the first time, so had to ship it back and wait three weeks for the new one.

Finally got the right powertrain and removed the stock harness. I installed the new RPM Extreme harness with the engine on jack stands. I followed a couple online videos to help identify where everything goes, I am not super familiar with the architecture of these engines.

I also mounted my Power Steering Pump adapter with the engine out of the vehicle. From what I understand the LT’s use an electronic steering system. I used my stock PS Pump with ICT bracket 551368. The included long bolts are too big for the holes on the stock PS Pump, do not drill them out! (Don’t ask)

Instead go to the store and buy some smaller diameter bolts and appropriate nuts. I used a 99.5” long serpentine belt with this power steering setup.

RPM Extreme instructions will indicate what radiator hoses you need, the instructions on the website are not updated.

I opted to go with the Radiator from RPM, which dropped in and made things easy, though it is certainly not a cheap setup. Only problem I had with it is a couple holes did not line up to mount the stock condenser, I just drilled new holes in the condenser bracket.

I used a JSS transmission cooler mount and a Hayden transmission cooler, that setup worked well as far as fitment. I just used self tapping screws for now, I couldn’t get the included rivnuts to cooperate.

I used the stock 8l90 transmission cooler lines and just hose clamped rubber transmission fluid hose to get to the cooler. Make sure you use transmission fuel hose for this.

I received the wrong AC lines from RPM at first, but after I got the right ones they installed well enough. Routing the lines underneath the engine and up to the condenser is tight, be patient. Blew ice cold after I had a local shop charge it for me, but I am still working out Idle RPM drop when AC is engaged.

I had to cut into my battery box a bit to reuse the stock power steering hose.

I went with the recommended engine mount location from RPM. Measure twice then weld. I can’t weld so I paid someone to do it for me. We tacked the brackets in place then dropped the engine in. We finished the welds after the clearances looked good.

Dropping the powertrain in was a nightmare. I suggest you remove the battery box and or the passenger side exhaust manifold if you are doing this with the body on. Clearances are tight and we dropped the whole drivetrain, including the transfer case, all at once.

I used an engine hoist from the front, and a transmission jack underneath the Jeep to easily change the pitch of the powertrain. Keep an eye on your harness and go slow, it is easy to pinch or tear a wire.

Most recommend a 1.75” body lift to add clearance. That is a must if you keep the engine covers on. I decided to remove mine and have just enough clearance for AC lines without the body lift. There is more clearance at the firewall than I thought there would be. Things are tight up at the radiator.





The transmission mount bolted to my stock crossmember, I did this while the crossmember was off the vehicle. Couple of the holes don’t line up perfect but all the bolts went through. Do not overtighten the bolts going all the way through the crossmember, mine started to collapse a bit. RPM and I believe Motech no longer include internal spacers.

Budget for custom drive shafts.

RPM included a nice enough fuel line, I used barracuda hose for the fuel vapor line. I got creative with the stock vapor lines to get them attached. The Fuel Line clips right in place. I put a braided jacket on both to protect from heat and abrasion. I also tried to route them high on top of the transmission, it was easy to avoid the exhaust with a two door. Be more careful with a four door.

I paid a local shop to do a full custom exhaust, I was charged around $1400 for cats and all, aluminized steel. This was in Fort Wayne

 

Programming/Electronics

This has been the biggest headache for me, by far. The 8l90 transmission is controlled by an external module, unlike the 6l80.

From 2014 to 2016 GM used a T87 TCM. This module can be tuned via HPTuners without any special procedure.

From 2017 on GM switched to a T87A TCM. This module must be unlocked before you can tune it. I never even tried the module in stock form, I sent it straight to HPTuners for the unlock service. My powertrain is a 2018 L83 with 8l90 from a Silverado.

When I received my TCM back from HPTuners I could not get it to respond to any range selection, it was like it was in limp mode. After emailing with RPM they made it clear that this has been a common problem with these modules, so they, as well as other forum posts, suggested that I try an older T87 in place of the T87A.

The first one I ordered was from a Diesel. That did not work at all so I swapped it out for a Cadillac car TCM. That one will shift but shifts hard to second and won’t go into fourth gear or beyond.

The second one I ordered was mistakenly a T87A from a 2017 Escalade. This module shifted the best, but still had some problems down shifting and harsh shifting 2-3 and 4-5. I decided to take that module to the dealer and have them load the stock OS and solenoid calibration data to match the transmission I was running. When that was done, that module would no longer go into gear! We had suspected that the HPTuner unlock service somehow threw off the stock module, but this makes me doubt that.

So I bought another T87 from a truck, and that module slips and will not shift right at all.

That is where I sit now regarding the TCM. I am trying to resolve it with RPM Extreme.

The only other problem I am fighting is that sometimes the Jeep runs rich. I have randomly gotten open injector 7 circuit codes, running rich codes, and misfire on cylinder 7 codes. I am still trying to resolve this, high grade fuel and cleaner makes it go away for a while, I fear there may be an injector problem.
I will update as I try to resolve the TCM problem, feel free to reach out with questions. Sorry for the lack of good pictures, it was hard to document and do this at the same time. Things will get cleaned up as I wrap it up.

 

Thank you for taking the time to write this all up. I always appreciate the willingness to help others that come behind when you get no real benefit yourself except for the satisfaction of being helpful.

 

I feel your frustration. I am shipping mine to Motech in about 6 weeks. From watching their videos it appears they do a pretty good job of testing before sending it back. But definitely on the pricy side. I hope you solve your issues soon.

 

You will love it! I will admit that these persistent headaches have soured my experience a bit, but that short time that I was able to drive it for real were the best I had in the Jeep. You realize pretty quickly how bad the stock powertrain is.

 

Tilly, I have the same setup but a matched set of 2016 Silverado and 8L90. Mine shifted perfect right off the bat but my harness was the stock Chev one modified by Motech. Only tranny problem I had was I could not get manual shifting to work and that was simply some codes to turn off in HPT on the communication system inside the Chev ECU. But the dash display will only go to D6 then it still shifts 7/8 manually then if you go one more it defaults to D Auto. I used the 2012/later shifter. Did you ensure each gear position was verifed by EG put in each gear individually then remove the shifter cable to ensure it was still in the transmission indent. I did this for each of the 5 positions. I have little things wrong and will be resetting the ECU modules with wiTech soon then checking grounds but engine and tranny shift and run perfect. Whose fan are you using? I run the Camaro SS from 2017 car. And you are correct - the engine tranny combo is almost perfect and makes the Jeep a true driving package like Chev designed it for a Jeep.

 

Their is one person in the US that can fix your trans issues he's the engineer that wrote the code for these trans.
I suggest you contact him, he was the only one that got my Tap shift to work..

Jim...

Performance 4L80e 6L80E 6L90E Transmissions, Transmission controllers

 

You are lucky with the 2016. Pre-2017 seems to be much more friendly to aftermarket tuning/applications. Tap shift worked fine on most of the TCM's I have used. I am using a pre 2011 shifter, the tap shift is integrated into the cruise stalk.

I am using the Pentastar PWM fan with RPM harness. It has kept the Jeep cool so far, but we will see how it performs when I finally get it on the trails. .

 

Thanks caryt! I will have to give them a shout if I can't resolve this.

 

Long overdue update. The Jeep has been driving fantastic over the past two years/30K miles.

TCM Resolution:
I took the aforementioned T87A computer from the Escalade and sent it off to James at customecm.com to have it reprogrammed to its stock OS. I then broke down and purchased a GM MDI tool and struggled my way through the factory programming services. Using GM Techline and James' VIN program application, I loaded the correct solenoid data from the transmission in the Jeep (Silverado 1500) into the TCM from the Escalade. The Jeep still shifted rough here and there but over the past two years it has smoothed out. I changed over to Amsoil Signature fluid early on. At this point I am convinced I could have left the Escalade T87A with its stock solenoid data and achieved comparable shift results. There was not a lot of end user information on the 8l90 out there when I did the swap and I am too burnt out from it to research it more now!

I went back and forth with several tuners to dial in everything and landed on something I'm fairly happy with on the engine side of things. It would run a little rich before. I also had some intermittent misfire issues. Found out my air filter had fallen off and I haven't had the problem since. I also switched to running strictly Shell 89 mid grade. I experienced better efficiency and less ping. I adjusted shift points to hold onto gears a little longer, it really kept RPM's too low on the highway for my 3.21 gears.

My custom aluminum radiator started leaking pretty quick. I pulled it out, had it repaired under the warranty, and put my stock radiator in place. I had to use some adapters/fittings to make this work with the coolant vent tube. I'll try to circle back with pics and part numbers. I am still running the stock radiator and will likely keep it in unless I notice any cooling problems. It currently holds steady at 210 even on the hottest of days here in the midwest.