Questions about pulling an LS motor

[86LK]

I acquired a wrecked 2003 Trailblazer EXT a few months ago and am in the process of pulling the LM4 drivetrain and a couple of other items. I'll then sell the carcass to the salvage yard.
I haven't decided which vehicle I am going to drop it in, but I figure I'll probably use ECM/PCM, engine, transmission, and tbi. The timeline for rebuilding/installing the LM4 will probably be over the next 5 years as I get various things done.

The questions I have are:
1) what exactly to pull out, and
2) why to pull things a certain way

For example, every video I've found they remove the wiring harness, even thought they state that they will be re-using it. Why? If I plan to use the wiring harness wouldn't it just be easier to leave it attached to the engine and unmount cables from the body? are there certain parts of the underhood fuse box or the wiring harness I won't be using?

I've looked for other YouTube videos but haven't really found anything that answers these questions.

 

[Rt Jam]

Another option is stand alone controller. If you go with OEM, you will be force to any changes made by someone that knows how to do it and that's not usually free.

 

[81cutlass]

An AWD gmt360 v8 is probably the hardest LS donor. A 5.3 is a tight fit in a chassis that was honestly designed for an I6. I had a 5.3 rainier for over 100k and worked on it a lot so am pretty familiar with the chassis. TBSS also get a bad rap because they are so hard to work on.

The front diff is bolted to the oil pan so you will need to yank the spindles and CV axles, and probably have to pull the transfer case off the trans at the min to get the engine out. You can pull the rad, bumper, and rad support and it might go forward but the oil pan will hit the frame hard so I doubt it will twist up at the angle you need unless you jack the thing pretty far off the ground if you leave the Tcase installed. The Y pipe can't go out the bottom (GM dropped the engine onto the frame and then the body on the top) so you need to hack some of the frame off to get the exhaust off. The manifolds are OK if you want to go turbo but they won't clear so honestly just sawzall it all off.

In 2025 I wouldn't recommend saving the harness, especially on a AWD 360. The Tcase stuff is heavily integrated, the ECU sits in a poor spot for a G body and its a 03-07 P59 ECU based vehicle so CANBUS gages so you can't even get the harness to feed an OEM gauge cluster like you can on a 98-02 harness.

A holley ECU or an aftermarket swap harness from a reputable supplier like PSI conversions would get you miles ahead on wiring.

I reused about as much stock stuff on a 02 Yukon harness for my 2+2 swap back in 2015 before the advent of affordable aftermarket ECU's and expensive aftermarket harnesses were all that were available. It took a lot to cut down and is not as nice as an aftermarket harness. I reused some of the OEM fuse box and 3D printed a housing for it (not worth it today)

The 360 p59 ECU also doesn't have the IAC driver for mechanical throttle so if you are going mechanical throttle you need a different ECU. Cruise is easy if you keep DBW but you need to wire in your DBW pedal and a TAC box which won't come on a swap harness. Most swap harnesses are mechanical throttle anyway.

The LM4 also doesn't have a hole for a dipstick so you will need to add one. The 360 V8's run the dipstick through the pan. Everything else has the block drilled.

 

[Tomeal]

every video I've found they remove the wiring harness, even thought they state that they will be re-using it.

I feel removing the harness just protect it during removal. Would suck to hit it off something and break or tear the harness. Plus you will most likely need to depin some wires out of the harness.

are there certain parts of the underhood fuse box or the wiring harness I won't be using

In the end that will be up to you. Ive seen some youtubers take the engine side fuse box in its entirety and use it. They just run the 12v and keyed 12v to it and power it back up. That route is pretty tuff to make look clean in the new car.

Look at some of the ls build threads here. There are some really nice ls engine bays here.

 

[87National]

I acquired a wrecked 2003 Trailblazer EXT a few months ago and am in the process of pulling the LM4 drivetrain and a couple of other items. I'll then sell the carcass to the salvage yard.
I haven't decided which vehicle I am going to drop it in, but I figure I'll probably use ECM/PCM, engine, transmission, and tbi. The timeline for rebuilding/installing the LM4 will probably be over the next 5 years as I get various things done.

The questions I have are:
1) what exactly to pull out, and
2) why to pull things a certain way

For example, every video I've found they remove the wiring harness, even thought they state that they will be re-using it. Why? If I plan to use the wiring harness wouldn't it just be easier to leave it attached to the engine and unmount cables from the body? are there certain parts of the underhood fuse box or the wiring harness I won't be using?

I've looked for other YouTube videos but haven't really found anything that answers these questions.

I've always disconnected the engine harness under the fuse box and left everything connected.....its not really in the way when pulling the engine.

I would not use the underhood fuse box. I would advise you to follow the instructions on lt1swap.com (he also has a pretty good YT video).

 

[87National]

The front diff is bolted to the oil pan so you will need to yank the spindles and CV axles, and probably have to pull the transfer case off the trans at the min to get the engine out. You can pull the rad, bumper, and rad support and it might go forward but the oil pan will hit the frame hard so I doubt it will twist up at the angle you need unless you jack the thing pretty far off the ground if you leave the Tcase installed. The Y pipe can't go out the bottom (GM dropped the engine onto the frame and then the body on the top) so you need to hack some of the frame off to get the exhaust off. The manifolds are OK if you want to go turbo but they won't clear so honestly just sawzall it all off.

Might be easier to pull the body off of the frame.

In 2025 I wouldn't recommend saving the harness, especially on a AWD 360. The Tcase stuff is heavily integrated, the ECU sits in a poor spot for a G body and its a 03-07 P59 ECU based vehicle so CANBUS gages so you can't even get the harness to feed an OEM gauge cluster like you can on a 98-02 harness.

So with an 03+ ecu, you can't simply feed the tach and speedometer wire from the ECM to an electronic aftermarket tach/speedo and scale the output with HPtuners?

A holley ECU or an aftermarket swap harness from a reputable supplier like PSI conversions would get you miles ahead on wiring.

Not sure if this applies to the OP....but for someone on a budget assuming N/A and keeping the stock cam......reworking the stock harness and sending off your ECM for VATS removal and scaling the speedo/tach signal makes sense. You are talking $100 vs $2k.

 

[81cutlass]

Might be easier to pull the body off of the frame.



So with an 03+ ecu, you can't simply feed the tach and speedometer wire from the ECM to an electronic aftermarket tach/speedo and scale the output with HPtuners?




Not sure if this applies to the OP....but for someone on a budget assuming N/A and keeping the stock cam......reworking the stock harness and sending off your ECM for VATS removal and scaling the speedo/tach signal makes sense. You are talking $100 vs $2k.

03+ you can get tach but no oil pressure or coolant temp. I guess you can swap to a 98-02 oil pressure sensor and leave the OEM wires dangling and pull new wires to a 98 style coolant temp but yeah, its possible to get there but its more work.

There are a lot of added costs (IMO) to get a factory harness working than just the $100 HPT credits (assuming you have HPT). Otherwise its $150-$200 or so to get a tuned mail order ECU or buying HPT new and learning the software. I've got almost 15 years of HPT experience and its still a PITA to get everything working.

A decent fuse panel, conduit to trim down the harness, a wideband O2 (assuming you are going to modify it, you can bypass that if its a stock swap) connectors, probably 20-30hrs to trim it down sure adds up.

Or its $500+ for a good aftermarket swap harness. You can get one for less than half through fleabay but they are always wired wrong.

99 - 07 Gen 3 LS/Vortec (24X) Drive by cable 4L60E-75/4L80E Standalone harness

APPLICATION: 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007 Chevrolet, GMC, Cadillac and Hummer truck engines (DRIVE BY CABLE ONLY) w/ 4L60E-

www.psiconversion.com

Not saying its impossible, just having done it, I wouldn't HPT stock swap a car again with Term X being an option.

 

[86LK]

An AWD gmt360 v8 is probably the hardest LS donor. A 5.3 is a tight fit in a chassis that was honestly designed for an I6. I had a 5.3 rainier for over 100k and worked on it a lot so am pretty familiar with the chassis. TBSS also get a bad rap because they are so hard to work on.

well, it's not an SS so there is that, and it's not an AWD. as for the recipient, I'll probably be debating until the last second whether to throw it in my Gbody El Camino or the '67 Impala

The front diff is bolted to the oil pan so you will need to yank the spindles and CV axles, and probably have to pull the transfer case off the trans at the min to get the engine out. You can pull the rad, bumper, and rad support and it might go forward but the oil pan will hit the frame hard so I doubt it will twist up at the angle you need unless you jack the thing pretty far off the ground if you leave the Tcase installed. The Y pipe can't go out the bottom (GM dropped the engine onto the frame and then the body on the top) so you need to hack some of the frame off to get the exhaust off. The manifolds are OK if you want to go turbo but they won't clear so honestly just sawzall it all off.

In 2025 I wouldn't recommend saving the harness, especially on a AWD 360. The Tcase stuff is heavily integrated, the ECU sits in a poor spot for a G body and its a 03-07 P59 ECU based vehicle so CANBUS gages so you can't even get the harness to feed an OEM gauge cluster like you can on a 98-02 harness.

A holley ECU or an aftermarket swap harness from a reputable supplier like PSI conversions would get you miles ahead on wiring.

I reused about as much stock stuff on a 02 Yukon harness for my 2+2 swap back in 2015 before the advent of affordable aftermarket ECU's and expensive aftermarket harnesses were all that were available. It took a lot to cut down and is not as nice as an aftermarket harness. I reused some of the OEM fuse box and 3D printed a housing for it (not worth it today)

The 360 p59 ECU also doesn't have the IAC driver for mechanical throttle so if you are going mechanical throttle you need a different ECU. Cruise is easy if you keep DBW but you need to wire in your DBW pedal and a TAC box which won't come on a swap harness. Most swap harnesses are mechanical throttle anyway.

The LM4 also doesn't have a hole for a dipstick so you will need to add one. The 360 V8's run the dipstick through the pan. Everything else has the block drilled.

no dipstick on the LM4? sonuva.....! I never even looked!

 

[87National]

03+ you can get tach but no oil pressure or coolant temp. I guess you can swap to a 98-02 oil pressure sensor and leave the OEM wires dangling and pull new wires to a 98 style coolant temp but yeah, its possible to get there but its more work.

There are a lot of added costs (IMO) to get a factory harness working than just the $100 HPT credits (assuming you have HPT). Otherwise its $150-$200 or so to get a tuned mail order ECU or buying HPT new and learning the software. I've got almost 15 years of HPT experience and its still a PITA to get everything working.

A decent fuse panel, conduit to trim down the harness, a wideband O2 (assuming you are going to modify it, you can bypass that if its a stock swap) connectors, probably 20-30hrs to trim it down sure adds up.

Or its $500+ for a good aftermarket swap harness. You can get one for less than half through fleabay but they are always wired wrong.

99 - 07 Gen 3 LS/Vortec (24X) Drive by cable 4L60E-75/4L80E Standalone harness

APPLICATION: 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007 Chevrolet, GMC, Cadillac and Hummer truck engines (DRIVE BY CABLE ONLY) w/ 4L60E-

www.psiconversion.com

Not saying its impossible, just having done it, I wouldn't HPT stock swap a car again with Term X being an option.

Mail order tunes to remove VATS etc. can be purchased for $60-100. Fuse panel $20. I've never tallied up the hours to trim down a harness......first one was probably 10 -15 hours....it takes significantly less time after you've done one. It's time well spent to better familiarize yourself with wiring, sensors, etc. for future troubleshooting.

Holley certainly has its place......but, as an example, on the 87 c10 I paid $1200 for, I'll go with the $100 solution vs $2k.