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I recently joined this forum as I have started the stressful task of converting my 1991 V3500 Crew Cab over to the LS platform!
The truck had a Gen V 454 that developed a rod knock after about 262k miles. I contemplated Gen VI 454 and then changed my mind and went with a 6.0 LS out of a 2004 GMC Yukon. I would like to swap to L92 heads and an LS3 intake, but also having to swap accessory drives and the like is just getting to expensive at the moment.
My truck is lifted 8" and runs on 35's so torque is a concern. I have 4.10 gears and the factory 4L80e, so I may go to 4.56 gears in the future since the LS is more high strung.
All of the little details are starting to make me want to pull my hair out!
couple of questions I have...sounds like there are no conversion headers for square-body 4x4s?! Is this true? and where is the best place to get full wiring diagrams for the 2004 GMC Yukon I got the LS engine from?
My 1991 V3500 came from the factory (only year for the 3rd Generation trucks) with a 4L80e and a NP205 transfer case with VSS.
My 4L80e does not have the top center bellhousing bolt that was added in the LS block and my 2004 LS block does not have the upper, passenger's side bolt hole! It has the pad that looks like there should be a drilled and tapped hole though.
My question is...can I just drill and tap that hole to get my missing transmission mount?
Everyone is saying it is okay, but they are street cars. I am a lifted 4x4 truck and I do go off-road! In stock configuration my 454 with the 4L80e had the aluminum convertor cover with the struts to the engine mounts, but I am worried these extra plates that go under the engine mounts won't work on the LS, plus with the adapter plates I would now have adapter plate, strut mount plate, then engine mount.
Whatever you do, DO NOT drill and tap that extra hole in your block, you WILL hit a water jacket! Just run it with the bolts you can get in, you'll be fine.
Cool swap dude especially with your truck already having a 4l80e and VSS equipped NP205. I have an 84 K5 that I'm gathering parts to do an LS swap on so I love seeing other square body swaps. To answer your original question, there are a couple of companies like Schoenfeld and Doug Thorley (links below) that make swap headers for the 4x4 square bodies but a lot of guys are also using f-body headers with no clearance issues either. https://www.summitracing.com/parts/sch-1302ls1https://www.summitracing.com/parts/dte-323y-c
A couple of things to consider/keep in mind with your swap:
-Like LS1Nova said, DO NOT drill and tap the top bolt hole for the bell housing. Lots of guys run older tranny's with LS swaps and leave that bolt out with no issues.
-Don't worry about the struts that go from the dust cover to the engine stands. You'll probably need to use a newer dust cover anyways since the bolt patterns for it are most likely different and the newer covers don't come with the strut mounts. At least I've never seen them used on any of the 6.0/4l80e swaps I've looked at... If you're at all concerned about keeping the engine in place, take a look at the LS swap engine mounts made by DIY4X, ORD, or TNA they're way stronger than the stock clam shell mounts and are used in a lot of off road rigs.
-I personally wouldn't go with the L92 top-end. The rectangle port (L92/LS3) vs cathedral port (LQ4/LS2) debate is something that comes up a lot on this forum but the general consensus is that if you're looking for low end torque the cathedral heads are the way to go since most guys lose a good bit on their bottom end with the rectangular port heads but trade it off for better breathing in the top end.
-You didn't say which 6.0 you have but assuming it's the LQ4 (most common) I would keep your stock 317 heads and have them milled to bump up the compression or if you have the money, send them to one of the sponsors like Texas Speed or Advanced Induction to be ported and milled.
-You'll likely want to replace your torque converter since the power band of a 6.0 LS will be vastly different from the BBC that you currently have. If you aren't swapping the cam shaft or are only doing a small one, then you should be fine getting a stock replacement from a newer vehicle. If you plan on doing a cam shaft at any point then it would be good to figure that out now so you can buy a converter that'll match the cam's specs. If you want to do a cam, take a look at Brian Tooley Racing and Cam Motion they both have "drop-in" cam shafts that are made for the LS trucks. Or an even better option is to call them (or some of our other sponsors) and have them grind you a cam shaft that's custom made for your application.
-You may want to check out Colorado K5 and 67-72chevytrucks for more swap info. This forum is great for LS specific questions but those other two have a ton of info on getting the newer power plants in to the square body trucks.
Having some minor issues getting my engine mounts figured out. I have the Advance Adapters brackets through Off Road Design. First...the bracket hits the harness ground bolt on the Driver's side. It is a minor issue, but what do you guys do with that ground bolt? Sandwich it between the adapter and motor mount?
Second...I had planned on using my stock Chevy Truck Clam-shell mounts, but the clam-shell is hitting the oil cooler lines on the driver's side! Anyone have experience with this?
Should I be using some other set of mounts that will give me better clearance?! I could totally redo the oil-cooler lines, but I am trying to avoid that. The factory routing is pretty sano.
Any clean mounting point in that area for that ground will be fine.As for the oil cooler lines,now is the time to ditch or replace them,those crappy GM lines are just going to leak anyways.
How do I get vacuum to my heater system? I noticed a quick disconnect on the back of the truck intake. Is there a mating piece I can get that allows me to tap into the manifold for vacuum? Or...how about that factory capped-off smaller port on the RH side of the intake, that is hidden by the vortec cover? It looks like I could snap off that molded-closed piece and have a port now for a smaller vacuum line.
Another question...where is the connector for the MAF on the factory harness? Is it on the break-out that goes down the RH, back of the engine? I am pulling off the harness and labeling everything, but the MAF connector for my 2004 LQ4 isn't jumping out at me!
Lastly...what is the connector on the tail end of the A/C Compressor connector? There is a round 3 pin connector that I am not sure what that went to. There is a Red wire, gray wire and I think black wire?
So I have a 1991 4L80e that has the standard plug for the internals and then it has an Input Speed Sensor (ISS), an Output Speed Sensor (OSS) in the tail of the transmission, and finally a Vehicle Speed Sensor (VSS, obviously) in the Transfer Case. I do not have a switch in the transfer case for Low Range.
My engine is a 2004 LQ4 out of a 2004 Yukon XL 2500. Apparently it must have been a 2wd because it doesn't have the VSS going past the transmission. What I have is two connectors that plug into the ISS and OSS like it was stock on my transmission as well as the big electrical connector.
My confusion comes that it looks like the rear plug might be a Pressure Control Solenoid Valve? The color of the wires is correct, but I am really thinking this must be the VSS on the 2004?!
What do I do with the OSS on my transmission? Is it not used? My 1991 V3500 had wires going to the ISS, OSS and VSS?!
I think I figured my transmission wiring out last night. Please correct me if I am wrong!
I used a dmm to confirm that my rear plug is indeed pinned to the VSS pins on the PCM. Strange...I am reasonably sure this was an engine out of a 4WD. Did GMC just use a jumper harness to get to the transfer case? I was confused looking at wiring diagrams about the PC Sol Valve. The diagrams made it look like it was a separate connection. I see that it is actually part of the big electrical connector on the transmission.
The PNP connector looks like all I need is the Park/Neutral wire and the Backup wire?!
I have this connector:
I still want to go through the pins on the big connector to make sure they match my 1991 transmission and no pins are swapped.
12-05-2016, 03:37 PM
1991 V3500 with 6.0l LS 
