Thread: 04 4T65E into 99 GT

I recently got a 1999 GT for cheap due to rod knock. I have not been able to drive the car to see if there are any transmission troubles, but with almost 200K miles it might as well get swapped out at the same time as the engine. Locally I found a 2004 3800 engine and transmission for a decent price. I know that the 3800 will drop in the 99 with some parts swapping. After lots of searching and reading here about transmission swaps I am more confused than before I started. If I am correct the 04+ transmission will go in, but there will be some work needed on the feed and return lines as well as a tune. Is this correct or do I need to stick to a 98-03 transmission?

just swap the motor trans and computer

Originally Posted by dsmuts

just swap the motor trans and computer

This would require being able to get all the wiring harness and the computer. I'll have to check if he has all that. If not, could I just have my 99 tuned for the new engine/transmission?

Edit: Just checked and it is from a 2005. I thought it was from a 2004. Does this make a difference or still just swap engine/tranny/and engine computer?

You can do the swap relatively easy.

You'll need a TB adapter plate to change the TB to your TB. Use a Gen V to L36 or L67 one.
I would switch over the rear exhaust manifold in case it's a slightly different outlet position to the downpipe.
Use the 05's evap soleniod, just bring the wiring from where yours is currently located (probably over front valve cover area) to the back next to the throttle body where the 05 has it. This also means you cap the vacuum port on the side of your TB that was for the evap.
Check to see if your trans lines are held into the transmission fittings by clips or if they are threaded. The 05 will be clips. If the lines aren't the same, get a set of 2000 or 2002 lines and put them in while the drivetrain is out.
Last piece, move your transmission PNP/Range switch over to the new trans and align it. Use the link below for info on it.
http://www.grandprixforums.net/how-a...ion-49297.html
Last thing..if the engine came with the motor mount still attached to the oil pan, use it. You need to grind a little area off your subframe to make it work.

Here's a modded subframe. Look at where the mount sits in the bucket on the engine mount side.


As I remember, there's a piece of the bucket that rolls toward the driverside and it needs to be trimmed for the aluminum oil pan to fit. The aluminum pan is much better as it drains 100% of the oil when you pull the plug vs leaving 3/8" like the steel and it's uber easy to do a oil pan gasket if you ever have to.

Thanks for the info Bill, just wanted I needed to know. Will I need the 05 wiring harness and pcm or just use the 99 and get a tune? Will the 99 axles work with the 05 tranny?

I would use the 99 harness and pcm