3.9L T-stat change (3.5l similar)
 

Swapped my t-stat not that long ago and took some pictures. My little manual says that the 3.9 and 3.5 should be similar, but that the V8 needs the exhaust manifold removed. I don't know about the V8 as I haven't worked on one.

Step one:
Drain the coolant, engine holds 3 gallons so make sure you have something that can hold that much if it all comes out.
The drain plug from above, I reached down with a pair of needle nose pliers to break it loose then turned by hand.
http://img689.imageshack.us/img689/7186/p1040419r.jpg

The coolant will drain from this little opening in the air dam underneath the passenger side.
http://img171.imageshack.us/img171/8853/p1040416.jpg
http://img525.imageshack.us/img525/3497/p1040417z.jpg

Step two:
Disconnect the lower radiator hose from the t-stat housing. Some coolant may come out depending on how much is still in the block.
http://img695.imageshack.us/img695/2978/p1040420.jpg

Step three:
Remove the t-stat housing. Two bolts, the upper will require a deep socket or wrench if you can get it on there. I think they were 8 or 9mm bolts (anyone confirm/deny?).

With the t-stat housing off the t-stat itself should just pull on out. If it's stuck try bumping it a little, I just tapped mine with a wrench.

I would like to add that when you are putting the new t-stat in that you need to carefully push the new t-stat into the housing, and make sure it seats correctly before you install the t-stat/ housing. (MAMONTE)

Installation is the reverse, don't forget to screw the drain plug back in!

Pictures of the inner gasket that went wrong with mine.
http://img220.imageshack.us/img220/4779/p1040422i.jpg
http://img98.imageshack.us/img98/5378/p1040423by.jpg
http://img62.imageshack.us/img62/4995/p1040424g.jpg
^That's the t-stat still stuck in the housing.

mine was disentergrated i had the shop do mine do to a bad experence when one stuck open and it ended up pluging up the new t-stat. the shop took off the one head to try to fish it out they didnt charge me for it do to it was a dealer and they said it was a manufactures defect and they wanted me to be pleased to know they got all of the peaces out of the engine. i wanted to ask them if they could do a port and polish while they had every thing apart getting the rest of the thermistat out of the heads. well thats my good experence the bad is it was stuck shut and my brother drove my 3.8 comaro 5 miles down the high way i was ticked

when my water pump stopped working, i had my shop replace both the water pump and the t stat. it sucks that gm is the only place to get the $75 t stat.

I actually got mine at Advanced Auto for $23 after a student discount :)

umm. well wtf did i pay 75 dollars for!

No idea dude. The GM dealer here actually quoted me at $50 for the t-stat, but about $350 in labor so I said screw that, I'll do it myself.

Yea, I paid $49.95 for my t-stat at the Chevy Dealer.

The 3.5L is identical to the 3.9L for the t-stat replacement. My original t-stat had the same innner gasket problem.

I would like to add that when you are putting the new t-stat in that you need to carefully push the new t-stat into the housing, and make sure it seats correctly before you install the t-stat/ housing.

Good call, I'll add that

Thanks for the tips; these helped me a lot.

I ordered my thermostat (and all my other parts) at RockAuto.com. Got the thermostat for my '06 for about $10.