How to change thermostat on 3.9L engine

[ratskat]

So my engine temperature gauge wasn't moving and my engine light went on. We used the OBD code reader to see what code came up. P0128 which is already listed on this site with explanation, basically replacing your thermostat will fix it 99% of the time, however they don't state exactly how to do it.

The part itself is cheap enough. Got it at Napa - Part #34054 and it cost $31.29 with taxes. Not sure if it would be the same part number in the US.

Schematics and instructions we found with a software called ALLDATA. It gives you the drawing of the engine which shows where the thermostat is located and instructions. They tell you to drain the cooling system first. Remove the air cleaner outlet duct so you have a visual of the hose which has the thermostat in it. We removed the passenger side headlight which gave us access to the thermostat housing. We didn't quite follow the instructions on draining the system, instead we had a large oil pan under the car which once the housing was unscrewed all the liquid drained into it as we removed and replaced the thermostat. Then we put everything back together and refilled the coolant.

It takes alot of coolant when refilling - don't be fooled that it is full right away. Pictures to follow by Born2ski as he helped us with this today. Problem solved, gauge working properly and no more engine light.

The only thing we could see different with the old and the new thermostat is that the seal on the old thermostat seemed worn - can't really explain it - wrippled or out of place.

[snowmanG6]

Re: How to change thermostat on 3.9L engine

how many miles are on the 6?

[ratskat]

Re: How to change thermostat on 3.9L engine

how many miles are on the 6?

72K - not sure if that is good or bad though - how long should a thermostat last?

[snowmanG6]

Re: How to change thermostat on 3.9L engine

are you still in warranty? 72k is a lot longer then I made it. I had mine replaced at 36k

[ratskat]

Re: How to change thermostat on 3.9L engine

are you still in warranty? 72k is a lot longer then I made it. I had mine replaced at 36k

Wow at 36K - ok I don't feel bad then. No warranty ended at 3 years or 60K - I hit both around the same time. Hubby and friend (Born2ski) did the work so only cost me the coolant, the new thermostat and tacos for dinner lol

We took pictures thinking if it helps someone else who may want to do it themselves then great. Born2ski is going to post them soon. Anyone with questions can ask and we can let them know to the best of our experience today.

[BORN2SKI]

Re: How to change thermostat on 3.9L engine

Disclaimer: This is how we replaced the engine thermostat on a 2007 G6 GT Vert (3.9L), your vehicle may be different, and there may be steps which have been inadvertantly omitted, which you may need to perform. Using this guide is at your own risk.

What's needed:
Various wrehcnes/sockets
Flat screwdriver
Large oil pan, or bucket to collect drained coolant
Engine coolant to refil lost coolant
1 beer, maybe 2

The first thing that we did was to remove the engine cover, and front pipe for the air intake to allow for visual access to the thermostat housing. This step is optional, and the only purpose it serves to to give additional visual access, and the replacement can be done without removing it, but we found it helps! The following picture is of an '06 3.5, but the removal of the front pipe is the same. Loosen the two clamps (circled in yellow), remove the bolt (circled in green), and remove the hose (circled in red). This will allow the front pipe to be removed.


Next we removed the passenger headlight to gain physical access to the thermostat.

Pic from directly above engine with front pipe removed. Thermostat is circled in yellow. You will see that some wiring harnesses have been disconnected, this was not required.


The next step is to detatch the wiring harness (circled in red) which is secured to the right bolt (circled in yellow) of the thermostat housing. This can be a little trickey to remove. We found that using screwdriver to guide it off the bolt helped. You do not need to disconnect, move, etc this wiring harness. The purpose of this step is to allow access to remove the right bolt.

The following picture is taken through the hole in the vehicle body which is exposed by removing the passenger headlight.


The next step is to remove the two bolts (circled in yellow) to allow removal of the thermostat housing. The left bolt is easily removable, but we found that the right bolt required a wrench as access is very limited, and the bolt is too deep to place a socket over.

The following picture is taken through the hole in the vehicle body which is exposed by removing the passenger headlight.


This is where it starts to get a little messy.
By removing the two bolts in the previous step, the housing for the thermostat is now loose, and can be pulled back therefore exposing the thermostat.

This is what you should see with the housing removed. That is the thermostat sticking out.


And with the thermostat removed.


And finally the thermostat after removed. Your eyes are not bad, the pic is a little blury.
You can't see it in the picture, but we believe the check engine light was caused because one of the rubber seals on the orginal thermostat was broken.


You can probably guess what we did next, install the new thermostat.
Here is where it gets a little tricky. When reinstalling the thermostat housing, you need to be aware of possible air lock by traping a pocket of air in the coolant line. Someone had mentioned that pouring coolant directly into the coolant line may reduce the risk of air lock, but we did not add any coolant while reinstalling the housing.

From here is it just a matter of reversing the steps above; secure housing with 2 bolts, secure wiring harness back to right bolt, reinstall headlight, and front air intake pipe.

The final step is to add coolant back into the system to replace what was drained. We used an approximate mix of 50/50 coolant/water as per instructions on coolant, but this may be different for you.

There is a signifiant amount of coolant which will have been drained, and we were not able to add it all at once. We added coolant to the "full line", allowed it to work its way through the system for 30ish seconds, then started the vehicle. This pumped coolant through the engine, and the coolant leval droped dramatically. We repeated this until the coolant level did not drop anymore, or approximatley 4-5 times in total.

Once we were finished, we used a code reader to clear the check engine code. The engine temp guage now works, and there is no more check engine light!

Hope this helps!

[ratskat]

Re: How to change thermostat on 3.9L engine

blame me for the blurry pics lol - I wanted to detail every step for the site and the actual "mechanics" namely my husband thought I was in the way - therefore I was trying real quickly to not be in "the way" LOL

And it was 2 beers, a few hot dogs and chips and then later a few tacos

[ratskat]

Re: How to change thermostat on 3.9L engine

Wow at 36K - ok I don't feel bad then at 72K.

Update to this...Our numbers never took in the fact that the 36K was probably miles and my 72K was kilometers, therefore the difference isn't as great as it was. Your 36K is actually 57.9363 - let's round up to 58K in km. Instead of being half of my amount it's actually only 14K difference.

[LoJac963]

Re: How to change thermostat on 3.9L engine

great how to, I woudln't be too concerned with this "air lock" since the system should be burped anyways to remove any air that may have entered into the system during the process

[Thirdgen89GTA]

Re: How to change thermostat on 3.9L engine

Disclaimer: This is how we replaced the engine thermostat on a 2007 G6 GT Vert (3.9L), your vehicle may be different, and there may be steps which have been inadvertantly omitted, which you may need to perform. Using this guide is at your own risk.

Hope this helps!

Thats clearly not a 3.9L GT/GTP engine. Anyone know if the Thermostat is in the same location on a 3900 as compared to the 3500? The intake hugely different, and the block is similar but has different bore spacing.

The Water pump for the 3500 and the 3900 are different part numbers too.

I need to change the thermostat in my GTP, its stuck open now, but its dark out so its hard to look. Figure I'll pick-up a stat tomorrow on the way to work when I pass Napa.

[BORN2SKI]

Re: How to change thermostat on 3.9L engine

Thats clearly not a 3.9L GT/GTP engine. Anyone know if the Thermostat is in the same location on a 3900 as compared to the 3500? The intake hugely different, and the block is similar but has different bore spacing.

The Water pump for the 3500 and the 3900 are different part numbers too.

I need to change the thermostat in my GTP, its stuck open now, but its dark out so its hard to look. Figure I'll pick-up a stat tomorrow on the way to work when I pass Napa.

Say wut? This How-To IS for a 3.9 NOT an LX9

If you are referring to this part...

...The following picture is of an '06 3.5, but the removal of the front pipe is the same. ...

It clearly says that picture is of a 3.5, but the removal of the front pipe is the same

[Neatong6er]

Re: How to change thermostat on 3.9L engine

Obviously reading is not required on this forum

[sabrhawk]

Re: How to change thermostat on 3.9L engine

Very Good How To, thank you! I used it tonight to replace the Thermostat on my 07 3.5L. I just wanted to add that I ended up adding nearly 1.2 gallons of Antifreeze and Distilled water mix. I have not driven it that far yet so more may go in but the reservoir is full. I had a heck of a time breaking the thermostat loose from the housing after removing the two bolts, I had to pry it out with pliers and ended up using a very thin layer of grease to get the new one to slide in properly, the gasket kept trying to roll off the external part of the housing when inserting back in.