: Thermostat housing leak

06-20-13, 10:09 PM
2009 CTS4 3.6L base
I’ve had a slow coolant/antifreeze leak for a few months. I couldn’t seem to locate it.
It was high and on the back of the engine… was my best guess.

Today it developed into a big leak. It blew thru about 3 quarts in 30 minutes. (That’s about how much you lose before the temperature gage moves above 220F.) It is leaking from what looks like a thermostat housing at the back/top of the motor. The aluminum housing has two steel tubes coming from it; one is a radiator hose on the passenger side and the other is heater hose sized, going to the coolant overflow bottle near themaster cylinder. Each of the tubes are held to the housing via a single bolt. I expect that there are o-rings sealing the tubes.
The overflow connection is the leaker.

I’m going to dive into it this weekend. I’ll try to pull the tube first. But it looks like the upper intake will come off pretty easy, and give me better access.

I searched for info, and found nothing. The pictures of thermostat housings I could find were not correct.

If anyone has any experience or advice in this area… speak up. Thanks!

My housing looks like this:
http://i2.photobucket.com/albums/y48/ZeeVert/2009%20CTS4/Thermostat/OEM_zps6861e168.jpg (http://s2.photobucket.com/user/ZeeVert/media/2009%20CTS4/Thermostat/OEM_zps6861e168.jpg.html)
http://i2.photobucket.com/albums/y48/ZeeVert/2009%20CTS4/Thermostat/rearthermhsgDrverSideframedsmall_zps76438665.jpg (http://s2.photobucket.com/user/ZeeVert/media/2009%20CTS4/Thermostat/rearthermhsgDrverSideframedsmall_zps76438665.jpg.h tml)

Driver side:
http://i2.photobucket.com/albums/y48/ZeeVert/2009%20CTS4/Thermostat/rearthermhsgDrverSidecloseup_zpsac48916f.jpg (http://s2.photobucket.com/user/ZeeVert/media/2009%20CTS4/Thermostat/rearthermhsgDrverSidecloseup_zpsac48916f.jpg.html)
Passenger side:
http://i2.photobucket.com/albums/y48/ZeeVert/2009%20CTS4/Thermostat/rearthermhsngPassSideb_zps55d21b0e.jpg (http://s2.photobucket.com/user/ZeeVert/media/2009%20CTS4/Thermostat/rearthermhsngPassSideb_zps55d21b0e.jpg.html)

C "T" ess
06-21-13, 11:50 AM
I would try to see the leak to know for sure if it is coming from the overflow tube or the housing. It may be a good time to replace the housing gasket and and hoses. You don't say how many miles you have but I would it could be covered under the 100,000 mile powertrain warranty.

06-22-13, 01:16 PM
No warranty -salvage/rebuild car.
I'll just leave this information for future generations...

The leak was definitely coming from the O-ring seal. I could see it.
I pulled tube off today. It wasn't too bad.
There are two bolts, as you can see from the pictures.
I used a deep 12mm, swivel, and +12 inches of extension to get the bolt on the thermo housing.
I used a 14mm on the lower bolt.
I pulled the tube out without disconnecting the hose clamp to the reservoir/overflow.
At this point, you don't have to remove the hose clamp to get to the O-ring.
I happened to have an a/c O-ring of the right size. I think it was an 11/16"x1/8".
You'll want to make sure you have both bolts started before tightening them. I tightened the thermostat bolt first.


http://i2.photobucket.com/albums/y48/ZeeVert/2009%20CTS4/Thermostat/P1030791smcopy_zps7c1274ac.jpg (http://s2.photobucket.com/user/ZeeVert/media/2009%20CTS4/Thermostat/P1030791smcopy_zps7c1274ac.jpg.html)
http://i2.photobucket.com/albums/y48/ZeeVert/2009%20CTS4/Thermostat/P1030808s_zpsedb4e8d2.jpg (http://s2.photobucket.com/user/ZeeVert/media/2009%20CTS4/Thermostat/P1030808s_zpsedb4e8d2.jpg.html)
http://i2.photobucket.com/albums/y48/ZeeVert/2009%20CTS4/Thermostat/P1030805s_zpsed9fd6da.jpg (http://s2.photobucket.com/user/ZeeVert/media/2009%20CTS4/Thermostat/P1030805s_zpsed9fd6da.jpg.html)

06-22-13, 03:22 PM
Thanks for the info zeevert! I'm sure it will help someone in the future. That's what this forum is about, helping each other!


09-25-13, 06:44 PM
Well, I have a 2008 with a slow leak. The mechanic told me it was the water pump and tried to schedule a time next week. I replaced the pump myself over the weekend, but still have the leak. This may be my weekend project next weekend. I am in search of the SMALL leak that has plagued me for more than a month. I can smell a faint odor of coolant when I get out of the car (or with the window down at the drive thru), but not puddles. It uses about 1" on the reservoir for a weeks worth of commuting, but it needs to be fixed. I will update when this is done and let you know it it was my problem... thanks for a new place to look to try and fix this.

09-29-13, 11:26 PM
That was exactly what was happening to me; a faint but regular coolant odor.
If the coolant was full, I would get an opccasional puddle, but couldn't trace it down.
Here's a picture of the old O-ring:
http://i2.photobucket.com/albums/y48/ZeeVert/2009%20CTS4/Thermostat/P1030797s_zpsb462589e.jpg (http://s2.photobucket.com/user/ZeeVert/media/2009%20CTS4/Thermostat/P1030797s_zpsb462589e.jpg.html)

11-23-13, 01:52 PM
I have the same leak in my 2008. Only issue is our engines look a little different. I have a sponge foam like material behind my air intake manifold so I cant see my housing. Nor does my booster connect straight like that towards my intake. Im going in right now and will try to see what my leak is, but i am prepare to replace the entire housing and o ring and pipe. Looks like you didn't really need to remove your intake, but i might have to seeing i have that sponge covering my entire housing.

edit; I had to remove the entire intake manifold, removed the foam from underneath and behind the manifold. Mine looks a lot more complicated. The pipe is located behind the engine which i cant see where to screw these bolts. Also i received the wrong housing from autozone so hopefully I dont need it. Have some photos , might upload later.

12-02-13, 09:35 AM
UPDATE on my leak..... I have been chasing the elusive leak and thought I had it found when I replaced the water pump. While visiting relatives over the holiday, waiting in traffic and idling in 35 degree weather, I heard a "pop" and saw steam coming from the hood. I found the coolant came from the left (drivers) side near the firewall. temp gauge remained normal. I drove the last 5 miles and let it sit til Friday. I removed the coolant reservoir and examined the two hoses and did not find a cut or tear. On a whim I filled it with water and put air pressure in the small opening and found the the leak. The reservoir is made of two pieces of plastic and they had separated along the back side (toward the firewall) and allowed the coolant to leak out. This an OEM part and does not seem to be normally stocked by the dealer. I drove to the closest dealer that had the part (180 miles one way), paid $68 dollars and by night fall had a new part ready for my long 700 mile return trip home. One note is that knowing that the coolant leaked, and knowing I was going to make a long trip, I may have overfilled the reservoir which may have contributed to this failure, but maybe not. I hope I have finally found my slow coolant leak. You can pull the reservoir by removing two hoses and 10 mm bolt and a 10 mm nut, and look along the edge where the two parts are bonded together for the tell tale signs of white chalky residue that DEX COOL leaves. This small leak is very annoying and we are all looking for a fix. Give this one a try.

01-06-14, 07:18 PM
Well, I am confident in saying I FIXED THE SLOW LEAK. It seams that the coolant reservoir was the issue. more than a month has past and the coolant level has not moved. (I marked it) Since this leak was in the rear of the coolant reservoir, and toward the firewall it was not easily seen as it usually dissipated and left no "Dex Cool" residue, just the smell when I parked the car. If you have exhausted all other opinions, try to remove the reservoir, put it under pressure and see if the top and bottom pieces separate at the back of the bottle. YMMV

Good Luck

05-23-14, 09:52 PM
Wow. It has been a while since my o-ring fix. It was definitely my problem. Haven't added a drop in 10-11 months.

11-28-16, 08:05 PM
I am having the same leak from the o-ring on the outlet pipe at the thermostat housing as you were, zeevert.
I am having a real problem locating the proper o-ring for the pipe. All the GM parts sites have horrible parts diagrams. It's impossible to pinpoint the right parts. The thermostat housings all look wrong too.
It looks like I can remove one bolt from the pipe, then remove the pipe from the thermostat housing. Should be easy enough once I find the proper o-ring.

04-26-17, 09:43 AM
hi. just wondering if anyone on this thread has part numbers. i'm having a hell of a time finding the o rings and the housing. I can find a different housing but i'm pretty sure i need the one shown on the initial post

thanks in advance

04-26-17, 11:35 AM
Rock Auto? Or there are lots of GM parts websites that have drill down sketches with part numbers. Takes some doing, but they are there.

04-26-17, 02:11 PM
Rock Auto? Or there are lots of GM parts websites that have drill down sketches with part numbers. Takes some doing, but they are there.

I've looked. Rock auto has a different style. From what I can see. I haven't taken anything apart yet to be sure. Car going in to get checked out and make sure the leak is coming from thermostat housing. Hopefully it won't be anything more serious than that