Thermostat replaced less than 5 months ago -- now leaking

Around last Christmas, I replaced my hoses, reservoir, reservoir cap, radiator, performed a coolant flush, and also replaced the thermostat while I was at it.

This past weekend, I was driving down the highway when I noticed the 'HOT COOLANT' message on the DIC and saw that my temperature guage was nearly at 260. Note that there were no flashing check engine lights or alarms/dinging sounds. I immediately pulled over at the next exit, popped the hood, and noticed the coolant reservoir was nearly empty. I couldn't located any obvious leaks, filled it with a container of Prestone 50/50 dexcool (that I had in the back on-hand), and hobbled on home.

The next day, I went to Autozone to get the pressure test kit and found what appears to be a leak between the waterpump and thermostat -- so I'm assuming the seal on the thermostat failed. I'm very surprised that this is the case considering that it's an ACDelco thermostat (part no. 15-11057) which I purchased from RockAuto. Since it's covered under the limited warranty, I should be getting it replaced free. I have since re-ordered the part from RockAuto -- will return the defective item afterward.

As added insurance, I'm going to add Permatex 22071 Water Pump and Thermostat RTV Silicone. I didn't do this last time, as I was under the impression that it wasn't necessary. Do you guys agree with this?

Does a seal failure with such a new part seem odd?

I don't have the part number handy but I replaced my thermostat and water pump with AC Delco parts from Rock Auto. The instructions that came with the thermostat said to use BOTH the rubber gasket around the t-stat and the paper gasket on the housing.

We did a test with only the rubber gasket and it leaked a little, so we added the paper gasket and that sealed it up. Didn't use any sealant. YMMV.

Quote from my dad, "If nothing else works, read the instructions..." and I'm finally taking him seriously. RIP.

Use this stuff on paper gasket. Makes dissasembling a breeze. Chemical resistant as well.

Curious. I've had just the rubber gasket on mine for the better part of 2 years. No leaks

SnowDrifter said:

Curious. I've had just the rubber gasket on mine for the better part of 2 years. No leaks

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When my waterpump went I bought the gates from Rockauto. All it had was paper so I went with it. If was rubber then no gasket should be needed.

There was an old deteriorated rubber gasket from the old thermostat that had to be scraped off since it was stuck. That seems to have come with the new water pump at the time (installed May 2016 along with a new thermostat). The thermostat (installed in December) didn't include an additional paper or rubber gasket -- other than the seal integrated into the new thermostat housing.

I looked to see if I could locate/purchase a paper or rubber gasket to add to the thermostat for the Tahoe, but my results yield replacement seals only...suggestions?

In most instances Its prudent to use at least a little rtv. If it seals well you won’t mind a little scraping in a few years.

Big Mama said:

In most instances Its prudent to use at least a little rtv. If it seals well you won’t mind a little scraping in a few years.

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Personal preference I reckon. I can't stand scraping RTV period.

I don't use RTV. My vehicle has it in 4 places and 4 places only - the four corners of the oil pan where it intersects with the front and back cover gaskets. Everything else has just been straight gasket with good prep work and quality gaskets.


Anyways... Preferences aside, the gasket I used is a circular one with a groove in it to fit the thermostat. It has provisions for appropriate thermostat alignment within the housing.

Put it on the thermostat, light coat of silicone grease, align t-stat in housing, then secure the two pieces together.

If other methods of sealing exist I've not seen them. When I walked in the parts store and asked for a gasket; this is what I got. Rock auto seems to echo this and has no other provisions for sealing

Thanks guys for the advice, much appreciated.

I'm going to use either silicone grease or RTV with the new thermostat that arrives, as suggested by a few of you.

I think I may also do another pressure test to confirm that it is definitely the thermostat and not the water-pump --- since no one seems to have experienced this sort of failure in a short period of time as myself. I want to make sure it's not a leak from something like the weep hole dripping onto the thermostat.

JetBlackNali said:

Use this stuff on paper gasket. Makes dissasembling a breeze. Chemical resistant as well.

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Pic is way small. What is it?