Thermostat housing leakage

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Behfra

SILVER Star
Joined
Apr 20, 2026
Threads
1
Messages
2
Location
Texas
Relatively new to DIY auto repair and learning as I go. I've had my truck for about 4 years and am slowly going through everything. I am replacing the thermostat in the cooling system but am having an issue with leakage from the housing. I am using the upper and lower OEM gaskets that I bought from cool cruisers of Texas. Not sure what I am doing wrong. Happy to give more info if needed but looking for some guidance. Thanks
 
Buy a silver star for $20/year so you can post pictures (click on your user name and it's under subscriptions)

We have no idea what you're doing wrong either (if anything) without someway of seeing it.
 
Not sure where the leak is coming from, but on my housing I had a leak coming from where the top half mates to the bottom. I used a large thick metal plate with sand paper to LIGHTLY sand the housing until true.
 
Thabks for the feedback. The old gasket was stuck pretty good on the lower part of the housing so I used a razor blade and sandpaper to get it off as best I could. Then I put thermostat -> lower gasket -> upper gasket -> upper housing. The area is circled is where it is leaking from when I fill with coolant.
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Try snugging up the two bolts that hold it to the head
 
The horizontal surface of the bottom housing looks fine.

You don't show the upper, so... Could either be that or the housing is warped, cracked or gasket has failed where the bottom of the housing bolts to the cylinder head. Be careful snugging up those two bolts, you break one and you're F'd.

If it's not leaking from the rear gasket, you could use a little Blue Permatex on the four-hole gasket -that will help seal any warpage in the housing.
 
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