Thermostat still leaks after replacement
- Thread starter Agolding85
- Start date
If the thermostat doesn't stay in the recessed area of the head and slips down, the housing will crack when you torque it up. You might get away with a tacky spray-on gasket dressing but most of the other types act like grease and cause the gasket to shift and split under torque. The Felpro gaskets work well when dry.
I also ran into a Stant thermostat that was a few thou too large in diameter. It looked like it was sitting in there but after two cracked housings, I discovered the bottom just a hair out of the head recess. Had to tune up the circumference with 320 grit on a flat surface to reduce the diameter .002". It was very hard to see the issue down in there when you're bolting it up.
I also ran into a Stant thermostat that was a few thou too large in diameter. It looked like it was sitting in there but after two cracked housings, I discovered the bottom just a hair out of the head recess. Had to tune up the circumference with 320 grit on a flat surface to reduce the diameter .002". It was very hard to see the issue down in there when you're bolting it up.
Last edited:
If the bolts look wallowed out replace them. Use teflon tape instead on the threads. After re-cleaning everything, including the threads. Use a small wire brush. I use brass gun bore brushes. Place the housing on the block and snug down the housing. No gasket, no temp gauge. Use a dental pick and try and find where the two have a gap. If you find a gap then get a new new housing. While at the parts store pick up some gasket paper. Not the cheap 
, get the good thicker (med-thin) cork gasket paper like 20-25 thousandths. More cork the better. It comes in different grades. Carefully, CAREFULLY, trace out the gasket onto the cork and cut out with a razor blade. Now go ahead and install the thermostat and housing using the cork. Teflon tape on the NEW bolts. 2-3 wraps. Tighten down the bolts back and forth to be sure the housing is snuggled down evenly. Again, evenly. Then use the torque wrench a 1/2 turn on bolt 1, half turn on bolt 2, back and forth until full torque. Wait a day. Check torque and tighten if necessary. Add fluid and you are done. If there is any drops then check torque again. The cork may need a few heat cycles to hold torque. Using RTV means you can't go back and retighten without breaking the thread seal.
, get the good thicker (med-thin) cork gasket paper like 20-25 thousandths. More cork the better. It comes in different grades. Carefully, CAREFULLY, trace out the gasket onto the cork and cut out with a razor blade. Now go ahead and install the thermostat and housing using the cork. Teflon tape on the NEW bolts. 2-3 wraps. Tighten down the bolts back and forth to be sure the housing is snuggled down evenly. Again, evenly. Then use the torque wrench a 1/2 turn on bolt 1, half turn on bolt 2, back and forth until full torque. Wait a day. Check torque and tighten if necessary. Add fluid and you are done. If there is any drops then check torque again. The cork may need a few heat cycles to hold torque. Using RTV means you can't go back and retighten without breaking the thread seal.
Years ago when I had to replace the OEM thermostat, I could not get the housing to seal. After several gaskets, I ended up lapping the mounting surface of the housing using sandpaper on a flat marble tile. The housing clearly was not flat. Once it was, it sealed very easily.
Jerry is right. All of these thick gasket, rtv, Teflon tape, yada yada yada fixes are just Band-Aids trying to cover up gasket mating surfaces that are either warped, not cleaned, or overly pitted (assuming you are not over/under torquing it). I have had even brand new housings be warped and leak.
I got a tile of marble from Home Depot (for the flat surface) and put a sheet of sand paper on it. Took the housing and sanded the mating surface to ensure flatness. I use no RTV, Teflon tape, or anything else. No leaks.
I got a tile of marble from Home Depot (for the flat surface) and put a sheet of sand paper on it. Took the housing and sanded the mating surface to ensure flatness. I use no RTV, Teflon tape, or anything else. No leaks.
racecar825
New Member
I recently swapped out thermostat/housing when i swapped out my leaky water pump and there was a huge leak out of the thermostat housing. When i took it off, there was a crack in the housing, probably the thermostat slipped out of the indent and when i torqued it, it cracked. I also found that the OEM housing was not completely flat because I could rock it when I put it on a flat surface. Not sure if it got warped when I torqued it, but I doubt it.
I just got a dorman thermostat housing with and it actually seems like similar or better quality than the OEM one. Installed it with just the gasket, and made sure the thermostat stayed in place by applying some RTV to the back of it. No leaks so far.
I just got a dorman thermostat housing with and it actually seems like similar or better quality than the OEM one. Installed it with just the gasket, and made sure the thermostat stayed in place by applying some RTV to the back of it. No leaks so far.
MountaineerTom
LJ Enthusiast, Retired USAF Weapons Loader
Supporting Member
Ride of the Month Winner
I put mine on dry after cleaning the surface of ALL signs of the old gasket. I used a scuff pad on a 90 degree air tool as suggested by @Boinked . I also did not put any sealant on the bolts, as suggested by @Jerry Bransford . This was on an my 2006 4.0, if that matters. A Fel-Pro gasket was used. No leaks since November.
MountaineerTom
LJ Enthusiast, Retired USAF Weapons Loader
Supporting Member
Ride of the Month Winner
Why is that? Yeah, I read your first post, and comprehended it. Your way isn't the only right option.
Pagrey, you are wrong, that is why I ignored it. Since you want to bring it back up though, no bolts from the thermostat housing go into or anywhere near the block. Both bolt into the head, and neither go into a water passage. If you have coolant leaking out of the bolt holes, it's for the reasons I said and coating them in RTV won't help.
There is a head bolt that goes through the head (obviously) and in to the block where a water passage runs. This bolt requires sealant per the FSM. The thermostat housing bolts do not per the FSM. Because they don't need it.
There is a head bolt that goes through the head (obviously) and in to the block where a water passage runs. This bolt requires sealant per the FSM. The thermostat housing bolts do not per the FSM. Because they don't need it.
I just saw this post right above mine. I wasn't trying to rip off your post, promise! I think I just replied at the bottom of first page. Great minds think alike though!
Similar threads
