I don't know if this is a common problem for the 4.0s but the thermostat housing always seems to leak, no matter how new the gasket is. It's been about a year since it's been replaced, but even after replacing it, a few weeks later I could see a little wetness at the bottom around the gasket. The housing is new, and there doesn't seem to be any cuts on the head surface.

Why does it still leak?? Should I ditch the gasket and only use silicone? This little leak is leading me to think that air is getting into the system and causing the rusty particles in the coolant.

 

Upon reinstallation, make sure that you CLEAN both old mating surfaces with a razor, and use a thin coat of RTV silicone on both sides of the gasket.

 

Usually they leak because the thermostat slipped out if it's slot on the head while the bolts were being tightened. The housing is aluminum and gets tweaked. When everything is true and clean there will be no leaks. RTV etc is not necessary unless trying to make up for some other imperfection.

 

^^This^^
Don't over tighten it or that can also cause it to leak too by causing the housing to bend out of shape.

 

I'm just cursed with an engine that will always loose coolant somehow. I did just about everything mentioned above. Im meticulous about cleaning the mating surfaces, I use RTV on both sides, and with the new housing I actually torqued to specs. Although I did use one of the gaskets with the red head on it... Not sure it that's cause for trouble?

 

Had the same problem. I used the gray RTV and it has been fine since.

 

All of these X3........

 

U r absolutely correct.......any leak at all will allow air to enter the system as the motor cools.

 

I need to correct myself. I don't use RTV on both sides, I just stick on the sticky side and RTV the other side....oops. I'll make sure to try that when I change my gasket soon. And I'll probably use the regular flat gasket, the last one I used had a red silicone bead that ran along side it, but it stopped at both bolt holes.

Going to do a coolant flush while I'm at it to remove anything that's in the system. I'm thinking do the gasket first then flush? I plan on applying the gasket with RTV, let it sit for 10mins, then install it and bolt it down slightly snug, then wait a few hours, then torque to specs. Then let dry for 24 hours. I read that procedure on a forum and they said they had leak free results. So I figure do that first so if any surface rust builds up over night the flush will remove it.

 

Do the gasket last. While the stat cover is off, remove the bottom rad hose from the water pump. Stick the garden hose in the stat hole in the head and flush crud out the water pump. Stick the garden hose in the top rad hose and flush the rad. Flush the heater core as well thru one or the other heater hoses. Now u r ready to install the stat, the new gasket and re-attach the hoses. Absolutely no need to wait 24 hours to let RTV cure.

 

Good tip, I've never needed to let RTV cure for very long.

 

Thanks for the tips. I was just trying to play it safe with the RTV, 24 hours is a bit too long I agree. So no prestone flush to wash off any rust that may be attached to the block or head?