[Dante Garofani]

I have a freshly rebuilt 400 engine that runs great except it takes unreasonably long to warm-up. After some investigation I found the culprit to be the thermostat stuck up into the thermostat housing. Anyone have this problem? I replaced the thermostat and made sure it seated correctly however I still found it up in the thermostat housing after running the engine for a while. Is there a shim or some reason the thermostat would be pushed upward?
Thanks for any help!

[Dante Garofani]

I have a freshly rebuilt 400 engine that runs great except it takes unreasonably long to warm-up. After some investigation I found the culprit to be the thermostat stuck up into the thermostat housing. Anyone have this problem? I replaced the thermostat and made sure it seated correctly however I still found it up in the thermostat housing after running the engine for a while. Is there a shim or some reason the thermostat would be pushed upward?
Thanks for any help!

[Poncho64]

I cant beleive this happened to you to, all my friends thought I was crazy, I was buying thermostats from autozone and advance and it just kept happening to me, go to advance and ask for the superstant thermostat it seems to be thicker and hasnt happened to me since I installed it

[Ironbreaker]

I just discovered this problem when I removed my upper radiator hose and noticed the thermostat was cocked inside the housing. I will have to try the thicker thrermostat as mentioned above, or a shimmed style gasket, or even have a couple of "stops" spot welded onto the base of the housing neck. I had some curious cooling issues with my '67 400 HO, and now I can see why. I hope more of you have run into this and have possibly come up with a solution! Oh yea, tried a couple of the oring type seals, and they didn't work...too thick.

[screamingchief]

I ONLY use the robertshaw "balanced sleave" hi-flow style of thermostats,never had a problem with them.

[johnsma22]

There are two basic types of thermostats currently available: You have the balanced sleeve thermostat, and the reverse poppet thermostat. Both types function in the same manner, but have distinct differences. The reverse poppet thermostat opens against the flow of coolant from the water pump. The coolant, being under water pump pressure, is used to help the reverse poppet thermostat stay closed when it is cool so as to prevent leakage.

The balanced sleeve thermostat allows pressurized coolant to circulate around all of it's moving parts, and equalizes the system pressures above and below the thermostat’s moving valve. Therefore the thermostat is pressure balanced and will respond to system temperatures only. It provides a fast temperature response time and increased coolant flow to the radiator. For a performance engine application it is recommended to use the balanced sleeve design for it's added flow capabilities. See photo below:

John

[RayK]

I had the same problem. I used one of those reverse poppet units and it jammed up sideways in the housing!! It was brand new and I couldn't figure out why my 455 was running about 200 + degrees. I even checked it before i installed it to make sure it opened up at 160 degrees which it did. I put in one of the balanced units someone mentionmed and haven't had a problem since.

[RAIV55]

Good information, had the same problem but made a shim to hold the thermostat in place.

I was aware of the two designs of thermostats but didn't know the balanced sleeve syle was the prefered one for performance use.

Thanks

[GoatDr]

Pardon me if I'm wrong, but doesn't the gasket fit above the thermostat and hold it in. Put the thermostat in, next the gasket and then the thermo housing.

[RAIV55]

Originally Posted by GoatDr

Pardon me if I'm wrong, but doesn't the gasket fit above the thermostat and hold it in. Put the thermostat in, next the gasket and then the thermo housing.

The Fel-Pro thermostat gasket didn't hold mine in place. Guess I just figured the housing would retain it, which it didn't... well not in the right spot. My fault for not checking to begin with.

[engineer]

the water outlet nozzle is designed to hold the thermostat in place in the groove in the intake manifold. there are several diameter thermostats and if you use the smaller diameter one, it will not stay in place. likewise, if the outlet is worn or corroded or some chrome ones are too thin and wide to hold, it will also not hold the thermostat. if it is not placed in the groove, that is also a problem. I checked all types of nozzles and NONE of them will allow a thermostat to fit inside of them.