84 iron duke

leaks coolant from thermostat housing.
the cap does not seal correctly. new one did not seal.

do i have to replace the thermostat housing???

where can i find one???

if i go to junkyard, what cars am i looking for besides fiero????

is there a different easy fix??

thank you.

Is the sealing surface of your thermostat housing pitted with rust?

By coincidence, today I sanded my thermostat housing because it was weeping from there.

I stuck some sandpaper upside-down on a flat surface (a piece of glass works), and then I sanded the sealing ring of the thermostat housing (I held the thermostat housing in my hand; it was removed from the engine).

I went from 150, to 320, to 600 grit sandpaper.

Depending on the amount of corrosion, only sanding may not be enough. Mine had one really big pit that needed to be welded before sanding.

This is basically the same idea as how some folks sand a cylinder head's mating surface:
https://youtu.be/a8ctywVmzK0?si=PzNuNfCs9QkIN5b7

quote Originally posted by frosTkilos: 84 iron duke leaks coolant from thermostat housing. the cap does not seal correctly. new one did not seal. do i have to replace the thermostat housing??? where can i find one??? if i go to junkyard, what cars am i looking for besides fiero???? is there a different easy fix?? thank you.

I think the problem you're having is that a lot of the newer coolant caps do not have a "full gasket" ... and it causes the thermostat housing to leak profusely through the cap. Let me see if I can find some examples...

You need one like this: https://www.ebay.com/itm/375363085668




Not one like this: https://www.ebay.com/itm/275357048873




Do you see the difference in the size of the gasket? Many of the newer ones have a much smaller gasket for some reason, and they will always end up leaking since the surface contact with the gasket is so small on the new ones. You can buy that (new) old one that I linked to... that'll solve your problem.

quote Originally posted by pmbrunelle: Is the sealing surface of your thermostat housing pitted with rust? By coincidence, today I sanded my thermostat housing because it was weeping from there.

The other thing to check for is to make sure the top of the housing hasn't been bent slightly during "forceful" removal of the thermostat housing cover/cap (ie with a monkey-wrench). Mine was tweaked somewhat, and I had to massage it back into shape before I then also used the sandpaper on a flat surface treatment. You'll know the top of the housing is ready when the sanding on a flat surface reveals a complete shiny ring around the top of the housing.

And one more tip: Be sure the thermostat is fully seated in its "home" at the base of the housing. Some brands of thermostats are just a tiny bit wider (apparently) and need a little more of a nudge to get them seated (you might feel it click into place). I found my cap was leaking around the rim and it was because the thermostat wasn't all the way down, and thus was keeping the cap ever-so-slightly away from being sealed. This is on the V6 but worthwhile investigating on the 4 also.

quote Originally posted by css9450: Be sure the thermostat is fully seated in its "home" at the base of the housing.

Good tip. Sometimes it's just a bit of corrosion down at the bottom of the inside of the housing that prevents the thermostat from snapping into place. Removing the corrosion is probably best, but applying a bit of grease to the thermostat's O-ring is often enough to help the thermostat slide into place.

[This message has been edited by Patrick (edited 04-24-2025).]

quote Originally posted by 82-T/A [At Work]: Do you see the difference in the size of the gasket? Many of the newer ones have a much smaller gasket for some reason, and they will always end up leaking since the surface contact with the gasket is so small on the new ones. You can buy that (new) old one that I linked to... that'll solve your problem.

This post prompted me to check my thermostat cap, which happens to be a newer one.

The raised sealing ring of the thermostat housing left a ring-shaped imprint on the rubber of the thermostat cap. The imprint was within the limits of the rubber, but it was close to the edges.

Probably a cost optimisation to fit more rubber circles in a sheet...

quote Originally posted by pmbrunelle: This post prompted me to check my thermostat cap, which happens to be a newer one. The raised sealing ring of the thermostat housing left a ring-shaped imprint on the rubber of the thermostat cap. The imprint was within the limits of the rubber, but it was close to the edges. Probably a cost optimization to fit more rubber circles in a sheet...

I think it was Ogre who clued me in on this... but I also wonder if maybe the thermostat housings for the slightly newer cars might have a slightly narrower opening (not sure if that's even possible with the thermostats likely being the same)...

There are still a few newer and OEM ones still available, but I always check now. I've bought a couple as back-up just in case.