What sort of thermostat do I need?

[WilliamFredrickson]

I had a really nasty coolant leak the other day after not driving my car for about a week. I think I tracked down the problem to the gasket around the thermostat housing, which I'll be replacing tomorrow. Hopefully that fixes the problem, because it was really gushing out a lot of coolant... We'll see... it may be the waterpump. But since I'm here and I already took out a bunch of stuff to get to it, I think I should replace the thermostat too. It seemed pretty dirty.

Which sort of thermostat do I need to replace it? I can't make out the writing on it, so I'm not exactly sure what type I should order. Here's two pics of it -





The car is a 1986 Mercury Grand Marquis LS 5.0 EFI

[1990LTD]

195 degree thermostat...I used a Stant branded thermostat in mine and have had no problems so far. You will have all kinds of fun problems if you go with something vastly different from 195 degrees.

[WilliamFredrickson]

Thank you! I'll see if I can pick that up tomorrow!

[1987cp]

I always just buy whatever brand they have handy at AutoZone. Ask for a 'stat for your car, or look up representative PNs online before you go. Default is 195 degree; I use a 180 degree. That said, there's theoretically no need to replace a thermostat that functions properly (can always heat some water on the stove to verify that it opens at the correct temperature).

FWIW, I don't use water neck gaskets; I just make sure the mating surfaces are clean and dry and use a nice layer of Ultra Black RTV. Other people use gaskets plus RTV. O-ring style waternecks can also be convenient, provided the mating surface on your intake is flat (I've had one that wasn't).

[1990LTD]

if you want to test the thermostat before you put it in, put it in a pot of water and heat it to 195 and see if it opens.

[WilliamFredrickson]

So just using RTV instead of a gasket wouldn't be that bad? It's safer though, to use the gasket and the RTV, right?

I'm going to heat up some water and test out the old thermostat too... I suspect it's not going to work, but I'm interested to see if it does. If it doesn't, that could have contributed to the gasket failing, I suppose. If it doesn't work, I'm not too concerned, because a new thermostat is pretty cheap, right?

[86VickyLX]

I used both. I used RTV to hold gasket on intake, while I installed the housing. It's good for sealing imperfections in the gasket surfaces. But I'd use both. And haven't had any leaks yet.

[PushnFords]

I've actually had the best luck with the cheap thermostats. I've had two trucks that the previous owners put $12 "Failsafe" thermostats in and they both went bad. Funny thing was when the one stuck open the truck overheated because it couldn't cool the water fast enough. I put a thin layer on both sides of the gasket to seal any imperfections and then put it together with some antisieze on the bolts.

[1987cp]

86Vicky hates Failsafes because he feels they can cause premature engine wear if they stick open and the car runs too cold for a long time. I always notice my AutoGage temp gage from time to time while driving, so I'd know immediately if my Failsafe had failed. But if you don't like Failsafes, get something else ... IIRC, I put a Duralast in the car I don't have a temp gauge in.

Anytime I've used a stock paper gasket on anything coolant-related, it eventually leaked on me. Water neck, overnight; water pump, after three years, but still irritating. Obviously, other folks' experiences have been different. :shrugs:

[91waggin]

If you have SEFI, you need to run the stock 195 thermostat. 1987cp uses a 180 thermostat because all his cars are carbed. Use black RTV on the compressed paper gasket and you'll be just fine.

[Piece-it pete]

I've never had a gasket leak on me. Cracking the &@#$* housing is another story... (tighten tighten tighten) <tink!> DAMMIT!

I've had bad t-stats out of the box before.

Pete

[91waggin]

I've had bad thermostats out of the box before too, I always test them on the stove. Never broken a housing before though.

[WilliamFredrickson]

Well, I managed to get a hold of a new thermostat... it's a 192.. that shouldn't be a problem, right? Tested it too beforehand, and it works fine. I also went ahead and bought new housing for it too, but it was a little different from the old one. It has an odd bolt that screws into the top of the housing covering this hole that runs through the unit, on the spout that connects to the radiator.. the other one didn't have this. Should I be worried? Will it be weaker?

[1987cp]

It's just a 1/4-NPT port for connecting some sensor or other if you need it. I haven't yet been able to determine what stock equipment needs it, but it should be a good place for adding an aftermarket temp sender if you still need the two stock senders.


(BTW, the concern over temp is that a given state of tune is done with a certain operating temp in mind, as well as the concept that higher temps can mean lower emissions. You could of course tune your SEFI car to run properly at a nonstock temp, but very few people on this site ever do any tuning on EEC-IV cars.)

[gadget73]

192-195 is stock temperature, depending on what book you look at.


What you're seeing is a pipe threaded bung for a temperature sensor or a thermostatically controlled vacuum valve on a carb car. Just put the plug in it and you're fine. I run one of those on my car because it was A) availiable and B) allows my stock in-dash temperature gauge to work with the aftermarket gauge installed.
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As for the temperature tuning, while theoretically possible, there is no real point to it. The engine wears least between 190 and 200F, so theres no real gain to making it run best outside of that temperature range because you're still going to experience increased cylinder wear.