Bad Thermostat?

[Pedigry]

Hey guys,

Jumped in the car yesterday for a spin. Temp climbed to 205, typically it will hold at 185. It was cold as well so it should have been running cool. I thought the fans might be out so I stopped, both fans were running and the waterpump was spinning away. I ran it up to 50 mph, which, if its the fans, the motor would cool down because of the increased air flow. But the motor continued to get hotter at higher speeds. It got up to 209. I backed it down to 30 mph and the temp came down to around 200.

I am guessing the thermostat isn't opening so I ordered one from summit today. The only other thing could be the water pump but I can't recal a water pump just losing it's ability to pump after 3 weeks of not running. Hoping it's not the pump as it's obviously a more expensive than a thermostat.



Any thoughts?

Thanks.

[ZZ430]

What kind of car are we talking about here?

[srode]

Vacuum leak would do that too, but it would probably be idling rough it was that though.

[dhutton]

If it’s a bad thermostat the radiator would be cold, wouldn’t it?

[Pedigry]

It's a 70 mustang with a stroker 351W. I checked the radiator and it wasn't cold, but it wasn't hot either. I thought thermostats had small holes drilled into them to let some water through which might warm up the radiator some, not sure. The windsor also has the bypass in the water pump but I'm not exactly sure how that works. I'll have to google it.

[Vimes]

Take the radiator cap off then bring it up to operating temperature. Bring the engine up to 2000RPM and look inside the radiator. If the thermostat is working, the water level should suddenly drop, and water should start flowing out of the tubes on top like a waterfall. If this doesn't happen, the thermostat is likely stuck closed. When you let off the fuel, let off it slowly to let water flow slow down slowly. Just cutting it off can cause water to come out of the open radiator.

The bleed hole in the thermostat has two jobs. Main job is to let air pass through, as air won't transfer enough heat to open the thermostat. Second job is to allow a little water movement to get hot water up to the thermostat faster. It's not going to let enough water through to heat the radiator much, and you don't want it to. If yours doesn't have a bleed hole, a 1/16 hole is plenty.

If the thermostat appears to be opening, replace the radiator cap before getting deeper into the engine. Knew a guy with a Mustang that had similar symptoms and he replaced the hoses, water pump, thermostat and radiator to no effect. Turned out the cap wasn't holding pressure. The only other thing I can think to check is the lower radiator hose as when they get old, the water pump can suck them closed and water won't flow. Not all hoses come with springs inside.

[79 Camaro]

Don't know if this helps but any new thermostat I install I drill 3 1/8" holes in the flange to let some coolant flow even when the thermostat is closed.

[Pedigry]

Vines,

Thanks. I have a BE COOL rad and it has the cap on the side. When I pull the cap off it has a little metal landing and then the water flows in the rad from the side. So it's stupid but I can't see the water in the rad. Ill try and peek in from the side tonight when I get home but I don't think I can see it. Below is a pic for reference. I doubt the hoses are collapsing but I will check that as well. I will also try the cap, you might be right, it might be that simple. Thanks for the ideas.

[Elbert]

Nice engine bay. Update us when you tried a new thermostat.

[Hotwire]

This seems simple but i got bit by it. Purchased a truck last year and did all the pm to get it road worthy but missed flushing the cooling system. It got cold here and whatever concentration was in the truck wasn't up to the task and froze. Got lucky enough to not have any plugs drop out or lift heads but it did get stupid hot from the coolant not flowing and me not realizing it until the check gauges light came on.. Was it cold out enough to freeze when you were having the problem? Know if your coolant mix is correct?

[Pedigry]

It never got that cold. I'l be dropping the new thermostat in this weekend, I'll post an update. I'm hopeful it will solve the problem.