Do thermostats have a break in period?

[ubtripn]

So I have always been fighting finding temp equilibrium on my 68. Standard 350 crate with a new 4 row radiator, new clutch fan and shroud. I just finally decided to run a 165 to keep from cooking her. I now have a new carb and some other parts so I switched to a 180 t-stat. Here is what it does:

Cold start - temp goes to 200 exactly (Mechanical gauge, laser verified) then drops immediately to 180. After I start driving in a normal way, 45 degrees out the temp moves between 180 and 190. Slowly but enough to watch it do it within a few minutes.

I installed it yesterday, it has a 16 pound rad cap. I started it this afternoon, 50 degrees out, it went to 200, dropped to 180 then cycled between 180 and 190 as before. Took it for a short 4 mile drive, parked it and the temp landed on 188 and stayed put. Yesterday it was moving between 180 and 190 in 15 seconds parked with no load. Now it seems to have an equilibrium once it does it a couple times.

1. Do t-stats break in over the first few runs?

2. Is it ok for my radiator to spit a little fluid at 188? If I put a reservoir in would it suck that fluid back in?

It is a good NAPA stat tested before installed, thanks for any help you can provide.

[Stocker]

If they do, that's a new one on me, I sure wouldn't think so. That said, I can't think what might be causing your situation.

A guess or two -- With your recent cooling system work, maybe there's air trapped in the system? Have you tried drilling a small (1/8") hole in the t-stat? Some guys say it gets trapped air out of the system, others say no. Either way, it doesn't cause any problems and *might* work as claimed.

To your last question, an overflow reservoir is a great idea and I would not be without one. Besides capturing and reclaiming any leakage, it lets you run a full radiator, which can help keep things just a bit cooler. Make sure you get a radiator cap for a sealed (closed) system or it can't pull coolant out of the reservoir as it cools.

[ubtripn]

Copy that-will do and thank you.

Its weird, it does the castellation thing for a few moments and then sits down on 188 but it bigs me so I will give those ideas a try, thanks!

[Andy4639]

One thing it may be is hanging up a bit. But no they don't have a break in period that I'm a wear of. It could be air also trapped in it but if it's opening then it should pass it.
I would jack it up as high as you can and let it idle for a while and see if it passes any air out.
A over flow would be worth every penny! Not to mention it would be the green thing to do.

[ubtripn]

What do you think might cause it to hang?

I did read the 200 thing is natural, warms the engine u fast and is designed to do it then drop to 180 but vacillating like that is weird.

[Andy4639]

Originally Posted by ubtripn

What do you think might cause it to hang?

I did read the 200 thing is natural, warms the engine u fast and is designed to do it then drop to 180 but vacillating like that is weird.

I have seen them hang on the lip of the gasket when not exactly in place until it tears it enough to open.
Yes the thing is not calibrated to open fully at 180/190 that's just when it should start to open by.

[geezer#99]

Of course it does that.
It’s just a temperature controlled valve. Always closing and opening slightly.
Rule of thumb is temp will run 10 to 15 degrees above thermo rate.

If you want to see it rock steady at 180 replace the thermo with an orifice plate.

[Chris_oz]

Are you sure it is installed the correct way?
I have seen them in backwards which can make then slow to open, then do funny things,
Confim it's in the correct way and the 1/8" hole is cheap insurance against air locks and slow opening.

[kehstr]

10-15 degrees in fluctuation for the temperature is fairly normal, and will not hurt anything. Remember, all the thermostat knows to do is open at a certain temperature, and close at a certain temperature.

Generally, when a thermostat is bad they stick open and the truck will not warm up to operating temperatures.

And when there is air trapped under the thermostat the motor will overheat, since air does not heat up like the coolant and water does to open the thermostat

Beyond that I would just run it and move on.

[special-K]

I have installed new thermostats that didn't operate properly before. What makes the old one bad is not moving freely/binding. A new one can have the same fault.

[ubtripn]

Thank you for all of the information if fluctuating 10 degrees is normal then I'm fine going down the interstate at 60 it stays at 188 let's just hope it does the same the summer :-)

[demian5]

Originally Posted by Stocker

To your last question, an overflow reservoir is a great idea and I would not be without one. Besides capturing and reclaiming any leakage, it lets you run a full radiator, which can help keep things just a bit cooler. Make sure you get a radiator cap for a sealed (closed) system or it can't pull coolant out of the reservoir as it cools.

People totally miss this and I'm glad it was said early in this post.

[VWNate1]

It sounds O.K. to me ~

1888 F isn't hot .

For maximum power, fuel economy and engine life you want to run it as close to boiling as possible but many fail to grasp this .

The squeezing out a bit of coolant when fully warmed up means you're over filling it ~ it needs some air space for compression .

I too prefer a surge or "puke" tank .

Remember ; the cap is different when using a surge tank, must have two rubber seals or it won't suck the coolant back in as the engine cools down .

[garyd1961]

You said crate engine, is it a vortec or does it have vortec heads? Do you have the heater core connected? If not do you have a bypass hose from the intake to the water pump?
My gauge on my vortec would fluctuate wildly even with a hole in my thermostat before I ran a bypass hose. Now with a bypass hose it's steady as a rock.

[Redneck Rydes]

When I get a new thermostat, I always boil some water on the stove and with a wire hooked to it drop in the thermostat then drop it in some cold water, i do this a couple of time to watch the cycle,actually bought a new one once and only opened a sliver,so cheap insurance to make sure it rully opens.

[brown7373]

You are missing something. It is expected that the temp will go up and down until fully warmed-up. With the t-stat closed, the engine recirculates the coolant IN THE ENGINE until it reaches 180 or above and the t-stat opens. Once it opens, it dumps a bunch of 45 or 50 degree coolant from the radiator and the t-stat closes again because the coolant at the t-stat is again cold. It repeats that up and down a few times until ALL of the coolant reaches 180 or above. Then it will register fairly consistent. I agree that the radiator pukes because of overfilling. There is a line on the radiator below the cap that says full, or fill line or something like that. Overfilling does nothing but wasting coolant and making a mess.

My 72 with the original 350 or the 350/290 crate in it now NEVER pukes a drop, even in the middle of summer with the A/C on in Florida. Coolant level at the fill line and system in proper working order.

[ubtripn]

Ok. Sounds like it's working normal. What pound of cap so you uae? Or should

[brown7373]

I'd have to look, but it's 15 or 16...whatever they sell at the restorations catalogs.

[toolboxchev]

Believe mine is a 16lb cap for the closed system. More pressure and it will cool better.

Noticed the same thing every time I installed a new thermostat. Stay away from the fail safe units. They will fail, safely, everytime, and stay in fail mode.

[Stocker]

Either 15# or 16# cap..... pretty sure it's 16#.

[HO455]

Where is the temperature sensor located in the cooling circuit? I am assuming this is an aftermarket guage. Mechanical or electric?

[garyd1961]

Originally Posted by brown7373

You are missing something. It is expected that the temp will go up and down until fully warmed-up. With the t-stat closed, the engine recirculates the coolant IN THE ENGINE until it reaches 180 or above and the t-stat opens. Once it opens, it dumps a bunch of 45 or 50 degree coolant from the radiator and the t-stat closes again because the coolant at the t-stat is again cold. It repeats that up and down a few times until ALL of the coolant reaches 180 or above. Then it will register fairly consistent. I agree that the radiator pukes because of overfilling. There is a line on the radiator below the cap that says full, or fill line or something like that. Overfilling does nothing but wasting coolant and making a mess.

My 72 with the original 350 or the 350/290 crate in it now NEVER pukes a drop, even in the middle of summer with the A/C on in Florida. Coolant level at the fill line and system in proper working order.

My temp never went up and down with the original motor and it doesn't do it now. It will slowly rise until it passes it's normal running mark just a bit and then the thermostat will open and it fall back a little and stay there. It doesn't move after the one drop when the thermostat opens.
If it's fluctuating you probably don't have enough water flow before the thermostat opens and may be getting hot spots in the heads.

[special-K]

In all the trucks and other water cooled vehicles I have owned the temp never went up and down until all coolant is mixed between block and radiator. They start out could, then climb to operating temperature in a few minutes. Once there, stays steady.

I also test new thermostats by placing in boiling water. I have had brand new ones not open and close properly

[demian5]

When replacing thermostats here is what you should do.

After replacement and topping off the fluid, turn on the heater so the heater valve is open and run the engine with the cap off for about 5 minutes and top off the fluid. I like to run at about 2000 RPM and hold for about a minute to get air out. Top off fluid and put cap on, and top off the reservoir (if equipped).

I put a large fender cover over the grille to prevent air from going through radiator for cooling so this goes faster.

Run vehicle and hold at about 2000-2500 while watching temp gauge, on newer cars, connect a scan tool and monitor the ECT sensor temp. Let idle here and there and raise engine RPM back up until the temp gauge goes up. With a new t stat sometimes the gauge or reading goes higher than normal the first cycle, but stick with it until the thermostat opens.

When the thermostat opens you will see gauge drop a bit or temp go down, run the engine for a couple more minutes at idle then shut it down. I like to confirm temp with a temp gun as well so i know what the gauge reading is at.

After the engine cools, check the coolant level in the cooling system (radiator or expansion tank) and top off the reservoir.

Some vehicles have a coolant bleed port that needs to be opened when filling the engine with coolant, or some vehicles like to have the front of the vehicle jacked up. The goal is to get the air out of the system. This is usually accomplished by having the filler part of the cooling system (radiator cap) higher than any other component in the cooling system.

[Mike72gmc]

On some vehicles, make sure that when bleeding the cooling system that the heat is turned on. Some vehicles have a valve that restricts or stops water flow through the heater core. Not necessarily needed in this case but as a general rule of thumb so that the whole cooling system has the air flushed out. Air in a cooling system is bad. Last note, use radiator shrouds as they force the air through the radiator rather then recycle air around the engine compartment.