Thermostat Operation

-
plumkrazee70

plumkrazee70

Well-Known Member
Joined
May 5, 2013
Messages
3,747
Reaction score
1,449
Location
Medford, Oregon
I have a 70 dart with '68 340 block bored .040 over with mild cam. Timing is set at 18* initial and 36* total. I have a 22" radiator re cored to a 3 row, 7 blade mopar fan with Jag clutch and shroud.

I have a 195* thermostat and was wondering about the operation. I realize this may be a dumb question, but at what temperature does it close? I understand that it opens at 195*

The main reason I ask, is because I think mine may be faulty.

I can start my car and watch it come up to temp, which usually takes 10 minutes or so, just idling, which I feel is normal. The temp will rise to about 195* and the thermostat will open, and the temp will fall to just over 180* or so. This all makes sense to me.

Now after the thermostat has opened the first time, the temp will rise to 205* and stay there while cruising and go to about 210* at stop lights.

I have been told that these temps are fine and not to be concerned, but was just wondering if my thermostat isn't closing after the temp lowers and then re opening when it gets back to 195*

Or is this not how it works?

Thanks and I apologize for the dumb question. I couldn't find anything online about the "Closed temperature" just the open.
 
It's pretty simple. It is fully open at above 195°, and won't fully close until the temp is well below 195°. Somewhere around 195° it is about 1/2 way open. It doesn't just pop open or pop closed, instead it modulates the temperature.
 
The t-stat is slow to react. But if your driving along at over 200 it won't close . A lot of times the t stat is at half open or three quarters and fluctuates around there. The jag clutch is weak I have one in my car it's made for a plastic fan that jag used .
 
EL5DEMON340 said:
The t-stat is slow to react. But if your driving along at over 200 it won't close . A lot of times the t stat is at half open or three quarters and fluctuates around there. The jag clutch is weak I have one in my car it's made for a plastic fan that jag used .
Click to expand...

I don't know if I can fit a regular clutch. Maybe I should switch to a fixed fan...
 
69_340_GTS said:
It's pretty simple. It is fully open at above 195°, and won't fully close until the temp is well below 195°. Somewhere around 195° it is about 1/2 way open. It doesn't just pop open or pop closed, instead it modulates the temperature.
Click to expand...

Ok. Thanks for the response.

When it first "opens" the temp drops to about 180 or so, is this not "well below 195*" enough for it to close"

I guess I feel that the temps are a tad high for everything being brand new. Build has less than 1k miles on it. I've seen others on this forum who run in the 190's consistently.
 
Thermostats don't snap to fully open and closed positions. Instead they slowly move to various positions. Takes only a pot of water on the stove and a thermometer to test analyze. You'll find some new ones ( elcheapo ) can be as much as 10 degrees wrong. The more expensive brands are closer to spec.
 
RedFish said:
Thermostats don't snap to fully open and closed positions. Instead they slowly move to various positions. Takes only a pot of water on the stove and a thermometer to test analyze. You'll find some new ones ( elcheapo ) can be as much as 10 degrees wrong. The more expensive brands are closer to spec.
Click to expand...

Thanks for this response. I understand how they work now.

Given my setup and my symptoms do you think that its the result of a cheap thermostat? (currently an autozone one)

or a cooling system problem?
 
That first open of the day when it drops to 180 Is due to the stone cold water in the radiator
 
Go to napa and get there super stat or a Stant super stat , you will see the difference in the heat motor, the round solid peice in the center of the spring on the bottom it will be bigger than the cheapo ones
 
EL5DEMON340 said:
Go to napa and get there super stat or a Stant super stat , you will see the difference in the heat motor, the round solid peice in the center of the spring on the bottom it will be bigger than the cheapo ones
Click to expand...

I didn't see anything labeled super stat on their site. But this one is their "Premium"

http://www.napaonline.com/Catalog/CatalogItemDetail.aspx/Thermostat-Premium/_/R-THM532090_0293168778

Also found the Stant one on Amazon:

[ame="http://www.amazon.com/Stant-45478-SuperStat-Thermostat-Fahrenheit/dp/B000CKPIAK/ref=sr_1_1?s=automotive&ie=UTF8&qid=1405612784&sr=1-1&keywords=45478"]Amazon.com: Stant 45478 SuperStat Thermostat - 180 Degrees Fahrenheit: Automotive[/ame]
 
I suggest when-ever possible to replace the thermostat with a "fail safe/open" type..

Patented safeguard against overheating. When overheating occurs, Fail-Safe automatically locks in the open position to permit maximum coolant flow. Manufactured from high quality components to meet the demanding conditions of the automotive cooling system. Manufactured with OEM specifications and processes. Copper case and wax compound assures rapid response to temperature changes in the coolant. Offset design, air relief valve, and OEM style seals included where applicable

Prevents harmful overheats,, and lengthly walks/tows at inconvenient times.

hope it helps
 
inertia said:
I suggest when-ever possible to relace the thermostat with a "fail safe/open" type..

Patented safeguard against overheating. When overheating occurs, Fail-Safe automatically locks in the open position to permit maximum coolant flow. Manufactured from high quality components to meet the demanding conditions of the automotive cooling system. Manufactured with OEM specifications and processes. Copper case and wax compound assures rapid response to temperature changes in the coolant. Offset design, air relief valve, and OEM style seals included where applicable

Prevents harmful overheats,, and lengthly walks/tows at inconvenient times.

hope it helps
Click to expand...

Thanks for the tip. I'm gonna swap stats and see what happens.

Thanks everyone.
 
plumkrazee70 said:
Thanks for this response. I understand how they work now.

Given my setup and my symptoms do you think that its the result of a cheap thermostat? (currently an autozone one)

or a cooling system problem?
Click to expand...

stant and robertshaw are among the better mfgrs of these type parts. That initial swing in water temp is relative to radiator exchange. You'll see things with real instruments and testers that the stock gauges could not show. Their response to change is too slow. Less worry / concern from operators.
 
RedFish said:
stant and robertshaw are among the better mfgrs of these type parts. That initial swing in water temp is relative to radiator exchange. You'll see things with real instruments and testers that the stock gauges could not show. Their response to change is too slow. Less worry / concern from operators.
Click to expand...

I am gonna get the stant one from Amazon. The factory temp gauge was disconnected a long time ago, so I have an actual temp gauge. Of course it could be off as well.

If I got a infrared laser heat gun would I point it at the thermostat housing for comparing temp gauge to actual?
 
Dont forget, its July and you have a new tight engine. This is the hottest it will ever run. Next July when you have 7000 miles on it will be a little different.
 
I have played with T-stats, sensors, and an IR gun on my 1984 M-B 300D. The IR gun can be suprisingly accurate, but you must fill the "view" with surfaces at the same temperature. The view is much larger than the laser spot (my HF one). When I shot it next to the head temp sensor, I got within a few deg C of the dash gage. Also, when I shot at the T-stat in a pot of water I got very close to a thermometer in the water (sees thru the water). When I shot at the AC return tube, I got 200 F even though it was frosted. It probably saw some of the exhaust manifold behind it, so use cardboard to block such. The reading can vary with the type of surface (black is best), but seems to not matter much so probably uses the better "2 color" method.

After determining that my dash gage was very accurate (have you verified yours?), I wondered why the engine ran at 60 C, when T-stat and "normal" is "80 C". I went further than testing "just see if it opens" in hot water. I carefully varied the temperature, comparing it to a new T-stat and a few Mopar ones (180 & 195 F). I found that T-stat was "sluggish", starting to open much sooner and taking longer to get full open (engineers call "low gain"). I swapped in the new T-stat and the car then ran close to 80 C all the time. Both T-stats were German w/ same PN. Those are a 2 spring design, so maybe one spring went bad. Thus, I proved not all T-stats that look good are fine. BTW, even a snappy "high-gain" T-stat does not control an exact temperature. As the load increases, it must open more to reach a balance point, which requires a slightly higher temperature. Any engineer who doesn't know this (proportional droop) should tear up their degree.
 
fratzog lover said:
Dont forget, its July and you have a new tight engine. This is the hottest it will ever run. Next July when you have 7000 miles on it will be a little different.
Click to expand...

I've considered this also, but wasn't sure if it would runner cooler after it was really broke in.
 
Also when running a fresh engine, water wetter is an option. Not running a 50/50 antifreeze mix in hot weather, but rather 80/20 ( water/antifreeze) and a bottle of waterwetter will aid heat transfer greatly. Just dont forget to swap back when freezing temps hit.
 
fratzog lover said:
Also when running a fresh engine, water wetter is an option. Not running a 50/50 antifreeze mix in hot weather, but rather 80/20 ( water/antifreeze) and a bottle of waterwetter will aid heat transfer greatly. Just dont forget to swap back when freezing temps hit.
Click to expand...

Thanks for the tip. I will report back what I find when replacing the thermostat.
 
You must log in or register to reply here.
-

Back
Top