Coolant Temp Fluctuation

SofaKing

SofaKing

Well-known member
I have a 1936 21 stud engine in my 39PU and it did not come with thermostats. This weekend I put 180F thermostats in the the top radiator hoses at the radiator. I have the original red fluid temp gauge only. Outside air temp was about 28F. I took the truck for a 15 minute drive after verifying the thermostats open as installed and the engine cyl head temp, using infrared thermometer, was 168F. What I observed is the temp indication climbs to about 80% of the scale and then drops to about 30% and repeats as the thermostats cycle. The temp indication never flattened out like it does on more modern cars. My inclination is to think this is due to the volume of coolant. My question is; do you guys see the same type of temp variations on your engines?
 
I have a 59 AB with new stats installed at the top outlet of the heads and my cooling system is in good shape. Regardless of different air temps, my two temp gauges change their reading as much as a quarter of their scale as I go up hill, then down hill, thru shaded areas, and less-so as I change speed levels.

Thinking about your concern ..... air temp is about 28, coolant passing thru radiator gets chilled, enters block and closes or partially closes the stats, coolant heats up again, stats open, heated coolant enters radiator, and on and on. The complete volume of coolant may never get a chance to fully and constantly reach the activation temp of the stats and thus the stats constantly modulating their opening and closing.

Comparing their higher power output to their lower coolant volume, modern engines generally contain less coolant than out flatheads and therefore the coolant volume can more easily stay constant.
 
Most modern vehicles are also equipped with thermostatically controlled clutch fans or electric fans which control the temperature of the coolant passing thru the radiator more evenly than a conventional fan that is governed strictly by engine RPM.
 
Drbrown in post #3 has your answer. You won't have the fluctuation in the summer. This is the reason Ford sold Winterfronts as an accessory, to reduce the radiator's ability to cool the water in cold weather.
 
Thanks everyone. It was very steady in warmer weather and no t-stats performing as described by DrBrown. I did top-off the radiator after my drive so next time I will test AlanWoodieman's theory of excess air.

What is a weatherfront? When I bought the truck it had what looked like a small window blind affixed to the radiator inlet shroud that could be pulled down in front of the radiator. It had the pragmatism of a farmer fix and I removed it. (probably should have left it in hindsight)
 
We would often cover part of the radiator in colder weather with something like cardboard to lower radiator efficiency. It is a trail and error process and changes a little with the outside temperature.
 
PS to my post above .... I should have noted that I have a 16 inch - 16 amp electric radiator fan controlled by an on-off coolant sensor inserted in the fins of the radiator core. The fan turns on and off depending on the sensor's upper temp limit setting. Generally, when on the highway in moderate summer temp's and at steady speed, enough air flows thru the radiator that the fan does not turn on. During the winter here, sometimes the fan does not turn on unless I get slowed by a lot of traffic/traffic lights. I have a 12v alternator that normally puts-out about 14 volts and when the fan turns on it drops to just about 13 volts - about the same effect as turning on the headlights.

I like the fan because (1) the engine warms up faster, and (2) the fan is much quieter than the after-market fiberglass 6-blade belt-driven fan that came on it.

That feature aside, the engine gauges still show that the coolant temperature varies depending on the over-the-road conditions and the load on the engine.
 
JSeery said:
We would often cover part of the radiator in colder weather with something like cardboard to lower radiator efficiency. It is a trail and error process and changes a little with the outside temperature.
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i cover 1/2 to 2/3s of the radiator with cardbord in my 36 pickup. about 40 deg out it will get up to 180 deg
 
Amen to those good ole days... Back in "the day" when vehicles were more uncomplicated, it was very common for car owners in the colder climates to carry "winter fronts" ie; be they cardboard or fabric in the trunk during the winter months
I still have the aftermarket Ford V8 branded fabric winter front in the truck of my '36. It has not been used since 1954.
 
Still the good ol' days for me. I have to cover the radiator [yes I have 180 thermostats ] the small cover I use up to 75F , the large one I use in winter going to and sometimes from work, about a 20 min stop & start run. I just cant get it hot enough in that time. Temps here range from 30F to 112F
 

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to eleminate an air lock I drill an 8th inch hole through the top side of each t-stat, it will also let some warm water flow through even when the t-stat is close. I think it makes for a little more stable temp because the warm water gets up to the thermostat sooner.. less fluctuation. chuck s.
 
Follow-up; I topped off the the coolant 2 weeks ago and today took it for a spin. Outside temp is about 45F. The temp gauge still climbs to about 90% and drops to about 50%. I don't believe I have too much air or trapped air, both t-stats have two holes drilled to allow some circulation. The stats are at the top of of the line where it enters the radiator. I have the steel tubes with the rubber collars on my top coolant lines. After a 30 minute drive I observed that the steel coolant line at the t-sat was 178F and the cyl head temp at the inlet to the water pump was 205F. (stats are 180F) I may try a piece of cardboard tomorrow but suspect that the mounting location of the T-stat is contributing to the issue; too much temp drop through the 12" of steel coolant line is giving false temp signal to t-stat. A 27F variation in temp between the cyl head and the t-stat can't be good.
 
Final report- Removed t-stats, went with cardboard. Temps at top and bottom of upper coolant line are the same, 168F, indication in cab is midrange. Low-tech carries the day, again. I think I might conjure up a cover like TalkWrench has, entertaining other materials and themes, maybe an animal hide. Perhaps assassinating one of the local mule deer that eats my wife's roses or a cat I saw forever sleeping on the road yesterday. Smelly work but might be amusing...
 
SofaKing said:
Final report- Removed t-stats, went with cardboard. Temps at top and bottom of upper coolant line are the same, 168F, indication in cab is midrange. Low-tech carries the day, again. I think I might conjure up a cover like TalkWrench has, entertaining other materials and themes, maybe an animal hide. Perhaps assassinating one of the local mule deer that eats my wife's roses or a cat I saw forever sleeping on the road yesterday. Smelly work but might be amusing...
Click to expand...

Just use the whole cat ..
 

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As Lawrie mentioned I had the same problem. My testing and final fix was to get rid of the High Flow stats and install standard types. With the High flow temp would rise, stats would open coolant would enter the block and lower the temp. This happened even on a long trip. No problem since installing standard stats.
Some may disagree with this but it worked for me.
Kpar
 
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