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Cooling system work should include testing the
Water Temp gauge accuracy

     The real story begins here! 
by Mark Hamilton 

Is it HOT, or NOT?

Maybe the new Water Temp gauge just simply is not accurate! 

When first firing up a newly constructed Hot Rod, it’s especially important to know if a Water Temp gauge is dead-on accurate or “far off the mark”.

First on the agenda with a newly constructed Hot Rod will be to run the engine at a moderately fast RPM to break in the camshaft and valve lifters.  Typically we do this engine “break-in period” while the car is sitting in place, but with the engine running at highway cruise RPM.  The friction of the lifters rubbing on the cam lobes will actually burnish the surfaces to a finish that must last for the life of the cam and lifters.  (“Roller Cams” are an exception.)

The significance of noting that the car will be sitting in place, rather than rolling at about 60mph, is that we will not have the airflow to assist in cooling.  Also, the fresh engine will produce more heat than it will after break-in.  After the break-in period we may tune the engine, which can further reduce the coolant “normal” operating temperature.

The break-in period is a critical time with the new engine, and sometimes it’s difficult to keep the engine cool while running in place at high RPM during the break-in period.  It pays to check the Water Temperature gauge before starting the engine for break-in.  By checking the gauge ahead of time, we will know where we actually stand with the water temperature. 

We should also consider that a “restofication” project might include many changes that can affect operating temperature of the engine coolant.  (Some of us use the term “restofication” to describe a restoration with modifications and up-grades.)

With a new engine, we may have a higher compression ratio, a new distributor may have a different advance curve, a new carburetor may have changed the air/fuel ratio, and we may have also changed rear-end, transmission, and tire diameter, resulting with a different MPH to RPM ratio.

All of the previously mentioned topics can affect the operating temperature of the engine coolant.  If we add to the car’s makeover a new radiator of different design, a different cooling fan, and a new Water Temp gauge—then it will be a very good idea to verify accuracy of the new water temp gauge.

The same is true when buying a complete, running and driving car.  It takes some driving time to learn what “normal” temperature is for a particular car.  Actually for any car, it’s important to know where we really stand with coolant temperature.


        Most of us already know that water boils at 212 degrees F, at sea level, in an open container.  But when performing this test please be aware that water in an open container boils at a lower temperature at higher altitudes.  The table below provides figures for doing this test at altitudes higher than sea level.


Elevation (in feet)

Boiling point of water

0 ft.  (sea level)

212.0 degrees F

1,000 ft. (above sea level)

210.1 degrees F

2,000 ft.

208.1 degrees F

3,000 ft.

206.2 degrees F

4,000 ft.

204.3 degrees F

5,000 ft.

202.4 degrees F

6,000 ft.

200.6 degrees F

7,000 ft.

198.7 degrees F

         As seen in the table above, the boiling point of water in an open container is reduced about two degrees F for every 1,000 feet we go up in altitude.  With a comparison between sea level altitude and a location at 6,000 feet altitude, the difference when doing this test at the different altitudes would result with a nearly twelve degrees change in the gauge reading.  Twelve degrees of change would be noticeable on a good Water Temp gauge.

        With mechanical gauges, the test may be slightly easier to perform before installing a new Water Temp gauge, but the test can also be done if the gauge is already in the car.    

         We did this test at our M.A.D. Enterprises workshop, which is at about 1,000 feet elevation, near Springville, California.  And as the photo shows, this gauge is accurate!

This gauge is an imported, low-budget model, by Auto Meter.  It is the mechanical model, which the author has found to be typically more accurate and more responsive than the electrical gauge models. 
It’s good to choose a gauge with a 270degree needle sweep rotation, as shown in the photo above.  Accuracy is typically better and small temperature changes are easily recognized with the greater sweep of the needle.

(Ditto for oil pressure gauges!  The author also prefers mechanical oil pressure gauges with a 270degree needle sweep, compared to the electric models and gauges with less needle sweep.  The quick response of the mechanical gauge can be helpful with spotting early warnings of an engine problem!)

It’s typical that any gauge from Auto Meter would be accurate, as Auto Meter is famous for bringing good equipment to us Hot Rodders. 

 But the author can no longer say the same good things about all models of Stewart Warner gauges.

Several years ago…  A “budget model” three-gauge set of Stewart Warner gauges was installed in a customer’s car.  The budget model Water Temp gauge was the model shown above.  Shortly after installing the gauges, the customer complained that his car always seems to run hotter than he would like to see.  After considerable money spent in attempt to see the engine run cooler, the gauge still showed that the car was running too warm, in fact borderline over-heating on hot days.  (A costly new radiator, and also a new thermostat and high performance water pump made no improvement in the Water Temp gauge readings.)

We used the boiling water test to check the Stewart Warner gauge in question, and with the probe in boiling water the gauge displayed approximately 235 degrees F!  During a phone call to Stewart Warner in hopes of getting a replacement gauge, the author was told “Not to worry; that model of gauge is only accurate to plus-or-minus 14%.”   Well that’s a handy explanation, since at actual 200degrees coolant temperature a 14% error would cause the reading to be 28degrees in error!  We might as well not even have a coolant temperature gauge if it cannot be more accurate than those measurements.  But not to worry, we tossed it in the trash dumpster where it belongs, and bought a better water temp gauge.  Happily the new gauge checked out fine and it pleased the car owner to finally see his car running at “normal” water temperature.

And the reason for the above discussion is not an intended bashing of all Stewart Warner gauges, but only to point out the importance of testing the Water Temp gauge for accuracy.  We use the Water Temp gauge to monitor cooling system performance, and we should know that the gauge is giving us correct information. 

        When performing the test be aware that the probe should be submerged in the boiling water, but not laying at the bottom of the container.  The electric hot plate beneath the cooking pan, or a burner flame beneath the pan, will likely make the metal at the bottom of the cooking pan hotter than the boiling point of water.  If the probe was resting on the bottom of the pan it may sense the hotter temperature of the metal pan, and then the test would not be accurate. 

         This boiling water accuracy test for the water temp gauge can be used with equal accuracy for mechanical and electric gauges.  With electric Water Temp gauges, the test should be done after the gauge and wiring has been installed in the car.  (Because a wiring problem could affect the gauge performance.)  When testing electric gauges that are already installed and wired, we can bring the electric hot plate and cooking pan to the car.  Either we can put an insulated cover on the air cleaner to support the hot plate, or put the hot plate on a stand next to the car.


A very important part of the test procedure for electric gauges is connection of a ground wire to the threads of the water temp switch for the gauge.  For this ground wire, the author uses a test lead with a cable clamp that was originally intended for use with small battery chargers.  (Because it fits over the large diameter threads of the temperature switch, but a small alligator clip will not fit.)  Connect the other end of the test lead to a good ground, in example a carburetor mounting stud typically works well.

Then, make an extension wire from the existing original wire that will allow the sending unit to reach the hot plate area.  18gauge, 16gauge, or 14gauge wire is fine for the job, and the calibration reading at the gauge will not be affected with the wire at extended length.   

 The test is very useful with factory gauges too, and especially practical when a factory gauge does not have degrees of temperature marks.  The factory gauge shown in the above photo is marked only at “COLD” and at “HOT.”   But with the boiling water test, at least we will know where the gauge will read with the coolant temperature at about 210 or 212degrees F.  (That is 210F or 212F with the testing performed at about sea-level altitude.) 

 One lap to go!

Please click here for Part 2; There’s some good photos and reading on placement of the probe for the Water Temp gauge.

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