Cooling system work should
include testing the
Temp gauge accuracy.
The real story begins here!
Is it HOT,
Maybe the new Water Temp gauge just simply is not accurate!
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”.
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.)
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.
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
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
(in an OPEN CONTAINER)
0 ft. (sea level)
212.0 degrees F
1,000 ft. (above sea
210.1 degrees F
208.1 degrees F
206.2 degrees F
204.3 degrees F
202.4 degrees F
200.6 degrees F
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
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
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.
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!)
typical that any gauge from Auto Meter would be accurate, as Auto Meter is
famous for bringing good equipment to us Hot Rodders.
the author can no longer say the same good things about all models of Stewart
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.)
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.
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.
GROUND THE “SENDING UNIT” WHEN TESTING ELECTRIC GAUGES
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
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
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.)
lap to go!
click here for Part 2;
There’s some good photos and reading on placement of the probe for the Water