MACS Monthly Newsletter - 2020 - APR3

stat is beginning to open. Typically, around
the 195-200°F mark, the actual measured
temperature at the hose should rapidly rise
indicating the thermostat is opening. Note
that the temperature value monitored on
your scan tool is often different than the actual measured temperature at the radiator
hose near the thermostat outlet due to the
measurements being "apples and oranges".
A thermistor INSIDE the cooling system is
not going to be identical to an external hose
temp measurement.
f. If the measured temperature of the upper radiator hose near the thermostat outlet does NOT seem to rise as the ECT data
PID gets beyond the stat's opening spec,
then you can suspect a thermostat that's not
opening.

On Vehicle Thermostat Testing

Motor Age

a. Use a contact thermometer (thermocouple and DMM) or non-contact infrared
thermometer to measure the upper radiator
hose's temperature near the thermostat outlet.
b. Have your scan tool in Global OBD-II
mode to measure the ECT sensor data PID
(Figure 6).

Off Vehicle Thermostat Testing

Figure 6: On-vehicle thermostat testing. Motor Age
Technical Editor Pete Meier, in his "The Trainer" video
series, gives some great tips on pairing up your meter
& thermocouple with a scan tool to observe thermostat
performance. http://ubmadvanstar.hs-sites.com/motorage-the-trainer-july-2019

Checking a thermostat off the vehicle is an old test. It's
often overlooked because the popular train of thought
is that if I must remove the thermostat from the vehicle
to test it, why not just replace it? Regardless, the test is
not that hard to perform and is fairly conclusive.
a. Drain the cooling system and remove the thermostat. That's a great time to be observant for any blatant
errors made by whomever previously serviced the
thermostat. Many of us have had the experience of
finding a thermostat installed backwards (sensing side
towards the engine, please!).
b. Place the thermostat in a container of water that
you can heat on a stove. Place a thermometer in the
water to read the temperature as you increase the heat
(Figure 7).
c. Within 3-4°F of the thermostat's rating (usually
stamped on it) the wax pellet in the sensing section

April 2020

Steve Schaeber

c. After a cold soak with the engine at ambient temperature, start and allow to idle.
Leave the heater and A/C off to minimize
any effects those systems might have on
your cooling system.
d. Monitor the ECT sensor data PID while
observing the upper radiator hose's temperature. The ECT sensor temp should
slowly rise but the actual radiator hose at
the thermostat housing should remain fairly constant. If both the measured temp at
the upper radiator hose and ECT data PID
begin to rise together, you can reasonably
suspect the thermostat is not fully closing.
There is the possibility the thermostat is being opened by excessive coolant flow volume / pressure from the water pump, but
this condition is very rare. You'd be more
likely to find this kind of problem on a vehicle that has aftermarket high performance
water pumps. A thermostat that's stuck fully
or partially open is a leading cause of thermostat performance DTCs such as the P0128.
e. As the ECT data PID begins to rise to the
temperature level of the thermostat's rating, the temperature of the radiator hose
near the thermostat outlet should also begin
to rise a little as an indication the thermo-

Figure 7: Time honored off-vehicle thermostat test. If
you have a stove available to heat a pot of water, your
customer's thermostat can be physically tested off
the vehicle. Observe the stat's valve movement while
watching a thermometer in the container as you raise
the temperature on the stove.
will expand and cause the thermostat valve face to begin to unseat.
d. After the thermostat reaches 15-20°F beyond its
specified rating, it should be fully open.

MACS Service Reports

http://ubmadvanstar.hs-sites.com/motor-age-the-trainer-july-2019 http://ubmadvanstar.hs-sites.com/motor-age-the-trainer-july-2019

MACS Monthly Newsletter - 2020

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