Any specifics below are referring to my MGA and you are making an
assumption that the IR thermometer is working correctly and the
temperature gauge is also working correctly. Also, my suggestions are
for making readings which you will simply "compare/contrast".
For starters and something of a "baseline" you could check the temp of
the thermostat housing with a "cold" engine. Make any readings on the
metal parts where hoses attach and not on the hose(s).
Before starting the engine the 2 connections on the heater box should be
the same as that of the thermostat housing. Start with the valve for
controlling the water going to the heater box in the OFF position.
Although a rising temp in the engine compartment temp may cause these
points to rise a bit you should be able to determine if the valve is
faulty as the temp of the water exiting the heater box would increase if
the valve was not shutting completely and conversely the temp shouldn't
go up if the valve was not opening at all. The heater valve on the MGA
has an area on the outside where you can see a flat spot at one end of
the ramp where the actuating arm is suppose to rest for a "full off"
position and you may need to use a screwdriver or something else to push
it to this position - it's not supposed to get into that position by
simply moving the heat control lever on the dash. In winter it's
supposed to be in the "not fully closed" position and easily moved with
the heater control lever on the dash.
Before starting, the metal for attaching the hose going to the top of
the radiator should be the same as any other metal under the hood,
perhaps even the ambient temperature. As the engine warms up there
should be a point where the metal part of the thermostat housing to
which the top hose is connected should show a rapid increase in
temperature, reaching the temp at which the thermostat is rated. As the
engine warms up the temp reading at this point will probably go up, up,
up as will the needle on the dashboard gauge. The metal connection on
the radiator for the top hose should reach that same temp and the temp
on the metal connection on the bottom of the radiator should show a
lesser temp as a result of the radiator cooling the water as it passes
through. You can also use the IR thermometer to check locations across
the radiator to see if there are any spots where it is considerably
cooler that other locations. That would probably indicate a portion of
the radiator NOT circulating any water - blocked??
After things start to warm up you can get the best indication of what
temperature reading your dashboard gauge should show by checking the
temp at the point where the gauge connects to the water pump - just
above the front dynamo mount and at the same time see what the dashboard
thermometer displays.
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