Quote:
Originally Posted by Lambrettaman
I've checked it with an infrared temperature sensor gun at various points on the hoses ...
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I shoot at the copper base of the temperature sensor. As I recall the fwd sensor in the head (driver's side) is for the dash gage and the aft one is for the EGR system. Keep in mind that the IR gun has a viewing cone larger than the laser dot. You want to completely fill its view with the surface to measure, otherwise it will average in lower temperatures such as from a wire or tube obstructing part of the view. Get it as close to the surface as you can. In theory, the surface material can matter. It is most accurate on flat black surfaces, though I seem to get the same readings on dull aluminum or copper nearby. My gun is the cheap Harbor Freight one.
But, another approach is to go by your T-stat. As the temperature rises, it should hold for a while at (or near) the T-stat setting. If over-heating, once the T-stat is full-open it will no longer regulate. If you trust your T-stat and you see it steady for a while at say 90 C on the dash gage, but your T-stat is an 82 C model then you know that is really 82 C. To trust the T-stat, place in hot water with a mercury thermometer (suspended in the water, not touching the pan bottom). I also shot the T-stat thru the water w/ IR Gun and it closely matched the thermometer. The T-stat setpoint is not exactly one value. That value probably means when it first opens. To open fully requires a slightly higher temperature. Control engineers term that offset "proportional droop".
I had a bad T-stat in my 1984 300D. The dash gage showed it settling at 60 C. Tested in water, it opened a bit sooner than a new one (and a Chrysler one I had, all same set-point) and opened more sluggishly. Replaced it and the car then ran at a perfect 82 C (slightly warmer in summer due to offset above). Had I just done the hot water test without precise measurements, I would have thought "works OK". I recall I posted the resistance values for the various temperature sensors on the engine a few years ago (search).