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Running Cool


Jedi Warrior
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In years past, my car would always run with temp needle right smack in the middle of the gauge, except when extremely hot ambient or sitting idling for a while. In those cases it would move about 3/4 up the gauge (mid way between normal and full hot).

However, late last year the gauge began indicating the engine was running cool. Fully warmed and under normal driving, the gauge is only reaching maybe 1/3 up the gauge. Other than that, car runs excellent as it had before. Spark plugs look great too. No complaints.

So, what could cause a change? The only two things which changed are:
1) I flushed the cooling system. When I did this I refilled it with 55% water, 40% anti-freeze and 5% Water Wetter (water and Water Wetter are much more efficient in heat transfer). In the past I had used a 50/50 mix, no Water Wetter. Also, for the first time, I jacked up the front of the car, opened the radiator cap and "burped" all the air from the system. When I did this, it actually took a fair bit more to fill the system.

2) I switched from using 20W50 to 10W40 oil.

Even if these changes led to cooler running, I would expect the T-stat to keep the temp regular regardless of these two changes - I have a 180 degree T-stat.

I do recall this happened to me when I first got the car 12 years ago. A change of the temp sending unit in the thermostat housing was a quick and cheap fix (5 minutes and $5). I'll start there again and perhaps check the voltage stabilizer (although my fuel gas reads fine). Should I suspect a bad T-stat? I understood they fail "closed" which leads to over-heating.


PS: anything wrong with an engine running "cool", if that is the case?

Geo Hahn

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I think you want the engine to get up to full temperature.

A starting point might be to verify what the gauge is showing -- an IR thermomter can give you a second opinion, I use it right by the sending unit to get the same data point.

As I recall some thermostats are designed to fail open which could certainly cause what you are experiencing. You can kinda tell when the thermostat starts opening by carefully feeling the top radiator hose -- it should stay pretty cool as the engine warms up until the tstat opens. If it starts warming right away that might be a clue that the tstat isn't closed. Again, the IR thermometer can help assess the temps in case 'by feel' isn't senstive enough.


Luke Skywalker
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Went through same symptoms with the gauge reading as yours. Replaced the sender and all was well.


Run the car to temperature and shoot the thermostat area with a laser thermometer to find out if it is an actual lowering of temperature or if the gauge is giving you a false reading.

Geo Hahn

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Oh, one other thought -- could a failing voltage stabilzer cause the gauge to read low?

I think so, in which case it should also cause the fuel gauge to read low.


Luke Skywalker
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RJS said:
and 5% Water Wetter (water and Water Wetter are much more efficient in heat transfer).

Except the thermostat is supposed to control the temperature. The only reason WW would make a difference is if the radiator is not working properly (or properly sized) and the WW actually succeeds in improving the cooling enough to make a difference.


Jedi Warrior
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Traced it back to a failing Voltage Stabilizer. Replaced it with a new solid state voltage stabilizer ($13) and now gauge reads accurately. Gauge now reads just below mid point. This is gentle cruising at 60mph in 4th gear with outside temp ~65 degrees. Infrared temp gun confirms this with a reading of 165 degrees on the T-stat housing. Top of radiator was 175 degrees.

Actually, it was a bit of trial and error. First I put in a new water temp sending unit. Gauge went from 1/3 on the low side to 1/3 on the hot side. (Interesting that there can be a 33% difference between two temp sending units?! I also noticed with this sender unit the needle on the gauge moved off cold almost instantly - when normally it takes 5 mins. Obviously, it was wrong.) I then replaced the old voltage stabilizer with the new one and gauge shot up to 3/4 to full hot!! So...switched out new temp sending unit with the old one and all is back to normal.



Jedi Warrior
The new temp sending unit may have been a China product. Good thing you kept the old one.
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