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Discussion Starter · #1 · (Edited)
In the past, when driving the 142 at highway speeds when the ambient air temperature was in the -5C or lower range, the dash temperature gauge was indicating that the engine was not coming up to the design operating temperature (82 - 90 C). I have an aftermarket electronic dash gauge which is marked with numbers as opposed to colors and I have confirmed that it is accurate within about 2 C.

We had a rather dry winter here so conditions dried early allowing a resumption of the vintage Volvo driving season with air temperatures that were definitely on the cool side. Same experience as noted, driving at around 80 km/hr with outside temperatures in the -5 C range, the dash temperature gauge is reading in the low 70 C range. This left me with the do I have; a faulty thermostat, a thermostat which is leaking a lot in the closed position, or this is just the way it is?

It was a nice day today so I decided to pull the thermostat and check its operation. The thermostat is a Calorostat 82C. I did the immerse in water bath test with a thermocouple to measure the temperature right inside the top part of the thermostat housing. The thermostat starts to open around 81-82 C and is definitely as open as it gets by 90 C. So, the start of open and fully open temperatures appear to comply with the values for the type 2 thermostat in the Volvo service manual.

I let the temperature of the bath drop slowly and the thermostat does not appear to go fully closed until about 78 C, so there does appear to be a little hysteresis or the thermostat is just very slow to respond as the temperature is dropping. Out of curiosity, I checked the depth of the by-pass tube and the movement of the flap on the bottom of the thermostat and the by pass is open when cold and definitely sealed off when the thermostat is at 90 C. It was not practical to determine at what temperature the flap would close off the by-pass because as soon as you pull it out of the bath the thermostat temperature starts dropping and the flap starts moving. But, I am confident that when hot the by-pass tube is as closed as its ever going to get which is important for cooling under high air temperature conditions.

With the thermostat in the closed position I pulled it out of the bath and confirmed that the center plate is closing tightly with the surrounding flange. So, with the exception of the jiggle valve there is no material leakage through the thermostat itself. I examined the thermostat gasket and there were no obvious signs of leakage around the gasket.

I am left with the conclusion that the thermostat is operating as designed and that I do not have an obvious leakage problem. I am further left with the conclusion that if you are running the heater at max on a really cold day at speeds over 80 km/hr you will struggle to meet the design operating temperature (82 C - 90C) for the engine. Since the measurements suggest that my coolant temperature was running below the fully closed temperature of the thermostat, the old school thought of putting in a hotter thermostat would not appear to offer a solution. I speculate that on the 140, at highway speeds there is enough cold air flow through the rad and the engine compartment that convective cooling of the engine block by ambient air combined with the interior heater operation is preventing the engine from maintaining its design temperatures.

A confounding factor may be that I am running a 4.57 rear end with 185/60 profile tires (smaller rolling radius than OEM), so the engine is running at an engine speed about 12% higher than original design. That higher engine speed equates to higher water pump speed which may contribute to some over cooling effect. As an aside, I am really thinking that with the 185/60 tires I have to ditch the 4.57 rear end and consider going to a 4.1 rear end which would get me almost exactly back to the engine speed with the stock m41 4.27 rear end with OEM 165/75 tires.

Volvo used to sell cold weather black vinyl closures for the 140 that would clip on to the front grill. The closures had little flaps you could open for warmer day operation. I think these probably worked because they reduced air flow through the engine compartment and convective cooling of the block. If I want heat on cold days I think I am going to have to find one of those old grill covers.
 
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