As you may know, I have a mildly modified B230FT in my 90 wagon. It's always kept it's cool, even on the hottest days, but it had developed a habit over the last 18 months of potentially overheating (I always catch it and pull over) on long, steep grades at low speeds (60 km/h or lower, heavy throttle/boost). It'll get so hot the cap will vent, but the electric fan always brings it down when I pull over and let the engine idle away.
Now, the cooling system on this car is perfect, new water pump, coolant bottle, green cap, coolant at correct ratio, new thermostat and giant electric fan.
This has puzzled me, as the engine will keep cool all day, even with the AC on in stop start traffic, a sure place any cooling system issues will show up.
Now, I know for sure this was not always an issue with this car, I've climbed serious hills in the past and not even had a hint of an issue.
So this winter I've been pondering the slow warm up of my car and why it will drop the coolant temp on very cold days at highway speeds.
I fitted a new aftermarket thermostat about 18 months ago, as part of my usual preventative maintenance regime, when changing the coolant.
My theory is, that the aftermarket thermostat is not completely closing the bypass below the thermostat, preventing full coolant flow through radiator when hot (due to some coolant bypassing the radiator and flowing back through the bypass ) and allowing over cooling when cold (due to the bypass staying a little open and allowing coolant to flow through the radiator when the bypass normally wouldn't allow it). I think the thermostat is just too short in it's overall length.
I am testing this theory in the next week or so by fitting an OEM thermostat, and will report findings.
I posted this up because all to often we see threads in here about cars with perfect cooling systems overheating randomly or over cooling.

Regards, Andrew.

 

Yeah Andrew I only use OEM thermostats..aftermarkets give me nothing but problems. Also, I am assuming you have put a IR temp gun on the thermostat and rad while the car is in the hot zone? So you've eliminated a bad temp circuit board or bad rad? If you haven't throw the IR gun on there, this will tell you immediately if the thermostat is working properly or not as well as your gauge and your radiator....I don't do anything or assume anything until I've took an accurate temp reading..Good luck let me know if I can be of any assistance.

 

I know the thermostat is doing it's thing, I've pulled it on a hot and cold engine.
Like you, I don't assume anything either, I've been chipping away at this intermittent issue for a long time now and it can only be thermostat related.
I trust the gauge as accurate, my IR thermometer says so.

Regards, Andrew.

 

Well, the new thermostat is in and the car is getting up to operating temperature in a third of the time. The new thermostat's bypass closing plate was about 4mm (1/8") higher compared to the OEM Volvo thermostat it replaces. However, the OEM thermostat's bypass plate was marked on the bypass sealing surface, so it had been sealing, when open.
Also, the new thermostat's cylindrical body protrudes about the same amount LESS into the cooling gallery below the thermostat mounting face, so that could help improve flow through the stat.
Will be interesting to see how the new stat handles heat.

Regards, Andrew.