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Maybe it's an old myth; or maybe it's one side of an ongoing debate. I read the faq you cited - on the commercial website that promotes a product line - and I looked at other sources and really what I found is a lot of articles that support your assertion and a lot that support mine.
As far as the car that started this whole discussion, I agree that there's a good chance that the radiator itself is a likely cause. It's also possible that his used water pump, with the cast impeller, is more effective than the new one with its stamped impeller. Those stamped versions are proven less effective. Unfortunately, the only cast version on the market (that I know of) is from Italy and pretty expensive.
I don't buy the notion that the fan used by Volvo was designed for a Swedish climate. These cars were exported all over the world and worked reliably in a variety of climates. I drove my own 444 with a 2.2 liter B20 and the original B16 radiator with fixed 4 blade fan in ambient temps greater than 100 degrees, and it never ran hot. Not at prolonged idle, not at freeway speeds. But that, like most of what's to be found online, is simply anecdotal.
I have experienced, firsthand, a Volvo B20 that ran hot without its thermostat and then ran within appropriate temperatures once the thermostat was installed, without any other changes to the cooling system. I won't say that this will always be the case, but others who work on and with Volvos have reported the same thing. That's not anecdotal, it's just what happened. My experience differs from your own. No big.
I don't think we can assume that your heater would work better or worse, as the coolant temp into the core is (or should be) regulated by the heater control valve. It's not just a simple 'open/closed' thing. Or if it is, it's damaged.
Re: all water pumps: They aren't all set up to increase in speed as engine speed increases. Belt driven pumps certainly do exactly that, and that's a function of their drive system. If we follow this logic, why are belt driven fans set up to increase in speed when engine speed is increased given that there's no need for a fan if you're moving at more than 35 or so miles per hour? I guess if you wanted to hold the engine at sustained revs without actually going anywhere, but there's not much point in that. Maybe the water pump and fan speeds are simply a function of their drive mechanism.
Electric water pumps typically offer a given flow rate in cfm that is unrelated to engine speed. It's safe to assume that the electric pumps are intended to move enough water to keep things cool when demand is highest.
Bottom line: I hope the original poster gets the cooling issue addressed. I don't think that enlarging holes in the coolant tube nor the head gasket are going to help, and I do think that having the radiator tested more thoroughly than running a hose into it is the very next thing to do. Probably a good thing to have done well before now.
Best regards,
Cameron
Portland
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