Yes good point, more air but only a small amount more air and the temp gage responds almost immediately to the increased 200-300 rpm. That is why I had always thought it was coolant related. Generally I find my engine very stable at speed and under load. I have driven the hell out of it and even on hot days hill climbing the temp gage barely moves hence the feeling that at low rpms there is a lack of coolant Final note is that my engine has a jobber water pump not the high$ volvo piece, I have never compared the vanes from one to the other so like the rest of this, this is pure conjecture.
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Patrick, '68 220, '92 Eurovan , '53 PD4104 (highway coach conversion).