[ErroneousBosh]

It's not variable, it just pulls in and out with a solenoid, either fully surrounding the pump or fully retracted.

I hadn't thought about the thermal shock thing but I did wonder how it could possibly help the coolant warm up if it's not circulating at least through the block. The engine doesn't warm up evenly.

[docjay]

Oh wow, it’s upsetting that it’s not variable. The total system might hold 2x (or more) of the amount of coolant in the engine water jacket. When the coolant around the engine gets up to ~200 degrees and the pump suddenly snaps to 100% it’s going to flood the engine with coolant at ambient temperature. Imagine getting the engine up to operating temperature then dropping it into a swimming pool; even in the kitchen you find out that’s what causes pans to warp and glassware to shatter, and the engine is just a funny shaped pan with bolts.

My only other guess is that it’s not 100% on/off, like maybe a bit is still allowed to flow when “off”, but then it would still need to bring the entire coolant mass up to temperature so I’m not sure how that would be a benefit for faster warmup. Either there’s some clever engineering I’m not seeing, or you’re buying a few points of regulatory compliance for them by needing to replace head bolts and gaskets sooner.