Why

[92EcoDiesel Jetta]

did VW put the water pump on the bottom of the engine? Most water pumps are located higher up. Let's hear your thoughts. There must be reasons why VW did this.

[Zulfiqar]

I think mostly because its an entirely separate unit - if the impeller housing corrodes, chuck it out and install a new one. AND most importantly it does not cavitate at all, because all coolant will always travel to the bottom naturally and secondly it also has a suction port directly from the coolant expansion tank. No stray air bubbles or empty pump any time.

On the other hand, lets take a toyota 2C diesel, 2.0 litre IDI - its coolant pump is bolted in the middle of the engine, at the upper edge of the block, it also serves as a mounting point for the IP - thirdly as japs think that half and inch above the top coolant pipe is just enough for the radiator to fill - it cavitates like hell if you climb hills or act silly when filling coolant. This causes the block to corrode on the impeller's concave face in the block, Then you have a permanently ineffecient pump. And when it fails wohoo slippy timing belt coated in coolant, not pretty

[92EcoDiesel Jetta]

Those sounds like good reasons. If the pump never cavitates, it should have a long useful life right?

The thermostat is also on the bottom of the engine. What actually opens it from a cold start? I'd imagine it's not coolant but engine oil since there is no coolant flow at cold start and heat rises.

[Zulfiqar]

when the thermostat is closed off, the coolant is circulated in the engine itself - through the bypass hose, travelling from the top radiator hose flange to the bottom i.e. in the coolant pump port below the suction pipe coming from the expansion tank. when the thermostat opens it blocks the bypass port and circulates coolant through the external cooling port i.e. radiator lower hose.

its pretty simple and very effecient