Coolant Flow.....MK2 1.6D??

[ftm1776]

Is my thinking correct???

When the engine is hot and the thermostat is full open, coolant flows throughout the system;
through the heater core,

into and out of the fill bottle

and from the head through BOTH the radiator and the by-pass hose to the pump?

with the pump pushing/pulling the coolant.

[vanbcguy]

As the thermostat opens the metal disc on the 'back' of it begins to block off the hole for the bypass inside the pump.  So the bypass does see a reduction in flow as the thermostat opens, but the thermostat needs to be fully open to have much of an effect on the bypass.

There is always flow from the front of the head to the fill bottle through the small hose - this is how air is purged as the fill bottle is the high point in the system.  Flow from the coolant fill bottle joins the return flow from the heater core.  Only a small percentage of the coolant takes this path but it is critical for removing air.

There is always flow from the end of the head through the heater core, returning through the hard pipe along the front of the engine to the water pump - this flow doesn't change regardless of thermostat position.

Likewise the oil cooler gets flow from the front of the head.  Its return joins in to the hard pipe on the front of the engine and goes back to the water pump - this loop also always has flow.

Flow through the radiator is blocked by the thermostat until it opens.  Hot coolant circulating through the bypass / heater core loops heats the back side of the thermostat.

The pump is capable of moving a huge volume of coolant - far more than can "fit" through the bypass / heater core loops.  Generally speaking too the thermostat is at most barely open unless you're sitting in traffic on a hot day not moving.  VW's setup is a little confusing to understand at first but it works extremely well.  The multiple loops provide excellent coolant circulation preventing hot spots from building up in the block.  The stock air purge system works very well too (as long as the small hose to the coolant bottle isn't clogged!)

[ftm1776]

I always thought that the thermostat was either closed or open....so there is more of a throttling effect depending on operating temps. Someone mentioned this in another posting, but it didn't sink in at that time. Thanks.

Ah, the "little hose". Yes, mine plugged...no flow....heater core exploded due to overheat. Just recovering from that......now it gushes under my watchful eye !!!

No oil cooler on my model MK2, non-diesel.

[vanbcguy]

Yep, the thermostat is progressive. I test my new ones in a pot of water. You can see it slowly open as the temperature in the pot increases.

I've had the little hose get clogged before too. Thankfully it didn't result in any sort of failure though.

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