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EPC shows two different coolant hose arrangements for late-prod years - one which has two hoses to the heater valve as normal, and one which has some sort of a bypass arragnement. What is that all about?

I haven't found any evidence of my 92 car having anything other than the regular setup (and that's what I'll be putting back when I replace a few hoses) but I'm curious as to what the bypass setup is all about and whether it was a market-specific thing like when APC was and wasn't introduced in turbo cars, and when ignition systems changed from points to reluctor to hall (I've had all three in my 900's).

It looks like a thermostat-type setup from studying the EPC diagram.

Craig.
 

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Late cars had a spring loaded heater bypass system in the hoses before the heater to ensure adequate circulation of coolant through the head before the main thermostat opened. Earlier cars had this built into the heater valve to allow bypass when the heater temp control was closed. Many people do without it because it it can cause problems. There are many posts on this.
 

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The notion behind it as I was informed is that, once water velocity creates enough pressure to overcome the break down point of the inner spring, most of the water is directed BACK towards the engine.
The water flow to the heater is not halted, but regulated to serve enough water to provide adequate cabin heating (if selected).
For what it is worth, in winter my car feels slightly warmer at idle than at road speed.

Older bypass valves tend to leak and the internal plunger can disintegrate and waste away, to which, the valve does nothing.

I changed mine after the leak became more than a 'dripping' flow.
I chased down a new unit before the dealer got on the last stage coach out of town, never to return.
 

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The later spring-loaded bypass valves are well known to give problems which result in the well known "cold heater syndrome". The spring loaded valve tends to gunge up and stick open, causing the hot water to bypass the heater all the time, resulting in no cabin heat. It is possible to take them apart to clean which normally solves the problem.
 
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