Hi,

The problem is that the temperature drop doesn't happen "briefly"... the
temperature guage stays reading low until I turn off the heater again.

If the thermostat starts opening @ 85C (approx) and is fully open @
105C, I'd assume that the 105 is where the "normal" spot on the
temperature guage.  Add the extra cooling of the front heater core, the
temperature drops and the thermostat should start to close & reduce the
amount of water flowing to the front radiator, thus increasing the
temperature of the water circulating in the engine & keeping the
temperature guage at the "normal" level.

I'm leaning towards the list wisdom about a malfunctioning thermostat.

Thanks for the feedback!

Cheers,
Steve W.

On 07/09/2015 10:16 AM, Dan N wrote:
> I think it's normal behavior. When you open the front heater, cold
> coolant in the heater core flows to the radiator, warm coolant from
> the radiator flows to the heater core - an exchange - and you see
>  small dip on the coolant gauge. This will happen briefly until the
> warm coolant in the heater core reach the same temp. as the coolant in
> the radiator.