Before I go deep down the rabbit hole on this I want to verify some things. My 2000 Disco isn't heating the cab in cold weather; I verified the mixing door and it's operating as it should; I did install an inline thermostat would that have any effect on getting hot water to the heater core? I have a feeling it's just got a plugged heater core and I may be able to backflush it to get it heating but want to make sure this modification didn't have any negative effect. I have two other Disco's with blown motors at this time; once I get them running I'm planning on installing the inline thermostat on them as well along with the ultra gauge like I have on this one.

 

If both heater core lines from the engine are the same temp you probably don't have a clogged heater core. Usually with a clogged heater core the inbound line will be hot, and the outbound will be cooler. When I had an inline thermostat on my 02 Kalahari I didn't get heat output like I did with the factory thermostat.

 

I have installed dozens on personal vehicles with no ill effects on the heat. The most important thing is to make sure you have done the bleeding per the inline instructions to make sure you completely flush out the heater core. Best is right, your heater core lines should be hot to the touch with the engine warmed up, especially the return that goes from the firewall down to the lower wye. If that is not hot you either have an air bubble or a blocked core. It should be too hot to hold your hand on once the engine is up to temperature.