I'm confused, title reads it's hot under steam and cooler at idle, thread reads not as cold (warmer) when idle......
If it's as per title, I'd suggest you have air in the system possibly or as you've stated a blockage, any temp issues? So when you're driving and the pump is pumping hard there's plenty of hot stuff getting into the core, idle not so much. Or your pump fins have rounded away, same effect based on revs. Or maybe the heater tap is stuffed (actuator) and under pressure it opens, less pressure it closes. Missing thermostat? Does the blower fan come on too early? Just some thoughts.
The temperature gauge doesn't budge. It stays on cold most of the time. When using the heater it blows cold with a small bit of warmth. However, when the gauge finally does decide to work or show a reading (usually very low but sometimes normal operating range) the heater actually starts blowing warm air.
I also noticed the engine doesn't get hot (although I did wait a few minutes). It's like it's not heating up. What could cause this?
I already have a new water pump, cooling hoses and thermostat ready to be installed using quality radiator flush and 6L Nulon 100% concentrate (the red, and expensive, stuff) with 6L demineralised water. But the weather hasn't been kind.
Might be a few more days before I can do the job. Have a bunch of other parts I want to do at the same time as well.