I have read all the threads about this and I’m still a bit stumped. This is my 4th Rover and first issue I can’t figure out. Temperate is normal and occasionally, only when I turn on the heat, the temp will rise. All my hoses get rock hard and the heat blows cold. After a minute or two (while panicking I’ll blow my new head gasket), all the hose pressure goes back to normal and the temp gauge drops to normal. Then the heat blows strong. The heater core is all I haven’t replaced. If done the head gaskets, radiator, hoses and cap. Fan relay, thermostat. The radiator is aftermarket. I have a coolant pressure tester and can’t get it to pressurize because of the tank overflow tube. I didn’t do the water pump yet. I tested it and it pushes tons of water. Also the pressure building is from the water pump. Blockage somewhere? Any ideas?

Try to park on an incline front high, Remove resovoir, raise 1 foot above engine (use hangar to hold it), turn on heater, and start car. Open bleeder screw, and add coolant if n cessary. You may have air in the system, or faulty thermostat.

Heater core could be blocked

let truck warm up and check temperature on both hoses going to heater core. If one is cold and other is hot you have blocked heater core

There was air trapped in the heater core. After the incline, I got the air to pass and it lowered the coolant. After a re prime, everything seems to be fine. Thank you! Very much.

Cheers! Keep an eye on it for a while....

Did you leave it Parked that way overnight? Was the bleeder screw on while it was parked?

I've been chasing the exact same issue for 2 weeks now and have replaced the tstat, all the hoses and topped off the system, parked on an incline while running the engine for 30 minutes and left the vehicle parked on the incline overnight. My avg temps range from 195-208 and will suddenly spike to 215-225 while I'm driving and the heat will only blow cold. I've found if I can hit 3500-4500 RPM for a short burst (1-1.5 seconds) it will cause the temp to drop back down to 195-208 suddenly and stay there. I think there is air trapped but thought it would have escaped after 2 weeks of troubleshooting and the incline parking.

Anyone have any advice? Next up I plan to replace the water pump and, if that doesn't work, the heater core.

Thanks, Dave03S. I'll report back over the next few days. Hoping mine is also just air still trapped in the heater core.

Update: Fixed! Parked uphill overnight and topped off before the morning commute. Temps stayed between 198-208 without any spike. Topped off again after work before drive home and temps stayed 195-202 consistently. Will do again in the morning but I'd say we're in the clear. Thanks for all the help.