[1Bishop]

I recently replaced the Radiator on my 98 RR after the water pump failed. While I was at it I changed the Thermostat too.

Now I'm getting a strange over heating problem.

After driving about 30 minutes at normal speed and temperature the water temp gauge heads for the hot side. After three short trips to test it I've noticed the heater changes from hot air and starts blowing cold air.
This morning we drove 20 miles and stopped to drop off some food for friends. the air temp outside is 40 degrees, so the heater was working great. The suddenly I feel cold air blowing on my leg from the heat vent and look up at the engine temp has started to move to the fight toward hot. I cut off the engine before it overheats and I let it stand for 20 minutes.
I can restart the engine, the engine temp is normal and the heater vent is blowing hot air again. Twenty minutes later and 20 miles it starts getting hot again and the cold air is coming from the vents.
I need some help with this mystery.

 

[1Bishop]

The water level is at the normal mark cold, and forgot to say I replaced the radiator cap too .

 

[RRToadHall]

Heater getting cold is a classic sign of not enough fluid circulating. You most likely have air trapped in the system.

 

[1Bishop]

I ended up replacing the new thermostat. I don't know if I was low on coolant but after I installed another "RAD" thermostat., I measured the coolant going back in. 2.7 gallons of fluid and that's all it would take.
When the 98 RR was overheating before Thanksgiving, as I was pulling over on the side of the road a couple of times before I shut off the engine the temp gauge went back to normal. I Had to stop 5 times on a 25 mile test run last week but twice the gauge went back to normal before I shut it down.
I waited each time about 15 minutes and restarted, and move a couple of mile it would get hot again. I babyed it home.
This past Monday the new thermostat came in from the RAD factory and it was installed Monday evening.
We made the first long run today without any overheating problems. The RAD brand fixed it.

The thermostat I removed was just 30 days old. I ordered the RAD brand through Advanced Auto parts in Lafayette Louisiana.
It may have been air in the cooling system too. The RAD thermostat cost $80.
I was getting tired of changing thermostats. Back in January 2018 I replaced the first thermostat. It only lasted 11 months. Three thermostats in 1 year. They don't make them like they used to.

 

[Reinhoud]

Most likely you have air pockets in your system.

Fill coolant with a running engine, with the aircon off and the heater flat out.

 

[RRToadHall]

With all the stuff you have replaced in bits and pieces you seriously need to drain the coolant in to a clean container and do the fill process over. As noted in RAVE DO NOT DRAIN the block.