dmoss90660

Although I did open the bleed valve, there must have been air in the system since this morning as I checked on the car it was low on antifreeze. No sign of leaking underneath the car or around the thermostat, so it must have been air. I refilled it, again making sure to open the bleed valve, and went for another drive. The temperature was stable and varied between 190 and 212 and produced plenty of heat. Thanks for the suggestion!

I've got a 2007 Fusion with 45,000 miles on it. I did a coolant flush last September, but I did not flush the heater core. Now that winter has arrived, I find that the heater is always cold, and I regularly get the P138 coolant under temperature code. I figured it was a stuck thermostat, so I just replaced it (plenty of fun! turns out the thermostat certainly needed replacing). I was hoping to have a victory lap, but all I really have is more confusion. There aren't any leaks and the coolant reservoir is filled up to the cold level. With the heater off, I went for a 50 minute drive with my scanner plugged into the OBD port. Prior to the drive, I let the car idle about 20 minutes and the coolant temperature was 230 degrees. Once I went out for the evening jaunt, it was 21 degrees outside and the coolant temperature was all over the place. My speed was 35 mph (except the few red lights I had to stop at), yet the coolant temperature was fluctuating wildly. One moment it would be holding steady at 234, then suddenly jump up to 259 (it looked scary on the temperature gauge--clear up at H!), yet continuing to drive at 35 mph for about 3 minutes and the temperature dropped to 219. Driving on a long stretch of highway with no stops going 35 mph, the temperature would again vary between 234 and 259. The few times I turned the heater on it was very cold (although not as cold as turning the temperature control all cold). Now the really weird part. Driving home I decided to leave the heater on to see if it would make any difference. The coolant temperatures now varied between 219 and 240. Yet any time my rpms were over 2000, the heat was warm. If I was under 2000 rpms, there was no heat. If the parking brake is on and car is in neutral at 2000 rpms the heat is still cold; if I take the parking brake off leaving the engine rpms at 2000, the heat is warm. Driving down the road produces heat if the rpms are 2000 or higher. I'm thinking the problem now might be a clogged heater core that I will need to flush. Does that sound like a valid diagnosis? Any other ideas?