Heater runs inconsistently

Craigd2599 · Post by **Craigd2599** » 14 Jan 2016, 09:41

I have a 2007 S40 2.4 The heater runs inconsistently on very cold days. When I am driving at highway speeds, it blows hot air, at idle it blows cold air. The engine has 90K miles, and timing belt, waterpump etc have about 20,000. New thermostat and housing as well. Any ideas?

Post by **alschnertz** » 14 Jan 2016, 15:00

I'm not familiar with pretty much any Volvo after 2005, but the first thing I would do is make sure your coolant is topped up.
Check your owner's manual for more information.
http://new.volvocars.com/ownersdocs/200 ... .htm#pg190

reflectorfilms · Post by **reflectorfilms** » 08 Feb 2016, 13:29

I'm having the same problem in my 2000 V40. I started noticing that at higher RPMs the heat would come back on, and then when getting off the freeway it would go cold again, off and on like this for long drive periods. The thermostat reading was always at 3 o'clock until yesterday when it crept awfully close to overheating. The radiator fan is always on for 3 minutes or so after stopping the engine, even on very short drives. I'm going to check coolant level and then see if ECT/thermostat replacements are in order. Then my breaks failed this morning. What a day!

reflectorfilms · Post by **reflectorfilms** » 09 Feb 2016, 11:47

Haha, so I feel dumb, but I'm glad it was just low coolant. All my heater troubles and more importantly, the overheating starting to happen, was simply due to very low coolant level. I topped it up and the car is running properly now. So I have to really watch it, because I must have a leak somewhere, even a slow one. I'll be carrying extra coolant with me in the car now, checking the level every time I get gas, and watching the needle like a hawk.

I'm not a mechanic, but I have had the ECT replaced twice in this vehicle before. Someone feel free to correct me, but I seem to remember that the ECT sensor is inserted into the top of a coolant passageway. The coolant level can decrease so that it could be cooling your engine satisfactorily, however it's not full enough to come into contact with the sensor and that can throw things off, like your car thinks it's cold. I think this had more to do with my in-cabin heating going hot and cold due to hot coolant sometimes coming into contact with the sensor (acceleration on freeway, or maybe higher RPMs increasing pressure and rising level of coolant?) and sometime not coming into contact. But I imagine the thermostat would still open up to circulate coolant through the radiator. Of course, until my depleting coolant got so low that the engine wouldn't sufficiently cool. It just seems that once I topped up the coolant (added about 2 quarts) the overheating of engine and non-heating of cabin both got fixed.