A couple months ago my car had a new engine put in because of a spark plug blowing up.

Anyway, now that its starting to get cooler, I have noticed lately that my heater takes a pretty long time before it starts blowing warm/hot air. maybe a good 10-20 mins of running.

Today, I was sitting in some traffic, and my engine went up to 5 bars, before I got off the road and got some air flowing. Engine cooled off right away back down to 3 bars.

Anyway, could this behavior be caused by air in the coolant system? I noticed my coolant is below the "low" mark on the reserve tank also, but I can still see it on the bottom of the tank (when the engine is cool).

 

What you see at the bottom of the tank is what the cooling system could not pull in because the pick up tube starts to pull in air at that point. Looking at a near empty tank is a bad way to determine the cooling system level. When the engine is cold, take the rad cap off and have a look. You may be surprised to see nothing in there. Make sure to fill it with the proper kind of coolant for your year of car. First open the bleed screw near the front of the intake manifold. Fill the rad right to the top, (close the bleed screw when fluid comes out), close the cap, fill the tank. Run the engine till operating temp. Then remove the upper bleed screw near the firewall till coolant comes out, close it. Drive the car every day till the engine gets up to operating temp. Each morning check the coolant tank and top it off to between the two lines. After a couple days, it should stabilize.