I have a 2003 g35 coupe 6mt
I recently bought it (3 months ago) at an auto auctions so it was sitting for a while. The heat use to work when the weather was hot. Took it to the body shop to get fixed it sat for another month. When I got it back the heat was only blowing cold air and Started over heating I took it to the shop and got the air bubble out. It stopped over heating started blowing heat when I was driving but when id get to a stop light or let it idle it would start blowing cold air again. it also had a burnt smelt when I'd let it idle. And yesterday out side temp reached 23° f degrees and I got no heat unless I was at 3000rpm and would heat up again. Right now I just drove it and it was blowing little bit hot air temp is about 45 degrees but still blows cold air at idle. I'm planning on selling the this week or next and don't want sell it if the heats not working properly

 

You still have air in the cooling system.These engines are a pita to get all the air out so you will have to take your time and bleed the system quite a few times.