2004 G35
I noticed I had a coolant leak when I saw smoke from it burning off the engine. I took it into one mechanic only for them to fix stuff other than the main issue and I didn't realize right away (unless they did and I somehow sprung another leak). The heating goes out about 300 miles later and I looked online that it could be low coolant. I look in the reservoir and couldn't see a drop. I filled it up to the max line and things seemed fine. Two days later the heating stopped working again and coolant was burning off of the engine again. I take it to a second mechanic and they fix the leak only for me to notice after they closed that the heating still doesn't work right. It only cranks out heat when I accelerate whether it's in drive or parked. While idle it slowly turns to cool and then frigid air. I had been thinking it was a clogged heater core but the vehicle has 167,000 miles and the system was flushed at 125,000. Bad heater core or air in the system?

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You might want to follow the troubleshooting steps in the FSM
Pressure test your cooling system
Replace radiator cap
Air in cooling system?
defective or failing water pump?
Incorrect coolant?
Replace hoses and clamps

Telcoman

Could be an air bubble in the system

mac, isn't that a VQ motor? Those probably got the water pump INSIDE the timing case , in the crankcase oil , and I believe the design is a DOUBLE outer O-ring (possibly double shaft seal too ) and when 1st shaft seal leaks , it dribbles in-between - the center both large outer o-rings - down and out to the pavement through a "dribble passage " in block.Very expensive time consuming fix. Couldn't have been done by mechanic yet , going by the EXPLANATION , and probably "steams-off-dry" quickly and not so noticeble yet causes all the above problems! Water pump drizzle -drippi 'n Constantly causing a low coolant level, G's accelerating allows coolant to temporarily fill the heater!