Hi all...I own a 1991 SE. First and foremost, let me remind you all that I am a 19 yr. old female who doesn't know much (if anything at all) about cars!! So please be patient and bear with me..
Anyway, once my maxima is fully warmed up and after being driven for a few minutes, my maxima usually has GREAT heat...most of the time I don't even need to turn the actual fan on to feel the heat pumping through the vents - I just need to turn the heat setting all the way to the hottest (reddest) and the heat just cranks out.
Every once in a while I've noticed now, that for no apparent reason the heater (with the fan on or off) will actually start pumping cold air - Even though, just minutes before the heat was cranking and the engine is completely warm and has even been driving around for a while. The problem seems to be completely random. As a matter of fact, it was just doing the same thing last night (pumpin out the cold air even though everything was hot and the car was bein driven for over 20 minutes). But then this morning, after warming up my car and beginning to drive, the heat seems to be working fine again!!??!
My main question is: When something is going wrong with the heating system...will the heat just stop working altogether? Or will it gradually fade like mine seems to be doing??
Has anyone else ever experienced this problem? Or does anyone have any insight as to what may be causing this?
Any and all feedback is appreciated! Thank you!!

 

When it starts blowing cold air check the temp of the car. Is it normal or is it cold. Even after it warms up does the engine temp get cold?

Also make sure the lever on the left side of the climate controls is down. If it is up cold air will come through the vents.

 

This is fairly common in these cars. most likely it's caused by air pockets in the cooling system. your car gets the interior heat from running antifreeze/coolant through the heater core- it's basically a tiny radiator inside your air vents.. The problem is that air pockets sometimes get stuck in the cooling system and will wind up in the heater core. thus you have no heat for a little while. eventually it gets filled up again and you're back in business...


the other issue is there's a small valve on your firewall that regulates flow of coolant into the heater core. this piece may be malfunctioning as well, causing the water to stop flowing through- thus the heat not working right. It should ONLY be closed when you have the thermostat control set to full cold (all the way to the blue). otherwise, you should have coolant running through it all the time.


Unless you're mechanically inclined, both of these are going to be a bit of a bear to diagnose, but neither are difficult- you just have to know what to look for.

to check the valve I talked about, start the car and turn on your heat.. let the engine warm up and wait until it's doing its thing.. when it goes cold, get out and feel the small rubber hoses on the firewall in the middle. you'll see a hose come off the engine and run into the valve assembly. it should be hot (not burn your hand hot, but hot enough you know there's hot water in it). feel the hose on the other side- it should be hot as well.. If the valve is stuck closed, the side going into the firewall will become cold..

of course, this may also be seen if the heater core has air stuck in it, but in that case, both hoses *should* be cold.

good luck. hope that helps.