Hello All.

I have a 93 Maxima and the heat seems to work fine until I begin to drive it. If I drive slow (about 30 mph), it will work fine as well, but if I do any highway driving at around 50 mph or more, the vents will begin to blow cold air. Actually, in addition, if I keep the heat fans on the 1st or 2nd speed it will blow fine. But once I turn it up, to the 3rd or 4th speed, it will get cold once again even after the engine is warm. The temperature gauge stays at normal temperature so I was guessing that it might not be the thermostat but I am still not sure. Can anyone possibly help me with this ASAP? Thanks in advance!

 

auto climate controls? or manual controls?

 

Could be a couple issues, but all are related to cooling system..

1. low coolant
2. stuck open thermostat
3. heater core clogged with engine gunk buildup.
4. thermostat control thingy on firewall is stuck near closed.

A THOROUGH engine flush (i.e. remove engine block coolant plugs and pull heater core hoses and flush every direction possible with strong water hose spray)

new t-stat, and coolant change should solve all 3. while you're in there, you can check #4 for proper operation.

That *should* solve your problem, and should be considered routine maintenance every few years anyway-- thus it certainly can't hurt the car to do it!

HTH.

 

For the archives: Anytime the coolant level has been lowered, an air bubble forms in the heater core or on top of the engine somewhere. I always have to make a sharp right turn up a steep hill to move it out to the radiator where it will burp out through the overflow. On flat land you might have to jack the front of the car up and rev the engine. If you ever take it to a shop, don't expect a mechanic to this either, it takes 15 min or so of driving with irregular heat output and slowly rising temperature to notice this bubble is there.

 

Manual controls..

 

Thanks I will check in to this!

 

Leaning towards open thermostat but could be any of the points Matt stated above.

What does your water temp needle do? Show too hot or too cold?

 

UPDATE:

Ok, so I never checked out any of the solutions posted because 1.. I have been busy, and 2.. the weather warmed up for the past few days so there was no need for heat. However, today I used the heat for a little while and I noticed that my side (drivers side) blows heat, but the passenger side blows cold air. Mind you I have a manual non dual zone climate control. So I have no idea as to why it does this. I will look more in to it tomorrow when I have the time. But does anyone have a clue as to what it is?