Heat blowing normal then cold

[skychair]

I have a 1999 Yukon purchased new, and have been able to diagnose most problems as they came up, But my blowers blow normal warm air (depending on setting) then will begin blowing cold air.
I then noticed that my heat gauge was ranging from normal.... almost to red line Hot, the interesting thing is that while is showing very hot on the gauge...it is blowing cold air.
The gauge moves back to normal and the air warms up?????

Hope I explained it clear enough for someone to offer an explanation.

I've driven a lot of years, but not had this happen before.
Thanks in advance for any suggestions, Appreciate it.
Ron

 

[lesterl]

I would change out the thermostat and the water pump.....

 

[treepete]

Bringing back this old thread.

I had this exact thing happen to me this morning.

My temp gauge got up over 220 then went down, then went back up... then down. I got to work and parked it, the temp normalized at about 185 or so, then the heater started to work.
I know for a fact that I am low on coolant, which I will address before I head home... but blowing out ice cold air??
It was just a stressful ride is all.

Can a thermostat sort of "stick" closed and not let warmed coolant flow through to the heater core? and also, if this happens again, am I risking damage to the engine by trying to limp it home?

 

[rockenthebowtie]

Lesterl, you know that I always back you up just like you do me and in some cases we cover areas for each other we my not of thought of at the time, we know but sometimes we didn't think of it at the time, Would you agree that maybe he should check for some type of a coolant loss that maybe causing air pockets in the cooling system like lower intake manifold gaskets, yes water pump, and or the quick diconnect fitting on top of intake manifold?

 

[Ice98]

with the guage going all over the place it may be a temp sensor, those are pretty cheap to replace, along with your thermostat.

There is a vacuum operated valve that switches the flow from the back heater loop as well, but I doubt that would be causing that sort of problem

 

[rockenthebowtie]

Ok, only a CTS isn't going to affect heater operation, that will cause starting, and drivability concerns as well, again will not affect heater operation or performance

 

[lesterl]

Ok, Thermostat closed would cause overheat.

Coolant is bypassed around thermostat for heater core.

Dash guage is in the front SIDE of the drivers side head, the CTS near the coolant neck is for the PCM so it knows how to fuel the truck.

The coolant being low could cause fluctuations in temps, I would top it off IMMEDIETLY.... keep tabs on it check for leaks, LIM leaking? Water Pump Leaking? Freeze Plug Leaking? Radiator Leaking? Heater Core Leaking (Front/ Back)?

 

[rockenthebowtie]

You tell it lesterl I knew you knew it.

 

[treepete]

yeah I was a gallon and a half low on coolant. I got a leak in the radiator (and one of the oil cooler lines to boot.) Ill start collecting parts I guess.
Just not in the mood for this truck anymore.
Poor thing.