Heater works, then it doesn't.

[blk87brd]

I've a 1999 Yukon SLT, 70k.
This morning the heater started to act up. It gave heat for the first five minutes or so than started to blow cold air. The rad is full, the temperature is normal, the heater blower works normally, the electronic controls switch the heater through the various functions normally. After driving several miles, the heater started to put out heat for a few minutes then returned to blowing cold air. It did this several times throughout the morning.
Usually when something like this occurs it can be traced back to low fluids, bad thermostat, or controls not working properly but they all seem to check out fine.
Anyone run into this before?

 

[treepete]

not me... man it really sounds like stickey thermostat.. but, I thought that once it got to running temp, it would stay open. Its weird that you have back and forth with the hot/cold air.

 

[blk87brd]

You are right, my first thought was a sticky thermostat. But the guage never moves. I let it sit for roughly an hour before checking the rad. But, just thinking about it, the rad felt cooler than you would think it should have after sitting for an hour. Maybe it is the thermostat. Hummm. Doesn't explain the off again, on again though does it?

 

[treepete]

yah I think that maybe you could start with the thermostat...its possible that the thermostat is stuck open. Thats my guess, anyway.

 

[NoEcm]

Here are some checks:

-Heater control valve. It is vacuum operated and the valve could be bad or there could be a small leak in the tiny vacuum hose.

-With the car at temperature, you could manually hold the valve open with a zip tie and see if you get heat in the passenger compartment.

-With the valve manually held open, check the hose going into the valve. Is it hot? Check the hose coming out of the valve. Is it hot? If yes to both, hot water is circulating through your heater core.

-If hot water is circulating through your heater core and you are not getting heat in the passenger compartment, then one of your actuators is bad.


(if your thermostat were stuck in the closed position, your Tahoe would overheat. No coolant would be passed to the radiator. If it were stuck open, you would still get reduced heat and would notice it on your temp gauge)

 

[blk87brd]

I agree, after taking some time to think about it. I was thinking the thermostat was one of the new fail open style, but the guage has been pretty accurate in the short time I have had the Yukon, so I should have seen the temperature drop if it were stuck. What puzzloes me is why the on again off agian . Thanks the the advise, I am going to look at it this weekend and figure it out from there.
Regards
Tom G.

 

[treepete]

Come back and tell us what it was, or if you find anything.

 

[blk87brd]

Last evening on the way home, I kept one eye on the temperature guage. It huffed and puffed about 10 degrees, slightly above and slightly below normal. I have never seen it do this before, although it is hard to notice. I am guessing it has a fail open thermostat that is on it's way out. I ordered a fail safe 195 degree last evening and will put it in this weekend. No sure that is waht it is but can't explain the huff and puff any other way.

 

[panther5]

i had a similar problem when i first got my tahoe, worked absolutely fine after i replaced the thermostat:thumbsup:

 

[blk87brd]

So here the deal. Thermostat is fine. I've traced the problem to where NoEcm is at, however, it is not there. It is with a switch on the firewall directly behind and above the inline vacum operated heater control valve.
There is a switch with a vacum line in, a vacum line out to the heater control valve and with an electrical wire coming into it.
With the vehicle running the vacum valve is closed, held there by the vacum. When I unplug the vacum line the valve opens and I get heat downstream from the heater control valve.
With the vacum line unplugged all of the heater controls inside work as they should, as does the AC.
My guess the firewall switch (I don't have a name for it) is gone. ?? Or does anyone have other ideas.
Thanks NoEcm you were right on the money.

 

[hippiechaser]

Bump for update on same problem- '97 tahoe
1st post noobie here, but with the same problem as blk87brd. Heat comes on for a short time, then cold air. Turn off heater, and turn on again, heat comes on for a short time and then off. blk87brd, what finally solved your problem? I am going to attack it in the next few evenings/weekend, but thought that the forum may be able to give some directed advice. Remainder of this thread has been a great start though!

 

[blk87brd]

It sounds like your problem is a little different than mine, Hippiechaser. My would come on then go off and stay off. If I understand your problem, it comes on, goes off, then comes on when the heater is turned off and on. Mine did not change when I turned the heater on and off, however, I fixed mine by bypassing the valve on the firewall, passenger side just above the right bank exhaust manifold. I took the vaccum lines off, joined them together and everything works as it should. I though I would have problems with either heat or AC but that was six months ago and everything seems to work the way it is supposed to. I bought a replacement valve but haven't put it in. Hope this helps.

 

[hippiechaser]

Blkbrd, thanks for the info. Checked the system out and noticed (to my suprise) that my coolant tank was empty. Hmmm. Oil was changed recently at my garage and they didn't mention anything, and I trust the guys there. Well, I filled the tank and the radiator (both were bone dry) and proceeded to drive it and PRESTO heat was fixed. After about 30 minutes of driving I stopped and was pleasantly suprised to find a coolant leak. Awesome. Some more exploration showed that the hose that takes coolant into the block (near the top) has the "valve" (crappy piece of plastic) cracked and broken and actually broke off on the connection with a small amount of pressure. Looks like fun to find, but hopefully replaceable without garage help. I will keep you posted, and thanks again for the input.