problem w/ '99 suburban - looking for confirmation

[Mikey von]

My wife tells me today that the heat did not work in her burb today. She said it was warm and that the heat blew cold for a long time and then suddenly started blowing warm right at the end (it was around 20 degrees today).

I took it for a drive tonight to check it out. It warmed up really fast, but it had only been sitting for a few hours. The temp went higher than normal, up to around 200 (it usually sits at 170 or so). The heat blew real cold. After a while (10 mins or so) the temp dropped way down, heat blew semi warm, and then the heat went quickly back up to 200. It did this about 6 times, then after dropping it leveled off at its normal 170 and the heater blew warm.

I believe this is the thermostat that is sticking. Am i correct?

 

[longbedder]

I believe this is the thermostat that is sticking. Am i correct?

Yes.

 

[72blazerguy]

I believe this is the thermostat that is sticking. Am i correct?

Yes

longbedder beat me to it

 

[Mikey von]

thanks kieth and 72!

 

[bigblock72]

Low coolant will cause the exact same symptoms.

 

[swettysblazer]

Try flushing the heater too, but it is most likely the stat.

 

[diesel4me]

If it has a hot water shut off valve in one of the heater hoses,it might be defective or losing vacuum to it,causing it to partially shut or close completely intermittently too,restricting the flow of hot water to the heater core..also if the "blend" door doesn't stay closed,cold air will be let into the heater box,colling off the heat dramaticaly..usually once a thermostat fails to close fully it wont allow you to get much heat no matter how long it is running,99% of the time they fail in the open position as a safety feature..