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KALAMYKID, Shop Owner/ Tech
Category: Cadillac
Satisfied Customers: 4343
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I have a 2003 Cadillac STS. A/C works fine however, heat

Customer Question

I have a 2003 Cadillac STS. A/C works fine however, heat blows cold, both sides. Using a simple temperature test I have verified coolant appears to be flowing through heater core, not 100% certain but trouble isn’t leading me that way. I have checked all the actuators I could find, 2 on driver side and one on pass side of console. One is bad however it is located lower rear left/drivers side and appears to function for rear controls and only affects the rear when changed manually.
I even pulled the cabin filter and while it was nasty and corroded, I believe sufficient air should have passed though. Anyways I testing without the filter, just reattached the cover so air would not leak out the cabin filter access port. Same no heat.
The only symptom other that when I call for heat bower comes on, blows cold. I can change from vent floor and other setting w/o problem. On occasion, if car is warm, I turn heat on vent mode I feel warm air (not hot) for a few seconds then it blows cold, same as outside temp.
I am at a loss and could use some guidance. I am now thing of taking apart the entire duct work assembly under the dash. I rather not since access it and the various screws holding its various parts will suck.
Anyone who believes the know what’s wrong chime in, I will pay for info that leads me to a solution.
Submitted: 10 months ago.
Category: Cadillac
Expert:  KALAMYKID replied 10 months ago.

After you have warmed the engine up to full operating temp, do both of the heater core hoses under the hood get hot? Are they both the same temp?

Customer: replied 10 months ago.
Both warmed but did not feel the same temp. I have a remote heat gauge I can test later today. I would think a little loss is expected (couple degrees). How close should they be? Is there a value inline on this loop of coolant? Havent been able to find detailed manual at reasonable cost.
Expert:  KALAMYKID replied 10 months ago.

No there is no cut off valve for this system. The two lines should be very close to the same temp. What color is the coolant in this car?

Customer: replied 10 months ago.
Both lines are classic radiator hose black with metal sections. Appear to be OEM. BTW it will be several hours before I can get back to troubleshooting.
Expert:  KALAMYKID replied 10 months ago.

I need to know what color the coolant is in the radiator please.

Customer: replied 10 months ago.
Sorry, when I answered it was very late for me and I clearly misunderstood. I am an electronic engineer by trade and given a schematic I can fix about anything. Problem here is no schematic.
Using a remote temp probe, running engine to normal temp, one side measures 139 the other varies around 110, averages 30 degrees less. The coolant appears to be classic antifreeze green. I’ve only owned the car for a couple of months. When I first got it I had to add almost a gallon of coolant which may have changed the appearance if you are thinking the coolant is old and/or heater core is clogged. I too thought that however even at a 110-output temp of core, I would think the heat would blow somewhat warm. Unless there’s some probe that drives other electronics/valves to changes that.
Don’t happen to have a schematic/drawing detailing the cars HVAC system, do you? I subscribed to one online which helped locate the actuators but doesn’t do a very good job with images and technical descriptions of operation.
Expert:  KALAMYKID replied 10 months ago.

That is not good. The coolant should be orange, " Dex Cool". What probably has happened is the green coolant has left mineral deposit in the heater core and stopped it up. The good thing about this car is the heater core can be replaced with out pulling the dash. There is an access panel on the bottom of the hvac box that can be removed so you can replace the heater core. The temp of the two lines should be almost the same no matter if the heat is on or off. I am willing to bet if you pull the cover off of the hvac to get to the heater core the center of the core will be cool to the touch.

Expert:  KALAMYKID replied 10 months ago.

I am sorry that should have been you do have to pull the dash out to get to the heater core, but you do not have to remove the hvac box.

Customer: replied 10 months ago.
That’s been my fear, the lines to the core are not too hot to touch and the output is somewhat soft like there’s no real pressure inside. To be clear, I do or do not have to pull the dash?
I owned one of these STSs years ago and been looking for another that didn’t have extremely high mileage. I got this one when it just rolled 100k for just $2500, well under book. I expected some issues. Could be worse I guess. Couple more questions:
1) Would it be possible to blow it clear is I somehow injected the core with a strong cleaning flush agent and blow it out with compressor?
2) Do you know where I can get a good set of manuals for this car? Found 2 book volumes that state they are the OEM best, ***** ***** prices are around $130, don’t want to spend that much.
Expert:  KALAMYKID replied 10 months ago.

Trying to blow the heater core out is a wast of time and money, and may cause it to start leaking. And yes you will have to pull the dash out to get to the heater core. Your local auto parts store should have a haneys manual for your car, and it will be about the best you can do for not a lot of cash, but All Data Diy may be a better option for you. You will also need to flush the cooling system very good to get out all of the old green coolant,. It must be very clean before you go to refill it with Dex Cool.