Mechanics

REAR HEAT STILL NOT WORKING PROPERLY.

1999 GMC Suburban • 145,000 miles

I have a 99 GMC Suburban K1500 4x4 5.7L and am still having problems with the rear heat. I replaced the heater control valve due to the plastic snapping when I moved one of the hoses. I got the hcv solenoid working so it draws vacuum for the hcv. I flushed out the rear heater core and it started flowing fine. Now, here is the kicker. I connected all the hoses and when the heater control valve is actuated, the heat up front goes from hot to warm or cold. The heat in the rear is still blowing cold! When the hcv is off, the front goes hot again. The only other thing I can think of is that the line(s) to the rear heater core are plugged, unless the rear heater core is plugged again. The system is just not that complicated of a system, but it is frustrating me that the rear will not blow hot. Any thoughts are much appreciated. Thanks.
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Dcplace2007
October 24, 2011.



Make sure the flapper doors that control air flow within the air box are moving properly. With the engine hot and the heat turned up, see if the hoses to the rear are hot.

Rivermikerat
Oct 28, 2011.
I took all the things apart to expose the heater core and watched the actuators work, and they moved properly. I felt the heater hoses going to the rear and they are not hot. I cannot remember which hose leading to the rear hc is hot, but only one is--I think it is the hose coming off the quickconnect. If I feel the two hoses at the rear leading into the hc, they are not hot. I was thinking air was trapped, but now I have no idea. I am really at my wits end on this thing. What could be causing hot coolant from not going through the rear hoses? The heater core is not clogged as I flowed water through it. Does that mean it is still not bad? What else could it be? I appreciate any help to get this resolved cause we are in for a cold winter.

Tiny
Dcplace2007
Oct 28, 2011.
Check the valve controlling coolant flow to the rear. Something is blocking the flow if neither hose is hot. Valve not working, plugged hose, or something similar.

Rivermikerat
Oct 30, 2011.
Ok, got home from after driving across town and the hose farthest to the rear is hot, and the other line is not. Also, my upper radiator hose is very hard now.I sure hope there is not a head gasket issue now. A service place in town told me there is another valve in that line at the rear heater core, but I don't see one. I will look again. The coolant comes to the heater core and that is it--it is not going through the heater core. A place in town told me my gauge should be showing hotter than it is. It is at the solid line between the 100 and the 210 degrees. There are three small lines in between the solid lines, so I am not sure where it. Approximately 155 degrees? What a ridiculous numerical division at that gauge. There has to be a reason that coolant is not getting through that rear heater core.

Tiny
Dcplace2007
Oct 31, 2011.
Yeah, 155 sounds pretty low. Normal operating range is between 180 and 200. The stock thermostat for your vehicle is listed as a 195 degree model. This means it opens at 195 degrees. None of my drawings show the heater control valve for the auxiliary unit. You'll need to trace the heater hose until you find it.

The upper radiator hose being hard may be due to increased pressure because the coolant isn't able to flow throughout the system.

Rivermikerat
Oct 31, 2011.
It's looking like you may have a plugged up rear heater core. You might try backflushing it with water pressure from a garden hose but you may ultimately have to replace the core.

Wrenchtech
Oct 31, 2011.
Absolutely it sound like that would be the case, but I have flushed it twice--twice at the heater core itself, and twice in the lines.I did the heater core first, then the lines separately with lines disconnected at the heater core and at the hcv area.I then connected the heater core lines and flushed at the hoses in the front in a "U". It all flowed properly. If it got plugged again, I can check that again. I am not sure what has happened. Perhaps the rear heater core is not freeflowing enough and needs replaced. Have you heard of this happening before exactly like this?

Tiny
Dcplace2007
Oct 31, 2011.
Yep, sounds like you need to replace it. Be sure to flush the rest of the system thoroughly first.

Wrenchtech
Oct 31, 2011.
Since the rear heater core is a part of the overall system, any contamination can be transferred to the rear heater core, as well as the front core and the engine block. Until and unless all of the contamination is removed from the whole system, it can very easily dislodge from where it is and move to the rear core.

Rivermikerat
Oct 31, 2011.
I took the lines lose again and flushed out the rear heater core. Unless there is still air trapped in the system, I don't see why coolant is not making it back to the heater core. I am almost positive that I hooked up the lines at the heater control valve properly. I don't think there is a way to hook the lines up improperly. Is there another control valve between the engine and the rear heater core? A place in town said there was, but I cannot find one. Thanks.

Tiny
Dcplace2007
Nov 3, 2011.
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