Dash heat only blows cool air

[rudedogfire]

I just bought a 1998 Newmar Mountain Aire with a Chevy 454. I turned on the dash heat, and it blows cool air. The air conditioning works great. The blower also works fine. Just no dash heat. Engine temp. is a constant 165. I tried to locate the heater core, to see if the controls were disconnected, but couldn't find it. Does anyone have any suggestions?

[rudedogfire]

Thanks for all the help. After much searching, I found a small shutoff valve on one of the heater hoses....hidden, of course. But, I now have wonderful heat

[APRSRVer]

Vacuum valve in heater hose is to stop coolant flow to heater when in Max Cool mode. It gets vacuum only then. Heat temp is controlled by coolant flow valve regulated by a cable to lever on dash control or by an air mixing door in heater plenum and also controlled by a cable unless vehicle has autotemp system. 165deg engine temp is too cool. You need at least a 180 deg or preferably a 192 deg thermostat.

[falconbrother]

Look at the 3 inch hoses where the heat comes out of the heater core and into the dash. Make sure they aren't split (assuming your MH has those).

[rudedogfire]

Thanks to all the above. I will start my check off list today. Hopefully, it is an easy fix.

[Daveinet]

trop-a-cal wrote:
Then there is possible low coolant which starves the heater core first due to it being at highest point in system. Always check when engine is cold and no pressure is present in lines. You may also have a mechanical inline heater valve in the hose going to the heater core that is not connected or stuck closed. That would be controlled by the dash heater lever and connected via a cable to the inline valve.

I would lean toward low coolant, especially if the engine temp never gets above 165 degrees. That by its self is a read flag that something is not right. Idle temp will be anywhere from 195 to around 205 depending on where the sensor is.

[C350amg]

On GM P30 series RV’s usually the heater are all working with vacuum. That is to say that all traps and the heater valve are operated via vacuum.
This implies that engine must be running and that there are no leaks in the vacuum circuit.
I own a 1998 Triple E on a GM P30 chassis with a 7.4 (454) and I had to make some repairs to that circuit because a line was disconnected.
So start by detecting if the vacuum circuit operates:
• Hit the button OFF on the heater
• Start the engine
• Put the heater blower motor to the highest setting.
• Push the button for defrost: all the air should come out close to the windshield
• Then push the button HEAT: all the air should go to the floor
• Try all the other function to see if the straps operate.

If you realize that the straps do not operate then you have a vacuum issue. Look at all the lines in the dashboard and under the hood in front. There should be a round sphere under the hood and that is the vacuum accumulator.

Now if all that works move to under the hood. The heater core placement is very easy to find just follow the heater water lines. It should be under the hood on the passenger side a bit higher than the radiator.

On the water line going to the heater core there is a valve also operated by vacuum (see my picture). With the engine running have someone (a friend) move the heater lever from cold to hot (and vice versa) and see if the valve operates. If it does not then you have again something wrong with the vacuum.

Finally, some of us actually add a manual valve on the circuit so it can completely be closed when in hot climates, this should be easy to find. If you look on my picture just above the heater control valve you clearly see mine in the middle of the red heater core tube.

When the engine is hot and the valve open the pipe going to the heater core should be pretty hot, do not forget to have the proper amount of coolant in the radiator and never ever open it when hot.

[donhoward49]

All good ideas above..Do them step by step to isolate the problem.. After running the engine, will the water temp gauge move up from "cold".. I am thinking a bad thermostat, stuck open.
Do all the above then come back for more ideas good luck

[dougrainer]

The AC/Heater Core is on the front firewall on the passenger side. You will see the heater hoses going to it. The odds are, the electric (12 volt) heater control valve is bad. Go to Evanstempcon.com and you will see all the parts for your Dash AC/Heat system listed. Doug

[sch911]

I also recommend you check the heater core valve. It's under the hood and is run by 12v or in some cases vacuum. It will be right inline in the heater core inlet hose. Most likely its not opening correctly. Very common issue.

[trop-a-cal]

Then there is possible low coolant which starves the heater core first due to it being at highest point in system. Always check when engine is cold and no pressure is present in lines. You may also have a mechanical inline heater valve in the hose going to the heater core that is not connected or stuck closed. That would be controlled by the dash heater lever and connected via a cable to the inline valve.

[Gasman_2]

Also check the coolant control valve that most likely on the outside of the firewall from the heater core. I think others have had problems with this valve not opening or closing.

[msredneck84]

First of all, I am no mechanic by any means. But your problem could have to do with the vacuum system that switches the airflow from heat to vents, to defroster etc. If that is your problem, then when you reset your control from say vent to heat it is not switching the vent internally. I had this problem on a GM passenger car once. Just guessing. Good luck!

[Davidlatham2012]

Flush cooling system, make sure heater core isnt bypassed. Make sure also that coolant level is correct. start engine cold with radiator cap off and watch coolant level. see if it drops when thermostat opens and then add proper coolant. Also be a blockage in heater core or a collapsed hose.