HVAC problems

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Fejjj951
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I have been having trouble getting heat out of the HVAC system for a while. The only time I could get heat out of the system was when I was on boost. The Indy shop that I had working on it said that the controller was bad that everything else was working properly. I never bothered to fix it as I live in the north east and the car gets stored from late October until the middle of April so I don’t use the heat very often.

Tonight I replaced the controller and still have the same problem. With the controller set at high heat it just blows regular temperature air until I get on the boost and then it blows out heat until I let off the boost.

Please help.


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Tom
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Fejjj951 wrote: Wed Sep 18, 2024 8:16 pm I have been having trouble getting heat out of the HVAC system for a while. The only time I could get heat out of the system was when I was on boost. The Indy shop that I had working on it said that the controller was bad that everything else was working properly. I never bothered to fix it as I live in the north east and the car gets stored from late October until the middle of April so I don’t use the heat very often.

Tonight I replaced the controller and still have the same problem. With the controller set at high heat it just blows regular temperature air until I get on the boost and then it blows out heat until I let off the boost.

Please help.
Has the heater valve been tested to confirm it works and is able to flow hot coolant into the cabin? Usually, if the heater valve is working, and the system is opening it, you get some heat in the cabin even if the ducts are working right. If the heater valve is ok, I'd run through all the diagnostics in the charts linked below to see why the system is not letting the hot coolant flow into the cabinet. Don't overlook testing the solenoids.

viewtopic.php?t=357


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The heater control valve was replaced previously when I wasn’t getting heat but will have to confirm it is working.

What would explain that I get heat while under boost?


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Fejjj951 wrote: Wed Sep 18, 2024 8:58 pm The heater control valve was replaced previously when I wasn’t getting heat but will have to confirm it is working.

What would explain that I get heat while under boost?
Does it blow out the vents hot when on boost?


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Fejjj951
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Yes.

Something else I thought about is my temp gauge stays at or just below the 1/3 line. Maybe the thermostat is stuck open?


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In reading Clark’s Garage and a couple other resources, I am thinking there may be air trapped in the heater core. I think I will start by bleeding the coolant system and then go through your EXCELLENT tutorial.


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Fejjj951 wrote: Thu Sep 19, 2024 9:16 am In reading Clark’s Garage and a couple other resources, I am thinking there may be air trapped in the heater core. I think I will start by bleeding the coolant system and then go through your EXCELLENT tutorial.
Worth a try, especially if you are getting erratic temp reading at stop lights and/or hearing a lot of gurgling when you try for heat in the cabin. I don't think a stuck-open thermostat would cause your issues. I ran without a thermostat for a long time and had no problem getting heat.

I'd open the hood and look at the heater valve arm. With the engine running and warmed up, and the heater set to high, the heater valve plunger should be pulled back all the way to open the valve and let hot coolant into the heater core. If it's not pulled back all the way, remove the vacuum line to it, and the plunger should pull out to the open position, which will allow hot coolant to pass into the cabin. One possibility is that the solenoid for the heater valve is bad and passing vacuum through to the valve 100% if the time, keeping it closed. And so when you go on boost, the vacuum turns to pressure, and the pressure forces the heater valve to open. If that's happening, you may also have a bad check valve on the vacuum reservoir and/or a leak in the vacuum reservoir.

To visualize the heater valve operation, see this video and the :45 to 1:30 portion in particular...



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There's typically a vacuum check valve between the engine intake manifold and the 'vacuum supply' for the HVAC controls including the heater control valve. Your symptoms point that valve being either non-op or installed backwards. The valve is a small plastic disk with hose nipples on the ends, two different end colors. You should be able to blow through (by mouth) from the HVAC side towards the intake side. Conversely, it should pass no air in the opposite direction.

In my cousin-to-944 car, there's a vacuum reservoir plumbed into the HVAC side of the system, so that there's sufficient vacuum to keep the system functional when the engine is under load and manifold pressure is close to (or in your case above with boost) atmospheric pressure. A vacuum gauge with some hose fittings will help with diagnosis. It's also very handy to test each of the HVAC actuators, including that heater control valve, to make sure they are actually leak-free and functioning correctly. I have a plastic Miti-Vac hand pump with a gauge, but I've come to use a metal version since mine gets used to test fuel pressure regulators and pulse dampers that will have fuel if they fail. I bought mine for a premium name-brand price, only to find the exact same tool and kit at Harbor Freight for a fraction of what I paid for mine. Regardless, getting the vacuum actuators and related plumbing correct is essential to restoring correct function. I'd start with that before diving deeper and $$ into the controller itself. The check valve is a very common piece you can find at any POLAPS, in case there's any doubt at all about it leaking through.

Tom has some valuable history restoring the climate control in his dad's Mercedes SL, and I'm sure will share war stories about all the actuators and valves, the heater control valve, the check valve, and the issues around vacuum system integrity. He also has great familiarity with the system in the 944. Heed his advice!


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Fejjj951
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Tom wrote: Thu Sep 19, 2024 10:02 am
Fejjj951 wrote: Thu Sep 19, 2024 9:16 am In reading Clark’s Garage and a couple other resources, I am thinking there may be air trapped in the heater core. I think I will start by bleeding the coolant system and then go through your EXCELLENT tutorial.
Worth a try, especially if you are getting erratic temp reading at stop lights and/or hearing a lot of gurgling when you try for heat in the cabin. I don't think a stuck-open thermostat would cause your issues. I ran without a thermostat for a long time and had no problem getting heat.

I'd open the hood and look at the heater valve arm. With the engine running and warmed up, and the heater set to high, the heater valve plunger should be pulled back all the way to open the valve and let hot coolant into the heater core. If it's not pulled back all the way, remove the vacuum line to it, and the plunger should pull out to the open position, which will allow hot coolant to pass into the cabin. One possibility is that the solenoid for the heater valve is bad and passing vacuum through to the valve 100% if the time, keeping it closed. And so when you go on boost, the vacuum turns to pressure, and the pressure forces the heater valve to open. If that's happening, you may also have a bad check valve on the vacuum reservoir and/or a leak in the vacuum reservoir.

To visualize the heater valve operation, see this video and the :45 to 1:30 portion in particular...


Tom,

Thank you for providing such detailed information.

We will start with your suggestions.

Where is the solenoid located?


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dr bob wrote: Thu Sep 19, 2024 3:11 pm There's typically a vacuum check valve between the engine intake manifold and the 'vacuum supply' for the HVAC controls including the heater control valve. Your symptoms point that valve being either non-op or installed backwards. The valve is a small plastic disk with hose nipples on the ends, two different end colors. You should be able to blow through (by mouth) from the HVAC side towards the intake side. Conversely, it should pass no air in the opposite direction.

In my cousin-to-944 car, there's a vacuum reservoir plumbed into the HVAC side of the system, so that there's sufficient vacuum to keep the system functional when the engine is under load and manifold pressure is close to (or in your case above with boost) atmospheric pressure. A vacuum gauge with some hose fittings will help with diagnosis. It's also very handy to test each of the HVAC actuators, including that heater control valve, to make sure they are actually leak-free and functioning correctly. I have a plastic Miti-Vac hand pump with a gauge, but I've come to use a metal version since mine gets used to test fuel pressure regulators and pulse dampers that will have fuel if they fail. I bought mine for a premium name-brand price, only to find the exact same tool and kit at Harbor Freight for a fraction of what I paid for mine. Regardless, getting the vacuum actuators and related plumbing correct is essential to restoring correct function. I'd start with that before diving deeper and $$ into the controller itself. The check valve is a very common piece you can find at any POLAPS, in case there's any doubt at all about it leaking through.

Tom has some valuable history restoring the climate control in his dad's Mercedes SL, and I'm sure will share war stories about all the actuators and valves, the heater control valve, the check valve, and the issues around vacuum system integrity. He also has great familiarity with the system in the 944. Heed his advice!

Bob,

Thank you for this!

What does POLAPS stand for?


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