5.7 tbi Running Cool and No Heat

[Dave_N]

Hi Everyone, new to the forum. I have a 95 Yukon, 350 tbi. It's a new project I'm fixing up to use as a hunting truck. I have a couple of issues with it, and I'm looking for some suggestions.

Symptoms:
1. No Heat
2. Running cool (160 F)


What I've done so far:
1. First thing I did was pull the thermostat (195F thermostat) and tested it on the stove... it opens and closes as expected. I reinstalled the thermostat with a new seal.

2. Next I noticed that the vacuum line going from the little actuator clipped onto the evaporator to the heater control valve was disconnected. I reconnected it, and it still didn't work. I replaced the actuator, and it still didn't work. I ran the vacuum source line directly to the heater control valve, and it then opened the heater control valve... not sure what's going on with the actuator... what actually controls that actuator? Interior temp control? Interior fan control?

3. After making the heater control valve open up, I let the truck run for a while, and felt both heater hoses... the heater core inlet gets hot, and the outlet gets warm, but not as hot as the inlet line. Should they both be about the same temperature? Also, still no heat at this point. I disconnected the hoses and flushed the heater core... flow seams restricted, so I'm thinking I might need a heater core. I would expect some heat in the interior heater air, though even with restricted flow in heater core. Is that a fair assumption?

4. I checked the blend door actuator and it is reacting as the temperature control knob is turned.

5. None of the other gauges on the dash (oil pressure, fuel, volt meter, speedometer) are working properly, so I thought the temp gauge or temp sensor might be bad. Since none of the gauges are working, I bought and installed a set of mechanical gauges... temp shows 160 F.... I guess that gauge was actually working properly. Oh well. no loss.

6. I noticed that the fan speed increases as the engine speed increases, so I thought the fan clutch might be seized. I turned off the motor, and the fan moves with very little force required, but does not spin freely. How much resistance should it have?


So now what?

The engine is still running at 160, and there is no heat.

I'm thinking I might re-test the thermostat, with a thermometer in the pan, to see exactly what temp it opens at. Should it just start to open at 195 or should it start opening sooner, and reach full open condition at 195?

Thanks in advance!

 

[Big Mama]

It’s been a while for me but is 195 the correct thermostat? The inlet hose sb hot but outlet a little cooler the heater core sounds like the culprit here if all other systems are working great job testing them especially the control valve and blend door.

 

[Dave_N]

It’s been a while for me but is 195 the correct thermostat? The inlet hose sb hot but outlet a little cooler the heater core sounds like the culprit here if all other systems are working great job testing them especially the control valve and blend door.

Thanks for the reply. Yes, from what I've read the 195F thermostat should be correct. My outlet heater hose is warm-ish, but not nearly as hot as the inlet, so I think I may stick a heater core in this afternoon. It can't hurt, and only costs like $35. I guess we'll see what happens.

 

[Big Mama]

I think you’re on to it. There’s not enough hot water going through the heater code to make the outlet hose any hotter. It’ll be interesting to find out how warm the core is.

 

[Dave_N]

I think you’re on to it. There’s not enough hot water going through the heater code to make the outlet hose any hotter. It’ll be interesting to find out how warm the core is.

Problems solved! Heater core was definitely plugged. It was only getting hot at the inlet. Stuck a new one in, and heat works perfectly. I also pulled the thermostat and retested it, with a thermometer in the pan. It started opening at like 155. I put in a new one, and the engine is running at 195 as it should. Thanks for your input.

 

[Big Mama]

Nice. Thanks for letting us know what worked it’ll make it easier on the next guy.

 

[Squirrelsmith]

nice investigating

 

[east302]

2. Next I noticed that the vacuum line going from the little actuator clipped onto the evaporator to the heater control valve was disconnected. I reconnected it, and it still didn't work. I replaced the actuator, and it still didn't work. I ran the vacuum source line directly to the heater control valve, and it then opened the heater control valve... not sure what's going on with the actuator... what actually controls that actuator? Interior temp control? Interior fan control?

!

Just to answer this question, the valve closes when the temperature dial is set to the “max” cold detent. This energizes the solenoid to apply vacuum to the valve, causing it to close. It is normally open. Test operation by pushing the spring loaded plunger “in” to close it. It should spring back down when released.

Closing the valve prevents coolant flow to the heater core and is meant to maximize a/c performance. My dash vent temps drop about 5-degrees when this is done.





Sent from my iPhone using Tapatalk

 

[east302]

2. Next I noticed that the vacuum line going from the little actuator clipped onto the evaporator to the heater control valve was disconnected. I reconnected it, and it still didn't work. I replaced the actuator, and it still didn't work. I ran the vacuum source line directly to the heater control valve, and it then opened the heater control valve... not sure what's going on with the actuator... what actually controls that actuator? Interior temp control? Interior fan control?

!

Just to answer this question, the valve closes when the temperature dial is set to the “max” cold detent. This energizes the solenoid (near the valve, on the firewall) to apply vacuum to the valve, causing it to close. It is normally open to allow coolant into the heater core. Test the valve’s operation by pushing the spring loaded plunger “in” to close it. It should spring back down when released.

Closing the valve prevents coolant flow to the heater core and is meant to maximize a/c performance. My dash vent temps drop around 3 to 5-degrees when this is done.





Sent from my iPhone using Tapatalk

 

[510man]

Problems solved! Heater core was definitely plugged. It was only getting hot at the inlet. Stuck a new one in, and heat works perfectly.

How difficult was it to install the new heater core? I understood you had to pull the dash to do it in that body style Yukon and Suburban. I have a '99 a know this day is coming at some point. Just curious if you had to pull your dash since you mentioned doing it in an afternoon.