Please help! Heat doesnt work!

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em5568

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Good morning all, and happy new year!

I just wanted to thank you all in advance for any tips and information!

I'm currently driving my 1999 Tahoe LE with the 5.7l Vortec, i've had the truck for 5 years, it currently has 180,000 miles on it and it's been an amazing truck. Next to no problems with the exception of a few small issues interior wise (as to be expected ;)).

The issue i am encountering now is that my heat does not work. This is my first time i've attempted to do anything hvac related on the tahoe but i'm fairy mechanically inclined. So, let me tell you the original issue i encountered and what i've done so far to perhaps help you rule out any issues. The heat does not work, it blows cold. for a while it would only blow hot air while driving (not sitting at an idle) which was bearable at the time, but now that it's 5 degrees in NY. Not so much anymore.

So far i've replaced
-Heater core
-Tstat (Checked opening temperature outside of truck)
-Lines to and from heater control valve
-Heater control valve itself
-Passenger side actuator
-Temperature sending switch (Not that it has anything to do with it, but i've used a temp gun along with
my gauges to make sure that the gauge was accurate, it wasnt working for a time. the temp is accurate.)
-Burped the coolant system
-no, i am not low on coolant.

A few other things i'd like to add to potentially make troubleshooting or ideas on your part easier.

The heat had worked temporarily as far as when driving AFTER i disconnected the small vacuum hose to the heater control valve.

I currently feel cold air comming from outside the truck while driving, I noticed the fresh air button/switch light is no longer working on my hvac cluster.

The heater core did get hot at idle, but not anymore. It now currently only gets hot when i bring it to 2500-3000 rpms in park or nuetral. it no longer gets hot while driving. it just doesn't work period.

Perhaps it's mechanical? I'm lost and in need of guidance, any help or ideas would be greatly appreciated. Unfortunately money is very tight at the moment and i wouldn't be able to drop it off at a mechanic to troubleshoot as it is my only car.


Thanks guys!...
- E:(
 

Doubeleive

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it is my opinion that if you can reach in somehow and feel the heater core and that if the vehicle is warmed up to normal temp that the heater core should then be hot at all times just like the radiator as these both run together and the coolant just flows thru the whole system with the only thing to stop it is the thermostat and water pump, there is no lever or mechanical part to prevent coolant from flowing in and out of the heater core unless you cap off the lines. so this would lead me to think that the heater core is either clogged or one of the lines leading into or out of the heater core is pinched or clogged up. if this is not the case then you still have a malfunctioning actuator or a broken door or both or the controller is shot and is not sending a signal to the actuator. there's really no other scenario.
 

Doubeleive

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also I would think you could verify if the heater core is getting blocked by feeling both the heater core hoses going into and out of the heater core against the firewall and if one is hot and one is cold that could then be the case, if both are hot then you have a different problem.
 

Fless

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When your heater controls (on the dash or console) are calling for heat, do you have vacuum at the heater control valve? Does the valve move when you change the dash control settings from cold to hot?
 

bottomline2000

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If your heater core isn' getting hot it could be a flow issue. Not sure how the hoses are routed to the core but if they aren't getting hot you may have a water pump issue.
 
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em5568

em5568

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Awesome, thanks a bunch for the feedback guys. I'm going to make an attempt tomorrow during the daytime to check my lines/see if i can feel the heater control valve working. It's a brand new heater core so i don't think that could be the case, but i guess a pinched or clogged line somewhere down the road is certainly plausible.
This may sound like a stupid question, but should the heater core itself and radiator both be roughly the same temperature at all times? If so id imagine i could narrow down where to look for a bad/clogged line.

Thanks guys
- E
 

Doubeleive

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Awesome, thanks a bunch for the feedback guys. I'm going to make an attempt tomorrow during the daytime to check my lines/see if i can feel the heater control valve working. It's a brand new heater core so i don't think that could be the case, but i guess a pinched or clogged line somewhere down the road is certainly plausible.
This may sound like a stupid question, but should the heater core itself and radiator both be roughly the same temperature at all times? If so id imagine i could narrow down where to look for a bad/clogged line.

Thanks guys
- E
yes they should be pretty close, there is t-connector that comes off near the water pump that lets most of the coolant go to the radiator but should still be close in temp
 

east302

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it is my opinion that if you can reach in somehow and feel the heater core and that if the vehicle is warmed up to normal temp that the heater core should then be hot at all times just like the radiator as these both run together and the coolant just flows thru the whole system with the only thing to stop it is the thermostat and water pump, there is no lever or mechanical part to prevent coolant from flowing in and out of the heater core unless you cap off the lines. so this would lead me to think that the heater core is either clogged or one of the lines leading into or out of the heater core is pinched or clogged up. if this is not the case then you still have a malfunctioning actuator or a broken door or both or the controller is shot and is not sending a signal to the actuator. there's really no other scenario.

Agreed with Doubeleive. See if the hoses up and downstream of the heater valve are hot and nearly the same temperature.

I'm fairly certain that this valve fails open...the only time it closes is when the temperature dial is set to the "max" cold detent to help with a/c performance. Feel the bottom of the valve to see if the plunger moves up and down freely. Up is closed.

342c44cdda00fde8210ecb52161d5632.jpg

It's either a coolant circulation problem or the actuator at the heater core isn't positioning the air damper/door correctly...either because it cannot or because it isn't being told to do so by the main panel.
 
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em5568

em5568

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I checked the core again today, i dont believe its the actuator only being because i can see it move and the heater core is currently not getting hotter than 'luke warm'. I believe it is a flow issue somewhere, one thing i did notice is that the hose comming from that vacuum solenoid thats connected to the heater control valve area is not getting vacuum in any position. I believe it may either be that, or perhaps i got a factory defective control valve. Unfortunately i only have a small window each day to work on it. So tomorrow, checking position on the control valve and potentially addressing the the vacuum solenoid!

Ill keep you guys updated, thanks!
 

east302

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one thing i did notice is that the hose comming from that vacuum solenoid thats connected to the heater control valve area is not getting vacuum in any position. I believe it may either be that, or perhaps i got a factory defective control valve.

Vacuum would only be applied to the valve (via the solenoid) when the temperature dial on the panel is set to the full cold detent (MAX). Otherwise, the valve will return/fail open to allow coolant to the heater core.

Having that valve close and shut off coolant to the heater core actually does kind of help with cooling in the summer. I have noticed a difference in mine since fixing the damaged vacuum hose.
 

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