Replaced heater core, now no heat

[Cam2363]

I just went through the painful process of a heater core replacement, and now I dont have heat.

Car comes up to temp just fine, but no heat out. When I set it to HTR, i feel a slightly warm air coming out, but thats about it.


Any help would be great!

[Rettax3]

Make sure there is no air in the cooling system -that is your most likely problem. Other than that, a kink in a hose, something left clogging the heater-core...

[Cam2363]

Make sure there is no air in the cooling system -that is your most likely problem. Other than that, a kink in a hose, something left clogging the heater-core...

Air is the first think I want to check. When I went to bleed it last night, air came out but no coolant did. I don't think the hose would be kinked because I tried to make sure that wouldnt happen, but it is possible. I'll have to look later

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[Cam2363]

It might be the temperature control motor but I'm not sure. I'll have to do diagnosis later

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[Money pit Beretta]

Do you have a temp guage on your car? If you do and it's reading too high(or much higher than normal) then there is still air in the system.
The controls a powered by vacuum. Keep that in mind when trouble shooting.

[Cam2363]

Do you have a temp guage on your car? If you do and it's reading too high(or much higher than normal) then there is still air in the system.
The controls a powered by vacuum. Keep that in mind when trouble shooting.

Yes. It reads like it always has. Sometimes it goes up, but it's just because the radiator fan hasnt come up. But I'll keep that in mind too. I did look at the flaps and I did see them move. And I hear the normal sounds, and it also moved direction so I dont think it's a direction problem. But I'll figure it out hopefully


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[Cam2363]

I found that if I have it on HTR it gives a little heat. But using a thermometer, it only goes to around 70. Which is wayyyyy to low lol. So either theres a problem with the temp motor, or the thermostat is bad, or the door that controls the temperature difference.

So what should I test?

I also noticed that both lines going into the core are hot. So I dont thing theres a clogged or kinked line

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[Rettax3]

Here is GM's diagnostic tree -this is from the '95 L-Body service manual, but should apply to your '96 for this system. I'm also posting the assembly schematic and parts identifying list. Note the 'Temperature Motor' is known as a 'Blend Door Actuator' in most of the industry in case you need to look for a replacement, but I am suspecting that something is binding or loose on your blend door causing it to fail. Of course, it is controlled by the temp knob, so a problem in the control panel could cause a failure to operate too.

[Cam2363]

Here is GM's diagnostic tree -this is from the '95 L-Body service manual, but should apply to your '96 for this system. I'm also posting the assembly schematic and parts identifying list. Note the 'Temperature Motor' is known as a 'Blend Door Actuator' in most of the industry in case you need to look for a replacement, but I am suspecting that something is binding or loose on your blend door causing it to fail. Of course, it is controlled by the temp knob, so a problem in the control panel could cause a failure to operate too.

HVAC Assemb1a.jpg

HVAC Assemb2a.jpg

Heater Diag1a.jpg

Heater Diag2a.jpg

Ok thanks. I tried to find the troubleshooting steps in my book, but honestly didnt even try that hard haha. Thanks for them though! I will look try and diagnose later. And thanks for clarifying the temp motor thing. I was very confused about what that was and where it was, but now it's more clear


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[Rettax3]

HVAC systems are hard. These systems were built off very old setups that were jury-rigged in the first place -most of this was put together like it was designed by a sleeping monkey who once saw an HVAC system in a car from 60 years ago. But, they tend to work for a very very long time with virtually no maintenance. Vacuum solenoids, electric stepper-motors, cable-operated controls, built layer upon layer and stuffed into an ugly box crammed under the dash so none of it very accessible, and without a full-on scan-tool it is really hard to figure out what the computerized modules are doing or even being told to do on the newer cars. Diagnostic procedures like 'feel this hose for heat' and 'listen for that motor' require you to have some control over your work environment too, and to utilize a lot of your senses and know intuitively if something seems right or not. The main thing to remember is every cause should have an effect, and that the most common failure of the control systems are the 'new' electronic controls or actuators, because they are cheaply made, put under too much load in the first place, and subjected to harsh and rapidly changing temperatures. Good luck.

[Cam2363]

HVAC systems are hard. These systems were built off very old setups that were jury-rigged in the first place -most of this was put together like it was designed by a sleeping monkey who once saw an HVAC system in a car from 60 years ago. But, they tend to work for a very very long time with virtually no maintenance. Vacuum solenoids, electric stepper-motors, cable-operated controls, built layer upon layer and stuffed into an ugly box crammed under the dash so none of it very accessible, and without a full-on scan-tool it is really hard to figure out what the computerized modules are doing or even being told to do on the newer cars. Diagnostic procedures like 'feel this hose for heat' and 'listen for that motor' require you to have some control over your work environment too, and to utilize a lot of your senses and know intuitively if something seems right or not. The main thing to remember is every cause should have an effect, and that the most common failure of the control systems are the 'new' electronic controls or actuators, because they are cheaply made, put under too much load in the first place, and subjected to harsh and rapidly changing temperatures. Good luck.

Thanks! I definitely feel that haha. A lot of things with cars are such a pain, but such is how cramming a lot in a small space is. I also work on cellphones a lot, and they can be such a pain because everything is so tightly packed. Thanks for all your help!

[Cam2363]

Fixed it.

I first got it nice and hot, then checked the heater core temp. I try not to be stupid all the time so I used and thermometer and it shot right up to like 160 when I decided that it was good. So I knew heater core was working well

The issue was that the temp door was getting stuck. It would be in the cold position, and only move a little and got stuck. Since I already had the heater core visible, I set it to all the way hot and screwed the cover back down, and now it works great! It will be so nice to have heat back now that the temp is like a high of 50


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[Money pit Beretta]

So do you think cleaning the pivot points of the door(in the summer) will totally fix the problem? Or do you think that the blend door motor is a bit weak?
As our cars have no cabin air filter there could be a lot of gunk in the pivot points. Hell one time a leaf came out of the dash vent. That was a crazy moment.
As to what could be used to clean that a guess would be something like electronics cleaner. Brake cleaner would work, but that stuff stinks real bad.

[Rettax3]

So do you think cleaning the pivot points of the door(in the summer) will totally fix the problem? Or do you think that the blend door motor is a bit weak?
As our cars have no cabin air filter there could be a lot of gunk in the pivot points. Hell one time a leaf came out of the dash vent. That was a crazy moment.
As to what could be used to clean that a guess would be something like electronics cleaner. Brake cleaner would work, but that stuff stinks real bad.

I know what you mean about the leaves -I've had a couple of them blow out at me over the years. One time, I had a dry leaf blast out of the fresh air vent in the footwell area of my old Plymouth -power nothing on that car and no A/C, but two doors and six windows that open (or vent) and the two fresh air vents under the dash. That leaf blew out like it was launched from Cape Canaveral, and blasted right up into my face.

Cleaning those pins could be done with one of those cleaners, but they are strong and could damage the plastic (brake cleaner can actually melt it, so no ). Simple Green on a paper-towel or even dish detergent are plenty strong enough for that job, then re-lube the pivots with a small dab of white lithium grease. Good to go, as good or better than new...

[Money pit Beretta]

Yeah Rettax3 it sure is a mind bender when that happens. Not hard to figure it out, just more of a how did that happen. Mine started with a noise from the blower motor, then just as you said it smacked into the vent hard. Lucky for me it came out of the center vent will less speed. A chunk would be one thing. It was fully whole and full sized.

Betting Simple Green would work just fine. That should get rid of the problem of getting heat out of the vent and not the floor. Well it will come out on the floor after it warms up. No trouble in the summer time LOL! Works just fine.
Grease and spray lube are covered. Could open a store with all that is sitting around the house.