How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Hal Your Own Question
Hal, Chevy Master Tech
Category: Chevy
Satisfied Customers: 1031
Experience:  ASE and Chevy Master , 31yr Dealership exp,
Type Your Chevy Question Here...
Hal is online now
A new question is answered every 9 seconds

Chevrolet Silverado 1500 i have a 1986 chevrolet silverado,

This answer was rated:

i have a 1986 chevrolet silverado, and it only blows cold air. I have replaced the heater core, blower motor, heater hoses, thermostat twice and flushed the entire cooling system three times. I have checked my heat blend door, and air flow direction door for proper operation and all are good.
So any ideas?
Hello...try to disconnect the two hoses that go to the heater core and run the engine to see if you have good coolant flow going inside the truck...Also are the heater hoses hot ? What does the temperature gage read after run the truck for a while ? Let me know...thank you...
Customer: replied 6 years ago.
ok, i actually did that last night i emptied the rad low enough to see the coolant coming back into the rad from the heater core and it was a constant steady flow the same size as the id of the hose itself, and yes both heater hoses are always just as hot as the rest of the hoses on the engine/rad, and my temperature gauge in the truck reads a contstant 100 or a bit higher with the cardboard in front of the rad and reads the notch exactly inbetween the 40 and 100 without the cardboard in front of the rad, which it has always read since i got the truck.

I think you have a problem with the blend door. you need to check where the cable goes and make sure it opens the door all the way...Let me know...thank you...

Customer: replied 6 years ago.
k, rechecked blend door and cable - cable is still attached and opens door all the way to hot and closes off passage for cold
Is your blower motor working?
Customer: replied 6 years ago.

yes its working

i am running out of ideas...are you sure that the door we are talking about is the one that by passes the heater core ? Because more or less you should have heat...
Customer: replied 6 years ago.

well i think so the one door that controls the hot and cold is run by a cable and when the slider is set to "hot" on the heater controls it closes off passage to the ac condensor or whatever its called, and when its set to "cool" the door closes off passage to the heater core, and its working properly and sealing completely against the heater box when set to "hot".

Then there is the other door which directs the air flow to which vents, wheather its defrost(windows), heat(floor), or vent(dash)

OK ,it sounds right. After the engine warm if you rev the engine do you get any heat ? usually there is option for a higher temp. thermostat do you know which one do you have ?
Customer: replied 6 years ago.

i've got the 195f/90c thermostat in it, and if the truck is idling i have heat and as you start driving it slowly turns cold, the faster you drive the colder it gets, until it blows straight cold air

are you using 100% coolant or 50/50mix (water+coolant) ? Does it get really cold where you live? 100% coolant reduces the heating performance.....Did you have any work done on this engine lately,like the intake gaskets ? let me know..

Customer: replied 6 years ago.

its got a 50/50 mix and ya during winter (right now) it gets to -30 sometimes which it currently is today, and no there has been no engine work done recently

Going back to the card board ....Do you have heat if you keep the card board there all the time ?On this engine you have one hose from back of intake to heater core and the other from heater core to the radiator right ?how did you loose the heat, it just did not work when you turn it on this winter ?
Customer: replied 6 years ago.
no it doesnt matter if the cardboard is there or not the heater does the same thing starts hot and as your speed increases the heat decreases, ya the hose comes from the front right corner of the intake manifold to the heater core, then from the heater core to the radiator, and i lost the heat when the old heater core plugged up during this winter, so i flushed the cooling system until i got clean clear water out of it and then changed the heater core and refilled the system with a 50/50 mix and ran the truck for 30 minutes without the rad cap on to make sure all air was out of system, and then rechecked the coolant level topping it up with a 50/50 mix
I am sorry, i will opt out maybe another expert can help you....
Customer: replied 6 years ago.

ok man, i appreciate the help and im at the same point ive tried everything i can think of, and i have my Interprovincial for automotive here in Canada.

thanks again

Hi my name is XXXXX XXXXX Your complete satisfaction is my goal. Please provide as much information about your concern as you can.


Have you tried back flushing your new heater core? maybe some stuff that was plugging the old core worked its way into the new core. It sounds like you have covered all the bases. This sure just sounds like you are not getting full flow through the core itself.. May be it is a defective core, I have had one or two that had partial core tube soldered over, on new heater cores. Just trying to throw out some things that might help.

Can you feel any heat on the heater case itself. near where the heater core is located??

Customer: replied 6 years ago.

Hello Hal,

yes i have tried back flushing the new core and i got good flow when back flushing, and I have thought of it being defective (hopefully not) and yes when i feel the heater box where the core is located it is hot .


Currently i have a duct removed that redirects the air to the vents so that i can feel what is actually coming out of the heater box, and when the heater is on full blow, full hot i can feel hot air coming out of 3/4 of the hole and the bottom 1/4 of the hole blows straight cold and it feels like outside air. I know I have to replace the top attaching stud as when I changed the heater core the stud spun inside the clip and I couldn't get it completely tight against the fire wall, so im thinking that i may be getting outside air sucked into the heater box past that seal. So I am going to to try that as soon as i can, but any other ideas would be great too

It sounds like you may have located you problem, I was thinking maybe the air box was not reinstalled correctly. Like i said before , you have done just about everything you could have. I think your on the right track, remove the heater case and recheck your installation , may be install some new foam seal between the firewall and heater case.


I live in NW Montana, and I know when it is cold out side, everything must be in good working order, and installed correctly , for the heater to work correctly.


Let me know how it goes. If I think of anything else I will post it.

Customer: replied 6 years ago.
Well i pulled the heater box back out and installed new foam between the firewall and heater box, and its still doing the same thing as before. when i stopped and checked what temperature air was coming out the the vent hole, im still getting heat out the top 3/4 and cold out the bottom 1/4. I also noticed around where the heater core is located in the heater box, the very bottom and a bit up the side of the box itself is cold like outside air cold, and the rest of it is hot. Is there anywhere else that i could be sucking in outside air directly into the heater box?

When you put the heater core in the heater box , did you make sure the foam seal that went around the core was in place??

The flow through the HVAC system is from the blower through the A/C evaporator,the inside the truck through the heater core So as long as you have the case sealed to the fire wall, it can't suck in outside air.


When you reinstalled the temp door and blend door. Were they working correctly ? and were the foam seal on the doors good. Look at the picture below and let me know how you adjusted rod #153. Air valve #163 and were the seals #170 in place This heater case looks simple , but it can cause concerns if these internal air valves are not in the correct place or adjustment.


If all this is good then I think you got a restricted new heater core. It can be partially restricted and still flow water. Even if cold air was getting in ,the core should heat up the air




Use this diagram when referring to components so were both on the same page.

This is out of 86 Chevy truck with A/C.

Customer: replied 6 years ago.

When i removed the heater box the first time, there was no foam around the heater core pipes nor was there any sign that there ever was, but i suppose now that ive disturbed it it could not be sealing that part of the box against the firewall, like it was before.


All doors are working correctly, and the seals on my doors are all rubber and sealing properly, as far as i can see. As for rod #153 I put it back on the same place it came off, and air valve #163 will only go back into the heater box one way, and it moves with air valve #171 (rod#153 hooks the two together), and is directing air in the proper direction when set to either deforst, heat, or vent. Seals #170 are all in place as I never had to move them or anything when I changed the heater core.


But now that we have a picture when I say I have heat out of the top 3/4 of the vent hole its the rectangular hole between numbers 162 and 164 and the air valve #163 is directing air through that hole and like i said the top two holes in that valve are blowing hot air and the bottom is blowing cold outside air.


Im thinking im going to remove the heater box again, and see where excatly the new foam I installed is sealing against the firewall, run some foam around the heater core pipes, and pull the heater core out of the heater box and hook it up to the heater hoses and run the truck and then i can feel exactly what part is heating up and whats not if the heater core really is partially restricted, and then go from there.


Im at home right now and wont be able to try this till monday after work, so ill let you know what results I get back

Thats exactly what I would do. If the bottom of that core is not getting hot you would have the results you stated. and since this started when this core was installed it is a very good possibility the core is at fault.


I don't run into this very often, but once you have had one you don't ever overlook that possibility again.


While you have the box out make sure the plastic retainer #168 is not cracked. This works the vent door, and is very a common thing to break. and it affects defrost mode .


Let me know what you turn up Monday.

Have a great week end


Customer: replied 6 years ago.

Hey Hal

My apologies as I haven't responded back in a timely fashion, the last week was crazy hectic as it got real warm like -2 to +5 and the winds picked up and drifted me in for three days.


I have finally done as I said I would, but like I said its now in the low positives for temperature and cannot really test the new heater core, I'm going to keep this session open and if it does get cold and the truck produces heat or turns into spring I will send my payment through, I'm hoping for the cold to return so I can put this issue to bed.

Thats OK, I was thinking about checking in with you today to see how it was going, but you beat me to it. Good to hear from you, and yes , wait until it gets cold again and test it out. Lets make sure this works. I want you to be satisfied 100%.

Have a good evening, and talk to you later.


Hal, Chevy Master Tech
Category: Chevy
Satisfied Customers: 1031
Experience: ASE and Chevy Master , 31yr Dealership exp,
Hal and 5 other Chevy Specialists are ready to help you
Customer: replied 6 years ago.

so its been -30 acouple times with the windchill, and the truck blew hot air like it used to all those winters before, thank you for your patience and understanding and thanks most of all for the help. Will definately use Just Answer again.

Hey I am glad that fixed it. And I'm happy I was able to help.

Thank You!!!