No heat, next steps?

[esebastian]

Hi People, I picked up an 86 C1500 recently with a new ZZ4 350 crate engine and the heat does not seem to get hot. I checked coolant levels, topped off the coolant and burped the coolant system. Replaced the 180 thermostat with a new one, and the blender door is working properly. Also no leaks... and still no heat. When I let it warm up in my driveway, temp gets up to 185 - 200 ish and the vents will blow heat but as soon as I start driving and the engine cools then boom....blows cool to cold. Only thing left is to replace the heater core. Before taking on that brutal task, wanted to see if there was any ideas if I am missing something or is that the next logical step? Looks like the heater core is a copper one and I am going to venture to guess it is still the original 86 one. Thanks

 

[SquareRoot]

Canada eh? Welcome. Are you saying the coolant temp drops below 180 when you start moving?

 

[esebastian]

Hi Mike, thanks for the welcome. Yes the coolant temp drops to around 182 or so (according to the gauge) when I start driving.

 

[SquareRoot]

That sounds normal with a 180 thermostat. Make sure your getting coolant flow thru the heater core. You could have the hoses reversed (unlikely and it should still flow albeit backwards), and there's no blockage in the core. If you take the radiator cap off BEFORE the engine gets hot you can see if coolant is flowing out of the bypass hole. If so, the heater core is not blocked. Only two things left to check; do you have a coolant shutoff valve in either of the heater core hoses? If so, it may be stuck closed or not opening. The last and most probable thing is the heater core airflow inside the case is blocked. It's common for debris to build up on the surface and block airflow. It's also less common but possible a critter has made a nest in there. Out of curiosity, whats the temp in your area lately?

 

[esebastian]

ok thanks Mike I will look into these things for sure. no bypass valves and the hoses are correct. But I have not opened the rad and then let it warm up to see if there is flow in the bypass. thats my next thing to check. Thanks for the tips.

Weather today is actually warmed up quite a bit. I am in mid west Canada and its -3 Celsius (24 F) but last week it was -35 Celsius (-31 F)

 

[esebastian]

That sounds normal with a 180 thermostat. Make sure your getting coolant flow thru the heater core. You could have the hoses reversed (unlikely and it should still flow albeit backwards), and there's no blockage in the core. If you take the radiator cap off BEFORE the engine gets hot you can see if coolant is flowing out of the bypass hole. If so, the heater core is not blocked. Only two things left to check; do you have a coolant shutoff valve in either of the heater core hoses? If so, it may be stuck closed or not opening. The last and most probable thing is the heater core airflow inside the case is blocked. It's common for debris to build up on the surface and block airflow. It's also less common but possible a critter has made a nest in there. Out of curiosity, whats the temp in your area lately?

Hi Mike, just wanted to follow up here. After replacing the stat, and ripping everything apart and replacing the heater core, making sure the blend doors were working and checking all the things you mentioned...Still was no heat. It wasnt until I started paying attention to the electric fans and when they were kicking in that I spotted the issue. See I picked up this truck with the electric fans already installed. And even though there is a cab switch to run them full time (if really hot etc) I did not realize that under the hood there was a switch to set when they auto kick in. This was set way too low for winter. so what was happening was the stat would kick in at 180 and open, heater core would heat up and boom had heat. At around 185 - 190 the fans would kick in and cool it back down to 175...stat would close and heater core would cool off. It wasnt until I adjusted the auto kick in for the fans to around 195 - 200 that it started to maintain operating temp around 190 and we are good to go now.... so safe to say my truck has some new parts that were not needed and I gained some knowledge on how to change them lol...thanks again

 

[SquareRoot]

Awesome. Glad you figured out the mystery. Stay warm.