Anybody want to take a guess?
2000 model automatic. Truck used to take FOR-EVER to warm up enough for heater in the winter.
Then, last summer, it started overheating when pulling hills towing a load.
I assumed poor circulation, radiator stopped up (that would cause both).
I drained the anti-freeze, refilled with water and some Cascade. Put around 100 miles on it, drained it, refilled with water, another 100 miles, flushed, flushed, flushed. THEN, installed another radiator. Inspected the old one and the lower 1/3rd of it was stopped up/restricted tubes internally.
No more overheating.
BUT, not long after, I began to notice what sounded like static noise coming out of the passenger door speaker with the radio turned off. AND, only while accelerating..??
Close inspection reveals the noise described above is coming out from under the passenger side of the dash, and only every now and then, and then only under acceleration.
Not easy to crawl under while I'm going up a hill though.
AND....The next time I tried to defrost some fog off of the windshield, no heat came out. No heat comes out anywhere, but, the vents still change from floor, to dash, to defrost correctly.

SO, Something either went bad about the time I flushed the cooling system and changed the radiator, OR, this has something to do my Cascade flush and radiator change. (more than likely the latter).

Do I have a vacuum blend/heater door actuator leaking vacuum during acceleration and not operating, OR, do I have a stopped up heater core?

Anybody care to guess before I tear into the wrong thing tomorrow? Because, It sure was a cold drive yesterday morning.

 

Heater core gunked up?

 

Low coolant level could kill the heat too. That is easy to check before removing most of the interior to look at the top of the HVAC box to check if some vac connection came loose.

 

Disconnect the heater supply hoses in the engine bay and run a hose through them both directions.

 

My plan was that, but with air. Didn't even think about a water hose till I read this. So, I used water hose.
Never noticed any gunk come out, but, I squirted high pressure from the hose thru it, both directions, several times.
Nothing else.
Reconnected and....I have heat as of right now anyway.

 

You probably had an air bubble in the heater core. If so you wouldn't get any coolant circulation which means no heat.

 

Is the hose to the heater core hot? Could be sticking or bad thermostat. I had a non-cummins one that had a burr on the shaft and it would work some times and not others. When I took it out it was stuck on an angle.

 

I removed the hoses, ran high pressure hose water through both directions, and...THAT CURED IT!
I now have heat again!

I wondered about air lock, but, it's been that way since first of summer. Sure thought an air lock would work itself out in that amount of time, but, I dunno?

It's fixed now though.
Thanks yall!!!

 

If it were air locked, the air bubble wouldn't have have worked it's way through. My 98 12 QC was lukewarm at best for a couple years until we pulled the upper hose and filled the heater core allowing the air to escape.

 

I now think I have an air bubble in my brain.
I've got the water hose...do I go thru the ear or nose?

 

Face down though the nose.
That way the bubble will pop out your ears.
47 psi max

 

LOL!