M923A1 Heater

[sd39u]

It's getting cold outside finally...and I've discovered that my heater isn't working. Drove the truck today for 10-12 miles and it never got warm air coming out of the defrost vents. Makes for a chilly ride in the 37 degree rain!

Stopped to get fuel, opened the hood and then opened the little valve on the side of the heater core and coolant flowed out. Any other simple checks before looking for a new heater core?

Thanks!

 

[juanprado]

That valve is to bleed the air out of the system on the side of the core. Check the shut off valves in the heater hoses. Follow both hoses to the motor as each one has a shut off.

Not uncommon to find the handles snapped off also.

 

[Ddmk18]

May have to flush out the heater core also. They will get crud in them.

 

[M35A2-AZ]

If the valves are off, I would take the hoses off and flush the heater out so the water does not go into the motor.
When I got one of my trucks the heater valves were off, when I opened them I found a leak in the heater core and all kind of crap came out of it.

 

[JohnnyBM931A2]

Mine has the same issue, though my truck is an A2. What does your temp gauge show after you have been driving for a while? Mine only starts to register temp by the time I reach work. That's 2 or 3 miles of driving, on top of 4 or 5 hours with the block heater operating.

 

[MtnSnow]

The A2 motors run cold anyways so those of us in cold areas usually need to put on a "winter front" which basically is the same thing as putting cardboard in front of the radiator to block the air flow.

Image is from member Jatonka who sells them on his website

 

[MWMULES]

If the winter front and core flush doesn't work I have a spare at the barn.

 

[74M35A2]

Surprised nobody mentioned thermostat. Is the engine running at 180F? A diesel will almost not even warm up at idle with no load. I changed my thermostat, and flushed the heater core, still no heat. A buddy said to change the heater core and that did the trick, even though it flowed well. Now in 37F temps like you say, I have to shut the heater off and roll the window down. Once thermostat is well and engine temp is at 180F, feel both heater hoses when on high heat. Inbound should be hot and return should be cooler. If both hot, your heater core may be mineral laden also.

 

[JohnnyBM931A2]

The A2 motors run cold anyways so those of us in cold areas usually need to put on a "winter front" which basically is the same thing as putting cardboard in front of the radiator to block the air flow.

Image is from member Jatonka who sells them on his website
View attachment 600752

Awesome. I had never seen those for the M939 trucks. I'll have to look into that.

 

[sd39u]

Problem solved! Heater core was full of crap.

I've got plenty of heat now!

 

[MWMULES]

I wonder if the crap in the heater core was some of the same that helped cook the first motor.

 

[Vintage iron]

The 8.3L motors are notorious from getting muddy coolant. I always flush my customer's trucks and put a coolant filter in them.

 

[JohnnyBM931A2]

Problem solved! Heater core was full of crap.

I've got plenty of heat now!

D*mn.. Maybe mine has the same issue. What was the temp gauge showing on your dash?

 

[sd39u]

Gauge was showing 180-190 after I drove it about 12-15min.