I just installed a new blower motor today and found that there was a lot of pine needles etc in the bottom of the heater box. If you're sitting in the passenger seat, the low spot in the box just to the left of the blower motor is where said gunk resides. I can't get even a small shop vac hose in there...there's some kind of grate/grill in the way. Does the heater core sit there? Is there any way to clean this area without removing the hvac box? Is my pitiful heat partly caused by this junk clogging up the hearter cores program? Any tips/hints would be great. I plan to upgrade to a/c in the future, and would rather wait untill then to remove the box.
Thanks,
greg

 

The heater core is actually after the Evaporator. If you have poor heat it's due to either the engine temp not getting hot enough, the blend door not traveling far enough towards the hot position, or the heater core is partly plugged.

Engine temp might require a Cummins thermostat. The blend door travel could be a couple of things. The heater core could be disconnected and back flushed with a hose.

 

I have back flushed the core. Seems to flow good. No evaporator in the way, as this is a non-a/c truck, if that even matters. I've got the Cummins t-stat, but it still wanders pretty bad unless I'm pulling over 10k. Radiator is brand new, but is huge (5 core) so its hard to keep it warm. Frnt of rad is completely covered with a big truck mudflap.

 

I saw the "I plan to upgrade to AC" but thought you meant "the AC" Anyway if there is no evaporator in there then sure the heater core could become plugged on the outside with debris causing the air to take the path of least resistance and bypass the core. AS the heater core is further into the plenum than the evaporator it would be impossible to clean properly without removal. As there is NO A/C to disconnect it really is a breeze to take the plenum out and clean or replace the heater core. Just make sure that the doors work properly before reinstalling the plenum in the truck.

 

Took you 5 years with that HVAC cleaning & upgrading to AC didn't it.

 

I have heard that plenty of garbage/junk/debris can make its way into the HVAC box. But that said, if you're not getting heat then its most likely a heater core issue and/or low engine temp. If it was lacking air flow then you'd notice that there's hardly any air flow coming from the heater vents.....but it would still be HOT air since the air isnt going to cool off just because the fan isn't moving enough volume.

Making sure the engine is at proper temps is important. Not only for heater function but also because if the engine runs too cool then its not running efficient combustion which will result in poor fuel mileage and power and excessive wear and tear from fuel wash.

Install a good quality thermostat. Use a 190* as the 180* is too cold for the 24 valve. Make sure its either a Cummins stat or is the proper Cummins type. The Cummins stat is expensive but NAPA generally sells a good comparison for about $30. It must have the o-ring and it must have the jiggle pins. If the thermostat is working properly then no matter how big the radiator is you wont need a radiator front unless the ambient temps are zero or less.

If the engine is running at normal 190*-195* temps at all times then check the temperature of the inlet and outlet of the heater core lines. They wont be quite that hot but more importantly, if they're the same temperature then the likelihood is the core is plugged up enough to not allow heat dissipation. You can get water to flow through them but not have the water flowing through the entire core. If the inlet is warm and the outlet is cool then the engine temp is too low. LOTS of threads on this very subject.

 

Would be great if there was an option for 195 degree stats in the first gen 12v.

 

I was under the impression there is a 190* thermostat for the 12 valve. I'm not positive though as I dont research 12 valve parts too often but I do remember there being talk about one.
Also, I know that supposedly the 195* thermostat for the early 70's Mopar 440 engine fits the 12 valve. If that temp runs too warm then they simply drill a small hole in the thermostat plate which drops the overall temp.

 

Any idea whether the big block gasser stat would still use the Cummins rubber seal ?

 

I dont have the answer to that but I would presume that if you needed the rubber ring then you could re-use the existing unless it was damaged.

If you're really curious, simply Google questions like Cummins 12 valve 195 thermostat https://www.google.com/search?q=Cumm...utf-8&oe=utf-8 and weed through the forums.

 

Good stuff thanks !

 

You're welcome.

 

I have a heater problem I can't chase down. I get no heat until the temperature dial is just past the half-way point, then it goes totally hot. So basically all cold or all hot. Where can I look? AC is ok.

 

Sounds like a thermostat stuck closed, if "totally" hot meant the gauge.
Vent door/vacuum valve sticking if "totally hot" meant the vent temperature.

 

Yes. Vent temperature.

I had the spacer, or whatever the connector is called, that opens and closes the AC door replaced because there was no cold air even though the AC worked fine.

Now the AC comes out of the vent fine, just need to chase down the heat adjustment problem. I get plenty of blowing air - it's just either cold or hot.