Can anyone tell me how the heater circuit works on these trucks?

My son just bought a 90, so after a 20 year break, I'm reacquainting myself with the infamous 2.2l FE. One issue im perplexed by is the lack of warm air from the heater. I suspected I wasn't getting any coolant flowing through the heater core; or if I am, very little. Even with the truck at full operating temperature, it blows tepid air and the supply and return hoses to the heater core in the engine compartment are cool.

I pulled the lines off and backflushed the core with a section of 5/8" heater hose connected to a garden hose. A little muddy water, but nothing overly concerning, and it appears as though I have full flow through the core. I hooked the supply hose back up to the heater core and backflushed it again and coolant came all the way back through the unhooked return line. So I dont think there's a blockage anywhere. Lastly, I left the supply line connected to the heater core and started the engine. I got barely a ****-stream dribble out of the heater core return. Is there a vacuum operated heater control module somewhere? It looks like the supply is fed off a coolant bung on the intake manifold. Am I correct?

Just looking for some insight.

 

First of all, I will say that my 1988 B2200 puts out GREAT heat, better than ALL my other vehicles. I mean hotter and sooner.

1. This is a huge clue, as these should be hot within a few minutes. This is where you want to focus your investigations.
2. it appears as though you have full flow through the core, 2nd clue.; this indicates that the issue is NOT in the heater box inside the dashboard.

I'd say there's an engine issue, like the thermostat is missing, always open, or maybe too-low a heat range. I use a 192/195F Stant thermostat in my B2200, and my heat was not as good when I used a 180F thermostat. And I don't run hot, even in Arizona.

I've heard of folks blocking off half their grille/radiator with a piece of cardboard for the winter.

Diagram of the cooling system: see Mazda B2200 Coolant Flow Diagram - mazdatrucking.com

 

Thanks Cusser.

Funny; while lying in bed last night, I started to suspect the t-stat as well. He just bought it Friday, so we haven't had much road time with it. However, I drove it to work today and about 15 minutes into my 20 minute commute, I started to get noticeably warmer air...not "hot", but warm.

Thanks for the diagrams. I'll look them over.

 

Important: on these trucks, especially important to bleed ALL the air out of the cooling system, and sometimes tough to do. I jack up the front end, take off the radiator cap, turn on the heat, and run the engine for like 20 minutes, topping off with 50/50 as necessary until system does not accept any more.

You just got this truck, and you just installed a thermostat, likely low on coolant due to such air pocket.