No Heat 78 3/4T - Help With Advice Please

[Chevy Chase]

I hate to start my first post with a tech question, but I'm going nuts:

78 3/4 Ton, No AC, 350/400 truck with NO HEAT!

The fan blows like a champ.
The radiator is full.
The truck has a working thermostat and runs great.
All the trap doors on the heater box work by the cables.
I have blown water through the heater hoses and core and nothing is plugged.

The original engine was swapped for a 87 350 truck engine and sports and Edelbrock intake and headers. Otherwise stock. I am down to maybe two possibilities:

1) Although the cables are working on the heater box, maybe something is broken inside? I this a common problem?
2) Some kind of air lock? Caused by the engine change/different water pump, ??

It kind of seems that the heater hoses are not getting hot to the touch, so I am leaning on #2, but don't know how to fix that.

Any advice appreciated   

[VileZambonie]

So you are saying it blows but only cool air correct? Post a pic of how your heater hoses are connected.

[FlatBlack77]

Post a pic of how your heater hoses are connected.

this. there are a handful of different ways they can be routed and not all of them work.

having an aftermarket intake could have cause someone to change something around(to save time/money on new fittings ect.) then maybe change it yet again when swapping the motor into an older truck.

[Chevy Chase]

Correct - just blows cold air. If the truck has been running for a long time, then I'll get slightly warm air. Hoses are still not hot to the touch, so it seems like fluid is not being pushed through them.

Crappy pic, but its dark and I have this one:

Red hose, engine water pump to lower heater core.

Black hose, radiator to upper core.

No valves, not nothin. Just straight hose.

[Chevy Chase]

Here is the original set up before changing the engine. Same routing of hoses with the new 1987 engine.

[Don5]

It could have the wrong thermostat in it. The thermostat could be opening too soon at a lower water temp hence the low air temp. I think there is a 165 degree thermostat as well as a 185 degree one for these trucks. You want the 185 degree one in your truck for the heater to blow really hot. The water is circulating in the engine but at a lower temp. This is my thought on the subject, I could be wrong. Don't ask me why I think this 

[jetmech]

Not sure if it matters but the red hose should be connected to the intake manifold and not to the water pump. Can't tell from the original pic but that is the only thing I can think of.

[bd]

X3

VileZambonie and Jetmech called it.

You have the heater hoses routed so both are connected to "alternate" return locations.  The 5/8" hose should connect to the intake manifold water crossover adjacent to the thermostat.  The 3/4" hose should connect to the radiator tank.

[Chevy Chase]

Thanks guys. I believe you got it.

When I got the 87 engine the temp sender was in the manifold and a hose was connected at the water pump. I didnt notice it when I made the swap and connected it the same way. I'm going to flip these tomorrow I bet that will take care of it.

 

[bd]

The temperature sender needs to be mounted in the intake manifold water crossover (or cylinder head, depending on the purpose of the sender) to properly sense engine temp.  The 5/8" hose routed to the heater inlet, also, needs to be connected to the intake manifold water crossover. 

Are you saying you intend to move the temp sender to the water pump and the heater hose to the water crossover?

[Chevy Chase]

That's what I did. I have heat now     and the temp gauge seems to work just like it always did. I guess it may not be perfect but it is working.

[bd]

I'm glad you have heat, now! 

But, a word of caution:  The temp gauge sender should be threaded into the left cylinder head between the #1 & 3 spark plugs, if there isn't a spare coolant port in the intake manifold water crossover.  With the sender mounted in the water pump, the gauge will read the temperature of cooled water exiting the radiator.  A motor could overheat before you have adequate indication a problem exists.  You should consider reconfiguring so that the sender is immersed directly in the engine's water jacket.