I realize this subject has been beaten to death "The Wild vents thread" so I'll try not to focus on that. I just have one pending issue at the moment.

So I finally got my 2001 B1500 back the other day. If you remember, the engine was bad and I was having a used Dakota engine installed. The van now runs great but I noticed there is now very little heat coming out of the vents compared to the other engine. Even though the temperature guage reads normal and the guy installed a new thermostat.

I can move the temperature controller to cold (vehicle has A/C BTW) and turn off all the heat. Heater hoses before and after the valve are warm to the touch but not as hot as the upper radiator hose. When the temperature controller is moved, my father said he never saw the valve under the hood move. Should the valve open and close when the temperature controller is moved or is it an on/off valve that is either on or off depending on whether the A/C is on?

My guess is if a vehicle has no factory A/C (like my 84 B150), then it uses a blending door to control the heat. If it has factory A/C it uses a coolant valve to control temperature by restricting coolant flow to the core.

My van suffers from the "wild vents" issue where the defrost vent opens under acceleration which I have read about on this forum. That's an issue in itself. So I'm not looking forward to tracking down this vacuum leak. Also the cruise control light "works" but the cruise control doesn't work other wise. So I think the two are related. Also the A/C does not work but that could be no refrigerant in the system. And also the fan on blows on high speed. I replaced the resistor piece and the melted connector at the heater box and it still didn't fix the problem! So my next guess is the fan speed switch. In other words, pretty much the whole climate control system on this van is a mess.

I just want heat for the moment!

Friday when I picked the van up it was raining and had to nap before going in for the night shift so I have not had get a chance to investigate why there is little heat coming from the floor vent.

I've got three theories:

1) There is little vacuum present on the heater valve. But barely enough to open it slightly. When the guy did the engine swap he disturbed the already mess up vacuum hose system probably from a cracked rubber connector. Not his fault.
2) The heater core is higher than the engine so there is a big air bubble in the core that needs to be purged.
3) Some junk got dislodged from the used engine and has clogged the core and restricted flow so the core need to be backwashed.

As mentioned there is a "dog's leg" coming off the heater hose with a cap directly above the engine. Doesn't anyone know what I am talking about? What is the purpose of this piece?

The heater valve is pointed in the correct direction so it's not backwards. I did trace the vacuum line down which is either white or gray and it goes into a corrugated wiring loom. From there I guess is ends up in the cabin at the control panel.

I wanted to drive this van this week but it's getting cold so I decided to park it until I had a chance to look at it. I'm going to pull a vacuum on the heater valve to see if I can get it to open so at least know if it's a vacuum issue or something else.

Any thoughts?

 

I believe the valve should close when the temp selector gets past a certain point. If you are having wild-vents issues, and that valve is vacuum controlled, I would expect that the two issues are related.

Also, it seems it is notoriously difficult to bleed all the air out of the cooling system on this generation of dodges.

 

That's kind of what I suspected. Most "American" cars I just fill the system through the radiator and top it off after the thermostat opens and that's all it takes to get the air out of the system. On foreign cars like my Fiat Spider it make 2 systems to get all the air out because the radiator is lower than the cylinder head and radiator core.

Having owned several cars with finicky cooling systems, I suspected there could be an air pocket in the core because it's just higher than the engine. I have disconnected heater hoses and actually filled cooling systems that way instead of using the radiator just to make sure I'm filling at the highest point in the system.

I've got one of those Lisle funnels that really helps I'm going to try:

https://www.summitracing.com/parts/l...xoCQHYQAvD_BwE

First I'm going to pull the heater valve and see what's going on, try to backwash the core then top it off through both heater hoses.

 

Yeah, I have one of those funnels as well. I think I bought it back in the early 90's. I still use it.

 

OK. I'm not sure if I made any progress or not but I figured out a few things.

First of all I pulled the heater valve out. It does not function as I expected. It is a fail open valve. In other words vacuum is what closes it. So the valve was open all along. I thought it was closed or barely open.

The temperature **** has no effect on it the valve. Instead it is either open or closed. It closes when the A/C is turned on.

My guess is the temperature cabin is controlled by a blending door (both A/C and heat) by pulling air from outside bypassing the heater core and A/C evaporator by turning the temperature ****. The question is why am I getting little heat after an engine swap? But I can shut off all the heat by turning the **** to cold.

I hooked up my Lisle funnel and let the engine reach normal operating temperature. It looked like the radiator was a bit low on coolant so I was hoping that might be the problem.

I measured the temperature coming out the vent and it was around 117F on a 50F day. So I've established a bench mark at a 70F increase in cabin temperature. I have not measured the temperature on my Subaru to compare but I think the heat should be more than that. Used to the heater would put out blistering heat.

I picked my daughter up from school which is about 1 mile and back. I had the heater running but it did not put out any blistering heat like it did before the engine swap.

So what is my next option? Backwash the heater core? Is it restricted from funk from the used engine? I'm not sure how to get to the blender door without taking the heater box out. Which I don't want to do (looks like a job) only to discover the blending door is working fine.

It would be great to know if the blending door is fail open or closed. That might save me some time troubleshooting.

 

Found the blending door. It's on top of the heater box and appears to be working when my wife turned the temperature ****.

Return heater hose doesn't feel as hot as the supply.

Next step is to backwash the heater core with a water hose and see what comes out if it .

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​​​​Also found a disconnected cabel near the box. Looks like a throttle cable so I think that's why the cruise doesn't work.

 

Yeah, back flush the heater core, and when you go to hook the hoses back up, keep as much fluid in the hoses as you can. I suspect you have an air pocket there.

 

Im dealing with the same BS James is...
There is definitely air in it.