Wildcat, you pointed out the "T" that I'm guessing runs from a coolant supply line, the heater core supply line, then to the auto trans cooler. My question is what if you have a manual trans do you need to make a by-pass for the mod?

OK, so I'm adding a small piece to an old thread. Here goes.

1. soaked floorboards.... easiest way to tell if coolant or condensation, is it glowing green or clear?

2. for prevention of heater-core stagnation, would there really be anything wrong with cranking the ball-valve down to about 5-10% and leaving it that way?... The hot-water flow would be greatly diminished, but, there'd be enough circulation to prevent stagnation. Instead of a 14*F reduction, maybe you'd see 12 or 13, instead, but, without the worry.

I'll add on to what 65stangboy and begle mentioned: why do you think they tapped the heater off the back of the head starting with the 3rd gens? And if you are seeing a 14degree drop you have blend door problem.

I did mine with a three-way valve to bypass the core and keep coolant flowing. That tends to avoid blown freeze plugs and melting #6. I tested it on long drives and never saw more than a 1-2 degree difference measured at the vent with a properly functioning blend door.

Interesting, was wondering what that was on my truck.

I was thinking about this mod on my 04, but maybe i need to look into the blend door idea....

Anyone know the answer to his question? Does a manual transmission need a by-pass to continue the circulation?

I looked at my truck(which has a manual transmission) and it appears that the coolant supply line goes to the heater core and then runs back to the water pump. Just one single loop. I don't think cutting off the supply of coolant entering the heater core will affect anything on the engine significantly. Maybe some of the Cummins Diesel Experts who know more about the manual trucks and cutting off the supply of coolant would comment.

I found the same thing 3/4 PEX ball valve takes up less space and easier to install.

Mine looks like the pictures below. You can get this heater shut off valve at Oriley's for $10 and change. Tell them to look it up for a 1971 Dodge D100 truck! Anyway, Oriley's heater shut off valve is made by Murry and is American made.

Attachment 31410


My Truck is a manual NV5600 transmission and the return line from the heater core goes towards the engine and then to the water pump. I don't think cutting off the flow to the heater core will hurt the engine.

Attachment 31411

What are you guys using to measure and compare the air conditioner temperature out of the vents?

Just did this works great cold cold cold👍

I did a little changing around! It seems that the use of two shut off valves may stop heat into the heater core from convection via the return line. I used 3/4" shutoff valves with 5/8" nipples. I probably could have used the 1/2" shutoff valves but did not want to go smaller than 5/8" hoses.


(The 3/4" shutoff valve is much easier to turn than screwing the other shutoff valve.)
Attachment 31409

I know this is an old thread but....
Won't closing off the heater core inlet hose cause the water pump to eventually fail because of the built-up coolant/water pressure not being able to circulate?
If so, wouldnt it be prudent to insert a T- or Y-shape bypass-like valve instead of a complete cut-off valve?