downset71

really couldnt find out in search mode. which hose is supply to heater core?front or back? (dont want to make a coolant mess on floor)

Schitzo

iTrader: (6)

The larger one(front or closer to the rad) is the return while the smaller one (back or closer to the firewall) is the heater core supply.

rao

iTrader: (18)

^^^^^ +1

bczee

iTrader: (12)

In general.. your discharge from pump to the heater core is usually smaller in dia. that would be your 5/8" hose and your return which is usually large in dia, 3/4".

If you look at your A/C and your Power Steering system, the high pressure line is usually smaller in size and the return line (usually low pressure) is usually larger in size.

downset71

good lookin out ...saved me 10 minutes of work...lol

67rsss

iTrader: (4)

GM usually set the heater lines up so that pressure is retained for the inlet and relieved for the outlet. This means the larger nipples are the return, the smaller nipples are the feed. Just connect the small one on the water pump to the small one on you rheater core.

Jaysz28

iTrader: (19)

Hrmmm, good info I was just about to start searching for! LOL

thanks

-jason

SirWarrior

unless your tubing sizes match, it really doesnt matter.
heater cores aren't directional, so even if you'd hook it up backwards, it would still flow fine.
so if you got 2 same size fittings on heater core, just use whatever works best/easiest.

67rsss

iTrader: (4)

I beg to differ on that statement. While I cannot say for all of them, there are several cores that sit higher than the coolant level of the engine block and would fill up with air if the coolant flow was reversed.

Thaniel

iTrader: (2)

On my swap the heater valve is unidirectional so hooked up backward there is no flow. Don't ask how I know.

Y-NOT

Vintage air's heater valve is directional... That is why I searched this site before I asked a question that was already asked.
Bill

Shawn MacAnanny

iTrader: (12)

I actually didnt know this. I thought it wouldnt matter. Mine had two different sizes anyway so i put 3/4" to 3/4" and 5/8" to 5/8"

67rsss

iTrader: (4)

And that will work!

Davy_Baby9

iTrader: (23)

What would one block off the lines with if they tore their heater core out? Im not really sure.

need ls1 not wankel

iTrader: (1)

davy check this link if u are talking about blocking out the ports at the water pump this should be the answer
https://ls1tech.com/forums/showpost....2&postcount=17

Shawn MacAnanny

iTrader: (12)

While we're on the subject how exactly does the heater core work? I mean the two lines coming off of the water pump transfer heated coolant through the heater core obviously but is that coolant run through before or after the thermostate. What i mean is that coolant based on radiator temperature or engine temperature. I ask because it seems like Duramax NEVER heats up and acts as though the entire radiator must heat up befor i get heat and yet my LT1 has heat in under 5 minutes.

rao

iTrader: (18)

It depends on the system, some have a valve that controls coolant flow into the core and some have full flow all of the time and use a system of flapper doors to direct air over the core or around the core depending on how much heat is desired.

67rsss

iTrader: (4)

The diesel also doesn't get anywhere near as warm as a gas motor. I keep mine plugged in all winter, and it still takes a while to warm up. Even with the front cover, it it is below 0, I will go out and start the truck, flip on the seat heaters, and go back inside until the truck runs a high idle cycle (this can take 25 minutes or so). Compression engines just don't generate the heat that spark engines do, this is a big part of the efficiencies and lifespan of a deisel.

SirWarrior

sure, but tell me why if you pull the thermostat, you could access both "feed" and "return" without much being in the way?

bczee

iTrader: (12)

There is a GM part number ( and other vendors) for a U shaped hose to act as a bypass loop for the heater ports on the pump.. if you search, I am sure someone has already posted the part number.