Previous owner rigged the heater core piping so now my heater core is always hot. I can't hardly stand the heat during a NC humid summer afternoon. I'm going to correct the entire circuit in the next few weeks, but there is a shut off valve in the hose coming from/going to the water pump. I'm not sure of the flow or if blocking the water through the heater core will cause over heating. I would like to turn off the heat, but I don't want to screw up my engine.

Thanks for the help.

Edited August 11, 201311 yr by 78sid

To temporarily block the heater core, just plug both engine hoses. This is the same as having the heater core turned off, so its safe.

The common mistake is to connect (short circuit) the engine hose connections. This reduces head cooling, effectively making the hose the easiest way for the coolant to get from the back of the head to the rad, rather than making it go thru the head, ie removing heat! Its ok to do for a short period, like in an emergency on the road, but don't leave it like that.

The heater valve is a restriction even when its wide open, so having the heater valve open is NOT the same as connecting the head connections with a hose.

Thanks and understood, but since the valve in the photo would close that circuit, wouldn't it effectively do the same thing as blocking both ends? I'm just trying to avoid draining my radiator this weekend, when I'm going to replace it in a couple of weeks.

This thread brings up a similar question for my 280ZX.

On the rear of the head there is an octagonal stud that sticks out that the heater core plumbing attaches to. Actually two hoses attach to it. One that CONNECTS to ANOTHER ONE from the front of the engine, at a T-fitting, (bypass??) and the one that actually goes into the heater core.

Is that just a solid, metal stud, with a connection between the two nipples, or does coolant flow out of the engine/head THROUGH that stud. (Making it a 3-way of some sort...)

If it's a 3-way, is there a check valve or restriction in the system somewhere? Like at the T-valve? If not it appears hot coolant could choose to completely bypass the heater core.

Anyone know the theory on how this plumbing nightmare (rivaling a Jaguar) is intended to operate?

Restriction in the T-valve, perhaps?

78sid, that setup looks fine to me. Closing the valve completely should stop hot coolant from going through the heater core and heating up the car in the summer which is what you want. Seems like an improvement to stock system. Also you could close it a little as a restrictor to protect the heater core from too much water pressure. Don't know if this is ever an issue for Datsun but I've read a lot about it on other forums (Late model Mustangs), where they bust a lot of heater cores. Looks like your line from the thermostat housing through the intake manifold was removed, if it's a 240 or 260.

Wade, could you post a photo of your setup?

The restriction is a temp valve on the back of the intake manifold (SU 240) feb by the pipe that goes around the back of the head. It closes when the temp gets high enough. Its just to warm up the intake, not to continuously heat it once the car is up to temp. Its a bypass on the thermostat as well when cold. Everyone should read the hybridz thread about cooling these engines and what might happen when you loose important things like manifold bypass and other "un-necessary" stuff..