Import cars almost always have a heater valve that controls coolant flow through the heater core to regulate heater temperature. But I know that at least SOME domestic engines (the Fold Vulcan comes to mind here) require a constant flow of coolant through the heater core. Temperature is controlled by bypass flaps rather than coolant flow. If coolant flow is blocked with the Vulcan, no coolant reaches the thermostat to make it open, and the engine overheats. Is the LS this way? I don’t want my engine to overheat when I first start it in my S13 swap.

 

I believe the LS heater ports need to be flowing to work right.

 

There are a lot of threads here on Tech about this. Use the search function. I’ll say this…I’ve seen a bunch, prolly two-three dozen LS powered off road vehicles that don’t even have a heater core in the vehicle, that never overheat. Now, the debate boils down to capping the nipples on the water pump, or using the short re-circ hose that feeds the coolant through the pump, and acts like a super short set of heater hoses. Again, I’ve seen guys in the real-world and here as well, do it both ways without issue.

 

Correct or not, I have a shutoff valve inline on my heater hose and it's turned off. Been that way for 8 years and 5,000 miles. My temp gauge is rock steady.

My 67 probably needs gaskets in the heater box to stop hot air leakage in the summer but this was my quick fix.

 

Pics of a pump cutaway I made.

 

I have the standard heater in my Miata, and the engine doesn’t overheat, no matter where the temp slider is set.

 

Required, no. Helpful, yes. People have done both, but connectivity between the two ports at the circulation pump will typically help it run more consistent from a temp perspective.

 

It looks to me like a bypass valve/hose you would see on Big Block Chevys. Once engine is in operating temperature not needed as much but when warming up keeps some fluid moving.

 

I’ve used on like this for years without problem. IIRC it’s a Mishimoto.

 

You need a CONSTANT FLOW between the 2 heater ports on a LS motor for the thermostat to properly function! ABSOLUTLEY do not block the flow...There are heater control valves like the one used on many Fords that allow you to cut flow to a heater core while maintaining flow between the motor outlet ports.

 

The gen III and IV LS differs from the SBC and BBC in that the thermostat is located in the inlet/cold side. It’s for several reasons, to avoid vapor pockets near the thermostat and thermal shock to the radiator among others. It means it takes longer for the t-stat to open if no ”hot” flow over it (ie plugged heater fittings).You can avoid this by using a t-stat as the above pic, it just takes a slight flow over the t-stat.
But if the t-stat seals good between hot/cold side and no holes in the t-stat you might have a initial overheat condition.

 

No heater core in my car , just have a loop on heaters ports, never overheated
I did just buy a heater core, my new place out bush gets to minus 4 during winter and i think its safer to be able to see where ya going

 

I have two motors running with the ports blocked off completely and I have one that was plugged and now hooked to the heater core in my S10 and there is no difference in operating temperature at all, They use stock water pumps and thermostats.

 

Don't forget the intake manifold tube runs coolant around the thermostat as well. I've noticed on engines with good logging that having some flow, doesn't need to be massive, make the temps more consistent. YMMV.. You wont see it very often on a standard gauge. Many of them are damped to only move on 5 to 10 degree changes...

I've also fixed more than one track car with overheating issues by just drilling a 1/8 inch bypass hole around the thermostat. Or by putting a fitting on both sides for 1/4 inch hose. I've had good luck on ls type motors putting reducer bushings in the heater hose locations and just looping it with a small tube, or hose. (1/4 inch... ) I'm sure that its the whole system as well, maybe some radiators have more or less back pressure, or something else in the system..