Have you seen the online flame wars about how Chevy LS thermostats work and whether it's OK to cap off the heater hose ports?
The LS engine series has been around for more than 25 years and these disagreements still pop up regularly.
Early LS engine swappers who weren't using a heater in their hot rods were faced with the question, "Is it OK to simply cap off the two heater hose barbs next to the thermostat housing or is a loop connecting them necessary?" Here are the two most common experiences LS owners have had:
- No loop needed. I capped off the heater ports and everything was fine.
- I tried capping off the ports and my engine had overheating issues. Then I installed a hose loop and the engine cooled as it should.
We wanted to know why this contradiction existed so we fired up the old Googlenator but got no definitive answers and no good explanation about why the LS thermostat is made so differently from conventional thermostats.
A traditional thermostat is a pretty simple device. There's a dis serving as a valve. This disc is held in place by a spring. Connected to the disc/valve is an area called a "wax motor." When the wax is heated it expands and opens the disc/valve, letting coolant flow.
Old school small block and big block Chevy engines have the thermostat located in the coolant passages in the intake manifold, keeping coolant in the engine until operating temperature is reached and then letting coolant flow out the top of the engine and into the top of the radiator.
The LS thermostat has a second disc at the opposite end from the main disc. This second disc is held in place by a smaller spring and appears to close off the 5/8" heater outlet from the 3/4" heater return port on the water pump.
It turns out the second disc allows a small amount of coolant to bypass from the 5/8" side of the heater outlet to the 3/4" side even if the outlet barbs are capped off. We surmise that's why some people had no overheating issue. The bypass let enough coolant contact the thermostat to open it.
For others, that small amount of bypass allowed the engine temp sensor to read high before the thermostat finally opened.
Bottom line: If you aren't running a heater then it's a good idea to use a hose loop between the two heater barbs to give the thermostat the best chance of reacting quickly to engine temperature. But, if you decide to cap the ports, eventually the thermostat should open.
Now, let's get back to what prompted this deep dive into LS water pumps and thermostats.
We have two OBS projects (1988-98 Chevy trucks) in the works. One with an EFI LS engine swap. One with a carbureted BBC. We wanted to be able to run a two-speed electric fan and not depend on the ECU to send the appropriate fan signal.
Our solution came in the form of a sensor that installs in a coolant passage and can send signals at two different temperatures. In a conventional SBC or BBC, that sensor can screw into the intake manifold ear the thermostat housing. But LS engines don't have that option. We found a "T" fitting that can be inserted in a heater hose and has a threaded port the sensor can screw into. We just wanted to be sure that the LS heater outlet would have the correct temperature for the sensor. Hence all this digging into how an LS thermostat actually functions.
If you want to learn more details on our fan control system, head over to our YouTube channel and check out our dual electric fan setup for our 6.0L LS swapped Chevy Tahoe. You'll see how the fans will come on based on coolant temperature but also come on when your AC system starts up. And you'll see where we mount the relay box and how we've made it a bolt-on, plug-in system with no ECU wiring or programming required.