Here is an important product from LOJ Conversions: The LS Swap Heater Bypass Block. US Patent Pending...
This version of the LOJ Conversions Heater Bypass Block is manufactured with a 5/8" and 3/4" Hose Nipples on each end of the heater block. It is supplied with a 3/8" NPT port plug and a 3/8" NPT to 1/8" NPT bushing for the installation of an aftermarket coolant temp sensor. It also includes a 3/8"NPT to M12x1.25 bushing to suit the installation of a factory Nissan 300ZX or 240SX Gauge coolant temp sensor. A set screw for grounding the block is also included, and often required when installing single wire temperature sensors. Two mounting holes are integrated as well, for mounting to the vehicle chassis if desired.
Why is this important, and why does it exist?
That answer is simple and complicated. This block installs in-line with your heater hoses. You will notice in this cross section picture, that the coolant passages form an "H".
The heater circuit on some Nissan vehicles (300ZX, 240SX) as well as many classic muscle cars utilize a heater control valve that blocks flow to the heater core when the heat is off. This completely stops flow through the heater core hoses.
The heater circuits on Chevy LS powered vehicles utilize a heater control valve that has an internal bypass. When the heat is off in a Chevy LS powered vehicle, coolant still flows through the heater hoses.
"So what?" - Well, it is a well known fact in the LS community that you can not block the heater ports on your water pump, they must be looped to always allow flow. If flow is blocked in these ports, you will cause a pressure differential on your water pump impeller creating cavitation, resulting in aeration of your coolant and an overheating condition.
This product is intended to solve this issue, while also adding a convenient place to mount your stock gauge coolant temp sensor, as well as an aftermarket CTS if desired. The block is available with or without a new factory style CTS, and with both a bushing and plug for the aftermarket CTS port. There is also a mounting screw for a grounding wire for proper gauge operation.
The coolant will always take the path of least resistance, which is a straight shot to and from your heater core, but will pass through the "H" when the heat is off. We have tested this product in 20* ambient temps and the heat works flawlessly.