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KEEPING YOUR COOL(ANT) | A Better Coolant Bypass Cap For Your Engine

January 29, 2020 3 min read

Engines are one of the most intricate pieces of a car. Because so much time is put into an engine's design, automakers find it beneficial to design engines suitable for multiple applications. Look at the GM LS/LT platform for example. If it comes from General Motors and has a V8, it is likely equipped with an LS or LT (or some other variant of Chevy small block). Although the engine can be shared, the vehicle size or space available in the engine may dictate the placement of certain components such as coolant plumbing. Rather than producing vehicle-specific engines, automakers save on development costs by applying this design philosophy.

Rubber coolant cap used to block off the heater core port on the engine head
The same engine from two different vehicles with entirely different coolant lines. The engine on the right provides coolant to the heater core via the port on the head, whereas the engine on the left blocks off the coolant port.

The factory installs rubber caps to close off ports where coolant hoses are not needed. The caps installed by the factory are one of the weakest links of any cooling system. Radiator hoses typically have a layer of fabric reinforcement between the rubber layers whereas rubber caps have no reinforcement at all. As rubber hardens and becomes brittle over time, the cap’s ability to hold pressure is reduced. With these caps under pressure for a long period of time, it can not end well.  When we noticed this potential flaw, we took it upon ourselves to produce a cap that can hold up to the job while reaping the benefits of silicone.

Common coolant leak issue caused by rubber coolant bypass cap deteriorating with heat and age
Here is a stock rubber coolant bypass cap deteriorating with heat and age.


The construction of our reinforced silicone coolant caps (RSCC) is what sets this product apart from the competition. We use silicone instead of rubber for various reasons. Primarily, silicone has a wider operating temperature range (-65 F to +350 F) compared to EPDM rubber which has a temperature rating of -40 F to +275 F. Like our HPS silicone coolant hoses, we reinforce our coolant caps with 3 layers of polyester (compared to zero reinforcement from its factory counterpart). The added reinforcement prevents expansion under pressure and makes the RSCC stronger than traditional coolant bypass caps.

HPS Reinforced Silicone Coolant Cap cross-section - showing 3-ply polyester reinforced layers
Close up cross section of the 3-ply reinforced HPS Reinforced Silicone Coolant Cap.

We have found many uses for The HPS RSCC during engine swaps, pressure testing, and race prep. Removing and bypassing the heater core is made easier with the reinforced silicone coolant cap whereas sourcing a suitable hose to loop the inlet and outlet ports can often be difficult. A popular (but controversial) mod is to bypass the coolant line to the throttle body to lower intake temperatures. The RSCC is not limited to the cooling system and can also be used to replace old and damaged vacuum port caps.

HPS High Temperature Reinforced Silicone Coolant Cap installed on 2JZ-GE engine running Borg Warner S300 Oil-Cooled Turbocharger
Our 2JZ-GE runs a Borg Warner S300 oil-cooled turbocharger. It does not need any coolant so the coolant port typically used to supply coolant to the turbocharger from the engine is now capped off with the HPS RSCC.

If one of your stock coolant bypass caps were to fail and you are thinking about replacing it with a vacuum cap from your local auto parts store, keep in mind this will only mend the problem temporarily. Although they look the same, vacuum caps are made of vinyl or rubber with a low temperature rating not suitable for coolant temperatures.

The HPS Reinforced Silicone Coolant Cap is a permanent solution for your leaky coolant bypass and vacuum ports. Don’t wait until it is too late. Preventative maintenance with quality parts will save you time and money in the long run. HPS RSCC are currently available from 1/4" to 1-1/4" to complement most of your coolant and vacuum port needs. Visit our website or contact your favorite HPS authorized dealer to order yours today.

HPS Reinforced Silicone Coolant Cap , Sizes Available from 1/4 to 1-1/4 inch, Black or Blue

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