Coolant Bypass Systems

In previous chapters, we discussed how thermostats worked to control the flow of coolant between the engine and the radiator. In this discussion, we will discuss what happens when the thermostat is closed, blocking the path to the radiator. 

When the thermostat blocks the path to the radiator, the water pump continues to work. If the radiator inlet was the only path for water to follow, flow would stagnate. This would have several undesirable effects. First, flow of coolant past the thermostat would cease, and it would take a very long time for enough heat to accumulate to open the path to the radiator. There would be no flow of hot coolant through the intake manifold, so fuel vaporization would suffer, affecting cold running. Hot spots may develop in the head.  The solution to is a bypass passage which allows coolant to flow around the engine and back to the pump inlet whenever the thermostat is closed. The bypass can be internal to the engine, or it can be implemented with external plumbing (as is the case in all Jaguar applications.) There are several ways to implement a bypass, we'll go through each of them in detail. 

Bypass History
Everything has a history, including thermostat bypass systems. Let's look at the historical development of bypass systems.

In the beginning...
The very first automobile engine to incorporate thermostatic temperature control was the Cadillac V8, introduced in September 1914 as a 1915 model. The thermostatic bellows element was supplied by Sylphon, and controlled the engine inlet:

The bypass in this early Cadillac comprised a small tube leading from the carburetor water jacket to the pump inlet. This was sufficient to provide some circulation when the thermostat was closed, the clear intent being to encourage fuel vaporization. However, the open bypass was not designed to limit flow when the thermostat was open. As a result, some hot coolant always recirculated through the bypass line. The only limitation on the volume of recirculated coolant was the small diameter of the tube. Although somewhat primitive, this thermostat and bypass system allowed the Cadillac engine to develop an enduring reputation for reliability and economy which proved to be the foundation of the brand. 

Not to be outdone, Packard's V12, introduced in the Spring of 1915, incorporated a Sylphon bellows unit which controlled both a primary valve and a bypass valve:

The Packard engine, like the Jaguar V12, was essentially two six cylinder engines built on a common crank. Each bank had it's own water pump and thermostat, so the Packard was prone to some of the same problems as the Jag. The Sylphon bellows was still not built into a self contained thermostat, but operated a bellcrank system on which two butterfly valves were mounted, one controlling the bypass and one controlling the radiator passage. This allowed full volume bypass circulation, a big improvement over the Cadillac system. Thermal control was comparable to a modern engine. 

During the late teens and early twenties, developement of bypass thermostats was sidetracked, as many cars were still being built with either thermosiphon systems or air-side shutters, neither of which requires a bypass. Where thermostats were used, they were often bimetallic valves. Some bypass circulation was provided by perforating the valve, or by allowing some circulation through the heater if so equipped. This circulation came at the expense of longer warm ups and poor thermal control. Bimetal thermostats were very susceptible to corrosion. 

By 1930, two events came together to advance cooling system design. The first was the development of the self contained cartridge thermostat. As we know it today, a brass or stainless thermostat includes the motor, valve, and support structure, and is built as a replaceable assembly. We don't even think of these things as separate parts today, but it was a new idea at the time. The other event was the acquisition of a series of thermostat companies by Reynolds Metals (as in Reynolds aluminum wrap), accompanied by a big sales push. Among the companies absorbed were Fulton Sylphon, Robertshaw, Bridgeport Brass and American Thermostat. A veritable juggernaut of thermal control. These companies would variously operate as independent or combined entities, aggressively dominating the thermostat market for many years. It's a very confusing corporate history. 

Buick 1931
Building on these events, a milestone in cooling system design was the 1931 Buick 8, which included a bellows thermostat. This relatively modern single poppet thermostat was new to the market, but for this story, the real news was the bypass mechanism:

The diagram is a little unclear, but what's going on is that a single poppet thermostat (T) controls the water exit to the radiator (R). But a completely separate spring loaded bypass valve (B) independently controls the bypass. The way this works is that when the thermostat is closed, it arrests flow, which creates a local area of high static pressure. This pressure opens the bypass valve, allowing coolant to recirculate to the pump (P) and back to the engine (E). When the thermostat is open, static pressure throughout the system will be fairly uniform, so the bypass valve closes.  Here are a few photos of the bypass valve:

Eventually, this system evolved into the open bypass systems used in most American cars of the 1960's. Open bypass will be discussed in the main chapter on bypass systems.

Chrysler 1934
The next stop on our history tour is the 1934 Chrysler Airflow straight 8, which incorporated a the first true dual-action bellows thermostat. This thermostat incorporated a secondary sleeve which closed the bypass as the main poppet opened. This influential design was adopted by most US manufacturers, and was used into the early 1950's. It is the direct precursor of the sleeve bypass thermostat used in British cars of the 50's and 60's. 

SS Cars 1934
Early SS (Jaguar) cars used a dual action thermostat made by Teddington. Teddington was a British company, operating under the name British Thermostat.  Rather than controlling the bypass with a sleeve or valve, their thermostat used the top of the bellows to do double duty and stop bypass flow when it's at full extension. There are several drawbacks to this system. The bypass port has to be oriented in the direction of flow, which makes it the preferential path in the event the seal fails. And the poppet lift shaft has to pass all the way through the bypass neck, creating another potential leak point. These thermostats were the direct ancestors of the modern dual poppet thermostat.

The top of the bellows will seal the bypass neck when fully extended. The shaft extends all the way through the bypass neck to lift the poppet. 


Meanwhile, what was going on in Europe? 

Thermostat, 1938 Steyer 220. Very pretty, but a technological dead end.

Open Bypass

Open bypass systems are typically found in American cars from the late 1950's to the present day. With this system, an unrestricted passage through the head or block routes hot coolant back around to the pump inlet. As the name implies, the bypass passage is always open. 

It may seem as if at least half the coolant will flow through the bypass at all times. But the physical layout is important. When the thermostat opens, the dynamic pressure of the fast moving fluid will carry it through the thermostat. The bypass passage is at right angles to the direction of flow, and is only exposed to static pressure. Static pressure in the thermostat capsule will rise if the primary path is obstructed (as with a closed or partially closed thermostat). Only when this happens will there be flow through the bypass circuit. 

Flow is a vector force, with magnitude and direction. Unless acted on by an outside force, a stream of water will not deviate from its path. In this illustration, only air pressure and gravity influence flow, and these forces are insufficient to overcome momentum. And so the water streams hit the targets. When the stream is stopped by an immobile object, energy is conserved and flow (kinetic energy) becomes static pressure (potential energy).

Why doesn't the open bypass arrangement result in endless recirculation of hot coolant? Newton's first law of motion. In the absence of interference, inertia carries the moving coolant through the thermostat. If the primary path is blocked,  energy is conserved and dynamic pressure transforms to a local increase in static pressure. This forces coolant through the open bypass passage. The problem with the open bypass system is that there is always some restriction to the primary flow path. If nothing else, the thermostat itself is a restriction, even when fully open. As a result, there will always be some undesirable bypass flow. But not nearly as much as you might expect.

Thermostat open. Because the bypass port is at right angles to the direction of flow, it is exposed only to static pressure at both ends, and won't have much flow. Dynamic pressure (Pd) carries almost all the flow through the primary path. This is also why many Jaguars with sleeve bypass systems survive with single poppet thermostats

Sleeve Valve Bypass

In the 50's, the British motor industry took a different approach to bypass design. Rather than a passive bypass, active control is achieved by a sleeve that is integral to the thermostat poppet. The  sleeve controls a bypass slot machined into the side of the the thermostat housing. The sleeve moves with the poppet, and cuts off the bypass as the thermostat opens. Coolant flows at a constant rate, either through the bypass or the main poppet. The thermostat may achieve equilibrium at some partially open position, so flow can be split between the bypass and the radiator to achieve steady head temperature. We'll address specific applications in later chapters.

For this system to operate correctly, a sleeved thermostat must be used. A visual identification guide:

Correct (sleeved)


  • Better regulation of head temperatures
  • Cannot fully seal bypass slot, because the sleeve has to be slightly smaller than the bore. 
  • Older bellows thermostats are relatively restrictive, raising static pressure. Where the thermostat is located at right angles to flow (to reduce lifiting), static pressure is further increased. So flow tends to leak through the bypass.
Ford Advanced Bypass

The Ford bypass system is a long digression from the Jaguar story, and will be discussed in greater detail in the section on the 351 Cleveland motor. Very quickly, it uses a unique thermostat originally made by Robertshaw to implement an unsual bypass mechanism. In it's native application, it resides in the intake manifold of a V8, and recieves incoming 'sideways" flow from the two heads. The primary path and the bypass path are at right angles to this flow. Depending on whether the thermostat is open or closed, flow is forced to turn either up into the radiator or down into the bypass passage . In a tribute to the earlier Teddington thermostat, the wax motor itself is used to provide bypass control.

This arrangement is discussed here because it's discussed around the web as a solution for slotted bypass applications. It isn't. For now, the important points are that it's based on the Robertshaw 330 thermostat, and that this thermostat Is designed for a very different sort of bypass system. 



  • High flow rate in its native environment
  • "Balanced force" design
  • Available in 160, 180, and 192F temp ranges
  • For British applications, 40mm sleeve is too small to provide bypass control. In the past, vendors have added a second sleeve, soldered around to first, but this limits flow.
  • For Jaguar applications, flow is constrained by 50mm housing: For optimal flow with this thermostat, coolant must enter from the sides
  • For Ford applications, dimensions and opening height are critical, and must be mated to a matching restrictor plate
Double Poppet Bypass

The dual poppet bypass is the most common system used today. There are two poppets mounted to the wax motor, and they both move in the same direction as the thermostat opens. There is a bypass port located parallel to the thermostat flange which is sealed by the lower poppet. This poppet is mounted on a light spring and floats on it's shaft, so that it can automatically adjust to variations in closing distance caused by machining error or wear. 

In vintage applications, the thermostat is usually used for "hot side" operation, meaning that its located at the top of the motor, near the water exit. In this location, the wax motor is sensing coolant at the hottest point in the jacket, and will work to keep the exit temperature relatively constant.

To complete the discussion of dual poppet thermostats, let's take a quick look at how they work for more modern "cold side" applications. Regulating the temperature of the exit water, as we've seen so far, doesn't directly control variability in the temperature of the inlet water. Some engines are sensitive to thermal shock when cold water enters from the radiator. And all engines benefit by uniform cylinder temperatures for efficiency and emissions. An approach to better thermal control is to make the bypass the primary path, and reverse the flow. This results in hot and cold coolant being blended before being pumped around the engine. 

As can be seen in the diagram, a dual poppet thermostat can be relocated to the engine inlet simply by moving it to the "bottom" of the engine and reversing flows. In actual operation, the thermostat is usually somewhere between closed and open, and the thermostat housing acts as a blending chamber where cold coolant mixes with hot coolant to keep the entire water jacket at relatively consistent temperature. Here's a look at a Mercedes cold-side system. The thermostat chamber acts as the blending point for hot and cold water before the pump moves it into the block.


  • Dual poppet thermostats provide more positive closure of bypass port 
  • Wax motor will tolerate much higher working pressures than bellows. Enables better pumps and caps, and elevates boiling point
  • Dimensions are less critical than a sleeve valve or the Ford bypass, since the bypass poppet is spring-loaded
  • Can be designed with high hysteresis, allowing cold-side operation
  • Modern engines require even finer regulation of coolant temperatures
High Flow Systems

Another brief digression here to discuss the high flow systems used for trucks and heavy duty equipment (and sometimes by BMW.) These are not compatible with any vintage Jaguar application. 

The wax motor is mounted below the flange, while the sleeve valve sits above the flange. When hot, motor pushes the sleeve up into the thermostat housing. Bypass flow is stopped when the sleeve seats in the top of the housing. This also opens the flow path to the radiator. A positive seal is achieved by use of o-rings. Note that while it's a balanced force thermostat, because it is not subject to flow lift.


  • Very high flow rates
  • Can be sized for any application
  • Full flow control thanks to superior sealing
  • Requires more space and extra plumbing in most cases
MAP Controlled Bypass Systems

I can't end this chapter without looking at some up-to-date, modern systems. As engines have progressed, so too have thermostats. We earlier said that the purpose of the thermostat was to keep the system warm. But there's complexity to everything. While engine wear becomes acceptable at 160F (70C), the sweet spot for emissions, power or fuel economy can be much higher, up to 230F (110C). What's more, there's no one right answer. The optimal temperature is dependent on transient operating conditions. 

For leading edge engines, this problem is solved by implementing a thermostat map, controlled by the engine ECU. In other words, the engine temperature is varied according to RPM and load under computer control. The mechanism for this is an electronic thermostat:

There is still a wax motor. But in this case, the "natural" temperature of the wax motor may be as high as 230F. This set point is a fail-safe, in case the electronics fails. The actual work is done by the ECU. It continuously reads a set of engine sensors, and implements a temperature map by varying the current through the heating coil, which has the effect of shifting the natural thermostat curve.


  • Very precise thermal control, mapped to optimal parameters
  • Lower emissions
  • Improved fuel economy
  • Greater thermal efficiency
  • Complexity
  • Additional plumbing and wiring
  • Additional potential failure modes
  • Temperature gauge in dash rendered worse than useless

All-Electric Systems

A proposed "future" system is the Davies Craig EWP. This is a combination of an electric fan, electric water pump, temperature sensors, and a digital controller. It uses no thermostat, no mechanical water pump,  and no bypass. Instead, the controller cycles an electric water pump on and off as needed. During warm up, it maintains a minimal flow in order to accomplish all the same things as a normal bypass system. Once the engine is warmed up, it increases air and water flow to match engine requirements. Although this is currently being sold as an aftermarket product, it should be considered a half-developed solution. 


  • Simplicity
  • Possibly reduces parasitic losses due to fans and pumps
  • Eliminates bypass entirely
  • Available as an aftermarket solution
  • Not endorsed by any car maker
  • Considerable effort may be required to retrofit any given application
  • Optimizing flow rates involves trial and error
  • Thermosiphoning may result in long warm up on some engines and very poor cold weather performance
  • Requires substantial alternator upgrade and harness modifications
  • The "cast in" flow path of any given engine may not allow bypass blocking
  • 'For every difficult problem, there is a solution with is obvious, elegant, inexpensive...and utterly wrong'
Schaeffler Thermomanagement Module

I  conclude this chapter with a brief discussion of the most advanced system in use today, the Schaeffler Thermomanagement Module. This system has entered service with some VW/Audi cars. As with the MAP controlled thermostat, the idea is to achieve precise thermal control, matched in real time to driving conditions. But while MAP controlled thermostats build on traditional wax-capsule thermostat technology, the Schaeffler system uses an electronically controlled rotary valve. The advantage is extremely fast warm up, and thermal regulation within ±2c. Rather than attempt to diagram this system, I simply refer my readers to the factory video:

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