How Oil Cooler Thermostats Work

An oil cooler thermostat is a thermally actuated valve that sits between the engine’s oil system and the external oil cooler. Inside the thermostat, a wax pellet or a bimetallic strip expands when the oil reaches a specific temperature, pushing a piston to open the valve. When the oil is cold, the thermostat remains closed, forcing oil to bypass the cooler and flow directly back to the engine. This allows the oil to warm up quickly, reducing engine wear during cold starts. Once the oil reaches the thermostat’s opening temperature, the valve gradually opens to direct a portion of the oil through the cooler, where it sheds heat before returning to the engine. The thermostat continuously modulates to maintain a stable oil temperature within the desired range.

Why Oil Temperature Regulation Matters

Oil serves multiple critical functions: lubricating moving parts, reducing friction, cleaning contaminants, and carrying away heat. Each of these functions is temperature-dependent. Oil that is too cold remains thick and viscous, leading to poor flow and increased engine wear. Oil that is too hot breaks down chemically, loses its lubricating properties, and can form sludge. The ideal oil temperature range for most performance engines sits between 190°F and 230°F (88°C–110°C). An oil cooler thermostat helps maintain this sweet spot by delaying cooler activation until the oil is fully up to temperature, then engaging the cooler only as needed.

The Full Temperature Range Spectrum

Oil cooler thermostats are typically rated by their “cracking” or opening temperature—the point at which they begin to allow flow to the cooler. The most common ranges encountered in performance applications are:

  • 160°F (71°C) – 170°F (77°C): Very low opening temperatures, primarily used in race-only setups where the engine is constantly under extreme loads and the driver wants the cooler to activate as early as possible. These are rarely recommended for street use because they prevent the oil from reaching normal operating temperature during light driving.
  • 180°F (82°C): A popular choice for modified engines with increased horsepower, forced induction, or track use. It provides a balance between warm-up speed and cooling capacity, particularly in hot climates or during spirited driving.
  • 190°F (88°C): Arguably the most common “all-around” setting for street-driven performance cars. It allows the engine to reach a healthy operating temperature while still providing cooling before excessive heat buildup occurs.
  • 200°F (93°C): Often selected for vehicles in colder climates or for drivers who rarely push their engines hard. It delays cooling to keep the oil warmer, improving efficiency and reducing friction in cooler ambient conditions. Some OEM performance vehicles come with a 200°F thermostat from the factory.
  • 220°F (104°C) and above: High-temperature thermostats are reserved for endurance racing, heavy towing, or desert driving where sustained high loads generate immense heat. At these temperatures, the oil cooler is effectively always active once the engine is fully warm. These thermostats require careful monitoring to ensure the oil does not exceed safe limits (typically around 250°F, depending on the oil type).

It is important to note that the opening temperature is just the beginning of the modulation range. Most thermostats are fully open at about 20–30°F above their cracking point. For example, a 180°F thermostat might be fully open at 200–210°F, providing maximum oil flow through the cooler at the highest temperatures.

Understanding Bypass vs. Full-Flow Thermostats

There are two main architectures for oil cooler thermostats:

  • Bypass thermostats: These have a built-in bypass port that routes oil around the cooler when the thermostat is closed. When the thermostat opens, it gradually closes the bypass and directs oil through the cooler. This design ensures constant oil flow to the engine even if the cooler is completely blocked (a safety feature).
  • Full-flow thermostats: In this design, the thermostat is placed in line with the oil cooler lines. When closed, oil cannot reach the cooler at all; when open, all oil passing through the thermostat goes to the cooler. These are simpler but require a separate bypass path in the system to maintain flow if the cooler is restricted. Most performance aftermarket kits use bypass-style thermostats for reliability.

Factors That Influence the Ideal Temperature Range

Engine Design and State of Tune

High-compression, boosted, or high-revving engines generate significantly more heat than a stock naturally aspirated motor. Forced induction (turbochargers and superchargers) also heat the oil directly through the turbocharger’s bearing section. A more aggressive tune that adds timing and fuel will raise combustion temperatures, further increasing oil heat load. In these cases, a lower temperature thermostat (180–190°F) is often recommended to keep oil from exceeding 220–230°F during sustained pulls.

Driving Environment and Duty Cycle

Track days, autocross, mountain passes, and stop-and-go city driving all impose different thermal demands. Track driving typically requires maximum cooling capacity, suggesting a 180°F or 190°F thermostat. For daily commuting in moderate climates, a 200°F thermostat may be sufficient and even beneficial for fuel economy. For extreme cold climates, some drivers opt for a 220°F thermostat to prevent the oil from never reaching operating temperature, but this must be balanced against the risk of overheating during occasional hard driving.

Oil Type and Viscosity

Synthetic oils generally withstand higher temperatures before breaking down, but they also flow better at low temperatures. A 5W-30 synthetic oil used in a high-performance application may still benefit from a 180°F thermostat to keep viscosity in the optimal range. Heavy oils (15W-50, 20W-50) often used in older or race engines may need a slightly higher operating temperature to thin out enough for proper flow, making a 200°F thermostat more appropriate.

Cooler Size and Mounting Location

The effectiveness of the cooling system also depends on the oil cooler itself. A large cooler with good airflow (mounted in front of the radiator or in a dedicated duct) will cool more aggressively than a small cooler tucked behind a bumper. If the cooler is oversized, a higher temperature thermostat can help prevent over-cooling on cold days. If the cooler is marginal, a lower thermostat may be needed to engage it sooner and keep oil temperatures in check.

Selecting the Right Thermostat: A Step-by-Step Guide

Step 1: Determine Your Baseline Oil Temperature

Before changing the thermostat, measure your current oil temperature during normal driving and during hard driving. Use a high-quality oil temperature gauge plumbed into the oil pan or the oil filter adapter. Note the peak temperatures under load and the typical cruising temperature. If you see temperatures exceeding 240°F (115°C) under load, you likely need a cooler thermostat. If your oil never reaches 180°F (82°C) on a cold day, you may need a higher temperature thermostat.

Step 2: Consider the Cooler System Setup

If you are installing an oil cooler for the first time, match the thermostat to the expected thermal load. A typical rule of thumb: for street-driven cars with up to 500 hp, a 190°F thermostat is a safe default. For 500–700 hp or track use, go with 180°F. For 700+ hp or sustained racing in hot weather, consider a 160–170°F thermostat but be prepared to install an oil heater if the car is driven in cold climates.

Step 3: Verify Compatibility

Oil cooler thermostats are available in different port sizes (e.g., -10 AN, -12 AN) and thread types (NPT, BSPP). Ensure the thermostat matches your oil line fittings. Also check the flow rate: some thermostats are rated for up to 50 GPM, while others handle only 20 GPM. High-flow pumps require a thermostat with a large internal passage to avoid restriction.

Step 4: Test and Adjust

After installing the new thermostat, monitor your oil temperatures over a few weeks. Log data in different conditions. If the oil still gets too hot, you may need to go to a lower temperature thermostat or upgrade the cooler itself. If the oil runs too cold (below 180°F during cruising), consider going up one step, or add a thermostat with a partial-bypass feature that allows some hot oil to mix with cooled oil.

Installation Best Practices

Mounting Location

Mount the thermostat as close to the engine as possible, typically on the oil filter housing or on a remote oil filter adapter. This ensures the thermostat reads engine oil temperature directly. Avoid mounting it near the cooler, where it might sense cooled oil and cycle erratically. Use a thermostat with a bypass port to protect the engine if the cooler becomes clogged.

Plumbing and Hose Routing

Use high-quality AN hoses and fittings rated for oil temperatures up to 300°F. Route the hoses away from exhaust headers and other heat sources. Keep the lines as short as practical to reduce pressure drop. Ensure the thermostat is oriented so that the valve stem is vertical or at a slight angle to allow trapped air to escape, preventing air pockets that cause erratic operation.

Air Bleeding

After installation, run the engine and check for leaks. Loosen the highest point in the oil cooler circuit (often the cooler itself) to allow air to escape. Some thermostats have a small bleed hole or a Schrader valve for this purpose. Once steady oil flow is observed, tighten the connection.

Maintenance and Troubleshooting

Common Symptoms of Malfunction

  • Oil temperature stays too low: The thermostat may be stuck open or opening too early. This can lead to condensation and fuel dilution in the oil.
  • Oil temperature rises rapidly and stays high: The thermostat may be stuck closed, preventing oil from reaching the cooler. This can cause severe engine damage.
  • Temperature cycles wildly: Possible air in the system, a failing wax pellet, or an incorrectly matched thermostat.

Inspection and Replacement

Inspect the thermostat every 12 months or after every race season. Check for external leaks around the valve body. Test the opening temperature by heating the thermostat in a pot of oil with a thermometer. Replace immediately if it opens more than ±10°F from its rated temperature or if it shows signs of corrosion or wax leakage.

When to Upgrade

If you have added power, switched to a more aggressive driving style, or moved to a hotter climate, it is wise to reassess your thermostat. A thermostat that worked perfectly for a 400 hp daily driver may not be adequate for a 600 hp track car.

Real-World Case Studies and Applications

Case Study 1: Turbocharged Street Car

A 2018 Subaru WRX with a Stage 2 tune (300 whp) was seeing oil temperatures of 255°F during summer canyon runs. The stock oil system had no external cooler. After installing a 25-row oil cooler with a 190°F thermostat, peak oil temperatures dropped to 225°F. The owner later switched to a 180°F thermostat after a turbo upgrade, further reducing peak temps to 215°F.

Case Study 2: Track-Only E36 M3

This BMW M3 was used for HPDE events. With a 200°F thermostat, oil would quickly climb to 240°F during 20-minute sessions and trigger a limp mode due to the ECU’s temperature protection. Changing to a 170°F thermostat kept oil below 220°F even in 90°F ambient temperatures. The owner also noted faster warm-up on cold mornings because the thermostat allowed some bypass flow until the target temperature was reached.

Case Study 3: Diesel Towing Rig

A 2016 Ford F-350 Power Stroke towing a 10,000 lb trailer in the Rocky Mountains experienced oil temperatures consistently above 250°F. The factory oil cooler included a thermostat that opened at 210°F. By installing a high-flow oil cooler and a 190°F aftermarket thermostat, the owner reduced peak oil temps to 230°F, well within the safe range for the heavy-duty diesel oil being used.

External Resources and Expert References

For detailed technical data on oil cooler thermostats, consult these trusted sources:

Conclusion

Choosing the correct oil cooler thermostat temperature range is a critical decision that directly affects engine longevity, performance, and reliability. By understanding the mechanics of how these thermostats work, the nuance of various temperature settings, and the factors that influence the ideal range for your specific vehicle, you can make an informed choice. Always measure your actual oil temperatures before and after a change, and consider consulting with a professional engine builder or tuning specialist when in doubt. With the right thermostat in place, your performance vehicle will maintain optimal oil temperature across all driving conditions, protecting your investment and ensuring consistent power delivery lap after lap.