Will an engine run hot without a thermostat?

Generally, no—most engines without a thermostat run too cool, especially in mild or cold weather. However, depending on the cooling-system design and driving conditions, an engine can run hot or even overheat without a thermostat. Because the thermostat also manages coolant routing and system pressure, removing it is not recommended and can create unstable temperatures, hotspots, and reliability issues.

What the thermostat actually does

The thermostat isn’t just an on/off valve that waits for the engine to warm up. It regulates temperature by metering coolant flow to maintain a narrow operating range, typically around 88–105°C (190–221°F) in modern vehicles. In many designs, it also closes an internal bypass passage when it opens to the radiator, ensuring most coolant goes through the radiator once hot. This coordination stabilizes temperature, maintains correct pressure in the cylinder head to prevent localized boiling, and helps the engine management system (fans, fueling, ignition) work as intended.

When removing the thermostat can make an engine run hot

While removing the thermostat often causes an engine to run cool, certain conditions and system designs can lead to higher operating temperatures or outright overheating without it. These scenarios are rooted in coolant routing, pressure control, and how the engine’s electronics expect temperature to behave.

• Bypass “short-circuiting”: Many engines use a bypass loop that recirculates coolant within the block when cold. The thermostat closes this bypass as it opens to the radiator. With no thermostat, the bypass may stay open, letting coolant skip the radiator and causing overheating—especially under load.
• Pump cavitation and head pressure loss: The thermostat provides restriction that helps maintain pressure in the cylinder head. Removing it can reduce head pressure, promoting localized boiling (steam pockets) and hotspots even if gauge readings look normal.
• Fan and ECU logic mismatches: Modern ECUs expect coolant to reach and hold target temps. If the engine never stabilizes, fans may not cycle as intended, and fueling/timing corrections can add heat under heavy load.
• High ambient and high load: Towing, steep grades, or hot days increase heat load. Without controlled flow and proper bypass management, the system may reject less heat than required and overheat.
• Air entrapment during refills: On some engines, the thermostat aids proper bleeding. Without it (or without a correct “blanking sleeve”), air pockets can persist and trigger overheating.
• Platform-specific designs: Certain engines (e.g., those requiring a blanking sleeve or with map-controlled/electrically heated thermostats) rely on the thermostat’s physical presence and behavior. Removing it can cause DTCs (like P0128), limp strategies, or unstable temps.

These issues mean an engine can indeed run hot without a thermostat, depending on how the system manages bypass flow, pressure, and electronic controls—especially under demanding conditions.

When it will tend to run cool

In many everyday scenarios, removing the thermostat makes the engine run cooler than intended and very slowly reach operating temperature—if at all.

• Cold weather or light-load driving: At idle and steady cruise in cool climates, the engine may never hit the ECU’s target temperature.
• Poor cabin heat: The heater core depends on hot coolant; without a thermostat, heat output can be weak or inconsistent.
• Rich fueling and efficiency loss: Engines run richer until warm. Staying cool prolongs enrichment, wastes fuel, and can foul plugs and the catalytic converter.
• Thicker oil for longer: Oil takes longer to reach optimal viscosity, increasing friction and wear during extended warm-up.

Running too cool is not harmless; it impairs efficiency, emissions, and longevity while offering no real protection under high-load situations.

Risks of operating without a thermostat

Beyond temperature instability, deleting the thermostat has broader consequences that affect reliability, emissions, and maintenance costs.

• Accelerated wear: Prolonged cold operation increases cylinder and bearing wear and can promote sludge formation.
• Emissions and catalyst damage: Persistent rich mixtures can overheat or poison catalytic converters and trigger fault codes.
• Cooling system stress: Cavitation, hotspots, and uneven flow can damage water pumps, gaskets, and radiators.
• Unstable fan control and DTCs: Modern cars may set codes (e.g., P0128) and alter fan or fueling strategies.

These effects often cost more over time than fixing the underlying issue a thermostat delete was meant to solve.

If you removed the thermostat and it runs hot: what to check

If a thermostat delete was used to diagnose overheating but the engine still runs hot—or got worse—verify system integrity and configuration. The following steps help isolate the cause and restore proper control.

1. Reinstall the correct thermostat: Use the OEM temperature rating and design (including any jiggle valve orientation or integrated seals).
2. Confirm bypass routing: Some engines need a blanking sleeve or specific thermostat housing to close the bypass when hot.
3. Bleed the system properly: Use factory bleed screws/procedures; trapped air can mimic or cause overheating.
4. Check the pressure cap: A weak cap lowers boiling point; replace if suspect.
5. Verify fan operation and ECU inputs: Ensure sensors, relays, and fan speeds work; scan for codes like P0128.
6. Assess radiator flow: Look for external blockage, internal scaling, or uneven temperature across cores.
7. Inspect the water pump: Check for eroded impellers, slipping drive, or leaks.
8. Evaluate mixture and timing: Overly lean mixtures or advanced timing under load raise combustion temps.
9. Test for head gasket leaks: Combustion gases in coolant cause rapid overheating and pressure spikes.

Systematic checks typically reveal a solvable root cause—often unrelated to the thermostat itself.

Best practice

Keep the thermostat. Replace a failed unit with the correct OEM-spec part or a quality equivalent, and follow the factory bleed procedure. Avoid “restrictor washer” fixes unless you’re dealing with a race-only cooling package designed for them. For vehicles with electronically controlled or map-controlled thermostats, use the precise part and expect fault codes and drivability issues if you deviate.

Special case: racing setups

Track-only engines sometimes use high-flow thermostats or calibrated restrictors to manage pressure and flow at sustained high RPM. These systems are engineered holistically (pump, radiator, shrouding, ducting) and don’t translate to street vehicles or mixed-use driving.

Summary

An engine without a thermostat often runs too cool, but it can absolutely run hot—or overheat—because the thermostat also governs bypass closure, pressure, and stable flow. Deleting it undermines engine management, increases wear, and can create hotspots. The reliable fix for temperature problems is to restore the proper thermostat and correct any underlying cooling system faults, not to run without one.

What happens if you remove the thermostat in a car engine?

Removing a car’s thermostat prevents the engine from reaching and maintaining its optimal operating temperature, which can lead to reduced fuel efficiency, increased emissions, and engine wear due to improper fuel-air mixtures and uneven thermal expansion of components. It also causes the Engine Control Unit (ECU) to malfunction, potentially triggering a check engine light and placing the car in limp mode. While the engine may not immediately overheat, prolonged driving without a thermostat damages internal components over time and compromises the effectiveness of the heater and defroster. 
Why you should not remove a thermostat

• Poor Fuel Economy: Without a thermostat, the engine runs too cool, which can cause the ECU to enrich the fuel mixture (add more fuel) to try and warm it up, leading to wasted fuel and decreased efficiency. 
• Increased Emissions: An engine that isn’t running at its optimal temperature produces higher levels of pollutants, making it less environmentally friendly. 
• Engine Wear: Engine parts expand and contract at different rates. Running an engine too cool for extended periods, without the uniform heat provided by a thermostat, can cause uneven wear, increased friction, and scarring between components. 
• Harm to Electronic Systems: Modern car ECUs rely on consistent temperature data from the coolant sensor. A constantly fluctuating temperature, without a thermostat to regulate it, can confuse the ECU, triggering the check engine light and forcing the car into a reduced power mode (limp mode). 
• Reduced Heater Performance: The coolant is what heats the cabin. A removed thermostat means the coolant circulates too quickly, preventing the heater core from getting hot enough to provide sufficient warmth. 
• Potential Overheating: Counterintuitively, a coolant flow that is too fast can prevent proper heat exchange with the air in the radiator, especially at higher engine speeds or when the car is stopped, leading to overheating. 

Will a thermostat prevent overheating?

To keep your engine running smoothly and prevent overheating, a crucial component plays a vital role—the thermostat. The thermostat regulates the flow of coolant between the engine and the radiator, ensuring that your engine maintains the optimal temperature.

Will heat run without a thermostat?

No. You just won’t get any heat out of your heater.

Can an engine overheat without a thermostat?

Yes, removing the thermostat can cause an engine to overheat because it removes a restriction in the coolant flow, making the coolant circulate too fast through the radiator. This prevents the radiator from having enough time to properly cool the coolant, leading to it returning to the engine at a hotter temperature. 
This video explains how removing a thermostat can lead to overheating: 1mDanny Johnson’s GarageYouTube · Jul 3, 2021
How the thermostat prevents overheating

• Regulates coolant flow: The thermostat acts as a valve, controlling the flow rate of coolant through the radiator. 
• Allows for heat transfer: By restricting flow, it ensures the coolant stays in the radiator long enough for the air to remove heat from it, allowing the radiator to function effectively. 
• Maintains optimal temperature: It helps the engine reach and maintain its designed operating temperature. 

Consequences of removing the thermostat

• Increased coolant speed: Without the thermostat, the coolant flows too quickly, preventing efficient heat exchange in the radiator. 
• Overheating: The coolant doesn’t get cooled sufficiently before returning to the engine. 
• Computer-related issues: Modern cars have computer-controlled engines, and a lack of proper operating temperature can cause the system to go into a “limp mode” or open-loop mode, affecting fuel management and causing performance issues. 
• Engine wear: An engine is designed to operate at a specific temperature. Running too cold can increase engine wear. 

In summary: While removing the thermostat might seem like a solution to an overheating problem caused by a stuck-closed thermostat, it typically creates the opposite issue: overheating due to insufficient cooling.