Frequently Asked Questions


Fuel Caps

What is a thermostat? What purpose does it serve?

The thermostat has two important jobs to perform; to accelerate engine warm-up and to regulate the engine’s operating temperature. A quality thermostat ensures excellent fuel economy, reduces engine wear, diminishes emissions and blow-by, improves cold weather drivability, provides adequate heater output, and detours overheating. This is accomplished by blocking the circulation of coolant between the engine and radiator until the engine has reached its predetermined temperature. The thermostat then opens as required in response to changes in coolant temperature to keep the engine’s temperature within the desired operating range.

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How does a thermostat work?

“Usually located within a metal or plastic housing where the upper radiator hose connects to the engine, most of today’s thermostats utilize the “reverse poppet” design, which opens against the flow of the coolant. Thermostats have a wax filled copper housing or cup called a “heat motor” that pushes the thermostat open against spring pressure.


As the engine’s coolant warms up, the increase in heat causes the wax to melt and expand. The wax pushes against a piston inside a rubber boot. This forces the piston outward to open the thermostat. Within 3 or 4 degrees F. of the thermostat preset/rated temperature which is usually marked on the thermostat, the thermostat begins to unseat so coolant can start to circulate between the engine and radiator. It continues to open until engine cooling requirements are satisfied. It is fully open about 15-20 degrees above its rated temperature. If the temperature of the circulating coolant begins to drop, the wax element contracts, allowing spring tension to close the thermostat, thus decreasing coolant flow through the radiator. ”

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How often does a thermostat need to be changed?

Stant recommends the thermostat be changed when the coolant is changed or whenever an over-heating of the engine has occurred.

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When does a thermostat start to open?

The thermostat starts to open at the rated temperature, plus or minus 2 degrees (F).

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When is a thermostat fully open?

The thermostat is usually fully opened at 15 – 20 degrees (F) above the opening temperature.

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Does Stant have a high flow thermostat?

Yes. Our Superstat™ thermostat has the highest flow of any thermostat available in the aftermarket.

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How do you test a thermostat?

A thermostat can be tested by threading a string through the valve and suspending it in a bucket of boiling 50/50 coolant and water. Immerse the thermostat in the hot/boiling coolant mixture for a few minutes; if the thermostat is working, it will fall off the string as it starts to open. Remove the thermostat. After allowing it to cool, the thermostat should close.

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Why does my engine overheat?

There are several possible reasons for an engine to overheat. Stant recommends that you start with the easiest to diagnose possible reasons at the top of the chart and work down until you find the solution(s) to your problem. Depending on your skill and experience level, it may be best to leave some of the diagnosis and repairs to a professional mechanic.


Working on a hot engine can cause burns. Use extreme caution or allow the vehicle to cool completely prior to starting work on it.

Cause For Engine Overheating



Low coolant level Check fluid level when engine is warmed to operating temperature. Fill coolant to proper level.
Coolant not in proper ratio / Coolant too old Most anti-freeze/coolant is recommended to be used in a 50:50 ratio with water. Too strong or too weak of a ratio can affect cooling performance. Check protection with anti-freeze tester or refractometer. Replace with new coolant in 50:50 ratio.
Worn radiator cap Check cap seals and gaskets for cracks. Check spring for proper pressure. Replace cap if needed.
Missing or loose fan/serpentine drive belt Check for belt, check for cracks in belt, check tension of belt. Replace or tighten belt as needed.
Broken or collapsed coolant hose Check radiator and heater hoses for leaks. Also check to make sure they have not been kinked or collapsed. Repair or replace hoses as required.
Contaminants in cooling system Perform visual inspection looking for rust, dirt or other particles and discoloration of coolant. Drain coolant, flush system and replace with new coolant in 50:50 ratio.
Thermostat stuck Examine thermostat and test to insure it opens at proper temperature. Replace thermostat if necessary.
Low engine oil level Check engine oil level. Fill oil to proper level.
Radiator blocked Check to make sure that road debris or other components are blocking air flow through radiator. Remove obstruction and repair if neccessary.
Fan shroud or air effects missing Check to insure that the shroud around the radiator fan is not missing. Check to insure that air foils and scoops that are designed to direct air flow through the radiator are not missing. Repair and replace if neccessary.
Faulty water pump Test water pump. Replace water pump if neccessary.
Retarded ignition timing Check ignition timing. Adjust to factory specifications.
Leaking cylinder head gasket Check for leaks. Repair if neccessary.
Faulty warning light or gauge If no problems are found and gauges indicate exessive overheating issues, check gauges and sending unit. Replace gauges if neccessary.

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Why doesn’t the passenger compartment in my car heat up?

There are several possible causes for not getting heat in the passenger compartment of a car. Stant recommends that you use the chart below, starting at the top, where the problems are easier to diagnose and repair.




Low coolant level Check coolant level. Fill to proper level.
Stuck thermostat Check thermostat. Replace thermostat and gasket if neccessary.
Clogged heater core Check heater core Drain and flush system or replace heater core.
Faulty heater control switch Check switch control. Replace switch if neccessary.

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Does Stant have a fail safe thermostat?

All thermostats will fail in either a closed or open position; there is no such thing as a thermostat that will fail in a “safe” position. Although some brands may claim a thermostat fails in a safe position, it simply locks itself open while it is in a full stroke open position. It will not spring open if it fails in a closed position.

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What does a jiggle pin do?

It helps bleed the cooling system of trapped air by allowing air to pass into the radiator and released from the system.

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My thermostat has a jiggle pin but the Stant thermostat doesn’t. Will a Stant thermostat still work?

Yes. Many thermostats have a “jiggle pin” that allows trapped air in the cooling system to pass through the thermostat and be released from the system. Some Stant thermostat do not use have a jiggle pin. These thermostats will have a “bleed notch” or other method of removing air from the system.

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Why do I need to replace my fuel cap?

If your cap has failed an emission test, is missing, the crown is cracked or bent or the seal or gasket is cracked or worn, you need a new gas cap. If your cap is over seven years old, chances are you need one as well. The Car Care Council estimates that one of six cars is in need of a gas cap.

Many states require an annual emissions test. A test of the fuel cap is part of those tests.

Stant manufactures a fuel cap tester. Ask your mechanic to test your cap for you.

According to Sun Oil Company research, a vehicle with a missing or inoperative fuel cap will allow 176 pounds (22 gallons) of gasoline to evaporate over the period of one year. With the price of gasoline at about $3.00 per gallon, a new gas cap will pay for itself very quickly.

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Can I get extra keys for my locking cap?

Yes. It’s easy to order additional keys by calling Stant customer service at 800-822-3121 ext 58330.

Please have the lock code that is found on the key or the lock handy when you call. There is a nominal charge for extra keys.

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Can I get 2 or more locking caps that use the same key?

Yes, keyed-alike locking fuel caps for multi-tanked vehicles or two car families are available from Stant. Look up the applications in a catalog at your favorite Stant retailer. Simply change the first two digits of the 5-digit Stant part number from 10xxx (for a boxed cap) or 11xxx (for a carded cap) to 17xxx (for a keyed-alike cap). Your Stant retailer can then special order the caps for you.

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What is a “pre-release” or Pre-Vent fuel cap?

Some Stant fuel caps have a “Pre-Vent” feature. These caps are designed to allow the controlled release of any pressure that may have built up in the fuel tank, so the tank is de-pressurized before the sealing gasket is lifted from the filler neck. This dramatically reduces the possibility of any fuel expulsion from the tank to your hands or clothing.

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Are fuel caps for today’s cars vented?

Fuel caps have not been fully vented since the 1960’s. Modern fuel caps typically have valves that relieve positive and/or negative pressure once it reaches a certain level.

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