Thermostat of the outboard motor: the principle of operation, diagnosis and repair

The thermostat in the outboard motor is a small but critically important node that is responsible for maintaining the optimum engine temperature. Mercury or Yamaha It can overheat in minutes, which can be costly to repair. Many owners of PVC boats and aluminum vessels forget about the existence of the thermostat until they encounter sudden damage. engine-heating or, conversely, his cold-swipe When the engine has not reached operating temperature for years.

In this article, we will analyze how the thermostat is arranged, by what signs it is faulty, and why. replacement of the thermostat on the outboard motor without draining antifreeze is possible only on some models with an upper location of the unitYou will also find a step-by-step replacement instruction, a thermostat compatibility table for popular brands and answers to frequent questions - from whether you can drive without a thermostat, to the nuances of choosing parts for old ones. Toatsu and Suzuki '90s.

How thermostat works in the outboard motor

A thermostat is an automatic valve that regulates the circulation of coolant (or off-shore water in open-loop systems) depending on the engine temperature. In the cold state, it covers a large cooling circle, causing the liquid to circulate in a small circle (through the cylinder block and head). 70–90°C (depending on the model), the thermostat opens by releasing the coolant through a radiator or heat exchanger.

In outboard motors, two types of thermostats are used:

• 🔹 Waxy The principle of operation is based on the expansion of the wax filler when heated, which presses on the stem and opens the valve. Mercury, Yamaha F-Series, Honda BF.
• 🔹 Bimetallic Less common in modern motors, but can still be found in older ones Johnson/Evinrude or Suzuki DTThey work by deforming the bimetallic plate when heated.

The main feature of the motors is that their cooling systems are often used. pumpless In the traditional sense, the impeller itself is the pump, which means that the thermostat must open smoothly, without jerking, so as not to create hydraulic shocks in the system, otherwise the risk of damage to the pipes or heat exchanger increases significantly.

Signs of a thermostat malfunction

On the breakdown of the thermostat signal two opposite symptoms: engine-heating or underheatingIn the first case, the thermostat is stuck in the closed position, and the coolant does not enter the big circle. In the second case, the valve is stuck in the open state, and the motor cannot warm up to operating temperature. Let's take a closer look at both scenarios.

Symptoms of jammed thermostat (closed):

• 🔥 The temperature arrow on the dashboard quickly goes into the red zone (over 100 C).
• 💦 From the engine comes steam or drops of water (on models with an open cooling circuit).
• 🛑 The engine suddenly stops with a characteristic “slap” – this is a sign of detonation due to overheating.
• 🔧 When tested, the upper tube (going to the heat exchanger) is cold, and the lower one is hot.

Symptoms of jammed thermostat (open):

• ❄️ The engine warms up longer than 10-15 minutes, the temperature arrow barely rises.
• 🔄 The squats are idling (especially noticeable on the two-stroke engines).
• 💨 White smoke comes from the exhaust pipe (do not confuse with steam when overheating!).
• 🛢️ Fuel consumption increases by 15-20% due to suboptimal engine performance.

How to check the thermostat of the outboard motor without removal

You can do it right on the water or in the garage without disassembling the motor, and you'll need an infrared thermometer or a conventional kitchen thermometer with a probe.

1. Start the engine and let it work at idle speeds for 2-3 minutes.
2. Check the temperature of the top pipe (going to the heat exchanger or radiator) it should be cold, which means the thermostat is still closed.
3. After 5-7 minutes, the pipe should start to heat up. If it stays cold, the thermostat won't open. If it's heated right away, the thermostat jammed in the open position.
4. For accuracy, measure the temperature of the cylinder block. During normal operation, it should stabilize within the limits of the temperature of the cylinder block. 80–95°C.

If you don't have a pyrometer, use tactile techniques: touch the pipes carefully (to avoid getting burned!), the difference between cold and hot will be obvious. four-stroke engines (for example, Honda BF50 or Suzuki DF9.9) the coolant circulation can be further checked by removing the expansion tank cover - the fluid must move slowly when the engine is running.

What to do if the thermostat is jammed in the open position?

Temporary operation of the motor is possible, but be prepared for increased fuel consumption and wear of parts. The main danger is corrosion of the internal surfaces due to constant contact with cold water (relevant to salt water). If the thermostat is stuck in the closed position, operation is prohibited: the risk of overheating and deformation of the cylinder head.

Step-by-step replacement of thermostat: instructions with a photo

The process of replacing the thermostat depends on the model of the motor, but the overall scheme is the same. Yamaha F25 (similar to) Mercury 15-40 hp and Suzuki DFBefore starting work, prepare:

• 🔧 A set of keys and heads (most often you need dimensions of 8, 10, 12 mm).
• 🛠️ Screwdrivers (flat and cross).
• 🧰 New thermostat (select the compatibility table below).
• 🧴 Sealants for gaskets (e.g., Loctite 574 or Permatex Aviation).
• 💧 Cooling liquid (if the motor is closed circuit) or distilled water for washing.

Step 1. Drain the coolant (if required)

On motors with a closed circuit (e.g., Honda BF150) the antifreeze must be drained.

1. Substitute the container under the drain tap (usually located at the bottom of the block).
2. Unscrew the faucet or remove the lower tube.
3. Give the liquid to drain completely (it may be necessary to tilt the motor).

Step 2: Dismantling the thermostat body

The body of the thermostat is usually located:

• Nana two-stroke engines - at the top of the block head (for example, Mercury 25-50 hp).
• Nana four-stroke engines - side of the cylinder block (e.g., Yamaha F70).

Unscrew the bolts of the case mount (usually 2-4 pieces) and carefully remove the lid, under which you will see a thermostat with a spring and a gasket.

Step 3: Installation of a new thermostat

Before installation, check the new thermostat in boiling water – it should start opening at the temperature indicated on the case (usually 80–88°CIf everything is fine:

1. Install the thermostat in the body so that the valve is directed towards a large cooling circle (see labels on the housing).
2. Apply a thin layer of sealant on the gasket and press the cover.
3. Tighten the cross-cross bolts with effort 10-12 Nm. (Don't pull it over!)
4. Connect the pipes and pour cooling liquid (if drained).

Step 4: Checking after replacement

Start the engine and control the temperature:

• The first 2-3 minutes of the pipe to the heat exchanger should remain cold.
• After 5-7 minutes, it should start to heat up, which means that the thermostat has opened.
• The temperature arrow on the panel should stabilize in the middle position.

Thermostat compatibility table for popular motors

Selection of the thermostat is a critical step: installation of the thermostat with the wrong opening temperature will lead to overheating or underheating of the engine.

Make and model of the engine	Original Art Thermostat	Opening temperature, °C	Analogues (brand/art.)	Notes
Mercury 9.9-40 hp (2T/4T)	8M0058457 / 8M0134357	82–88	Sierra 18-3334, Globe 78012	For motors until 2010, it is suitable 8M0058457And then -- 8M0134357.
Yamaha F25–F115 (4T)	6E5-12761-00-00	80–86	Sierra 18-3306, Quicksilver 8M0134357	On the engines. F150–F300 thermostat 6E5-12761-10-00.
Suzuki DT8–DT225 (2T)	15210-95J00	78–84	Globe 78005, Sierra 18-3320	For DF9.9–DF140 (4T) — 15210-95J10.
Honda BF5–BF225	19085-ZE1-003	83–89	Sierra 18-3340, Quicksilver 8M0134357	On the engines. BF150–BF250 It uses the art thermostat. 19085-ZE2-014.
Tohatsu MFS25–MFS225	391-50501-1	80–86	Sierra 18-3306, Globe 78012	Similar to thermostats Yamaha same period.

When buying analogues, pay attention to seat-piece and valve-walkFor example, thermostats for Mercury 2.5-6 hp have a diameter of 40 mm and for Mercury 115-300 hp Also check if the thermostat is suitable for your cooling system (open or closed circuit).

Frequent errors when replacing the thermostat

Even experienced mechanics sometimes make mistakes that cause the thermostat to break again or damage the engine, and the most common of these are:

⚠️ Attention: Never use car thermostats in outboard motors! they are designed for different pressures in the system and may not open on time due to differences in pump design.

Error 1: Tug the thermostat body bolts

Excessive force during tightening leads to deformation of the gasket or even cracks in the body (especially important for aluminum blocks). Mercury and YamahaOptimum force. 10-12 Nm.Use the dynamometer key!

Mistake 2: Incorrect orientation of the thermostat

Many thermostats have flow direction (indicated by the arrow on the body) If you set it "back-to-back", the valve will not open completely, which will lead to overheating. Always check the tags!

Error 3: Ignoring system flushing

If there is deposit in the cooling system (especially after using ordinary water instead of antifreeze), they can infuse a new thermostat after a few hours of operation. Liqui Moly Kuhlerreiniger).

Mistake 4: Using an inappropriate sealant

Vinegar-based silicone sealants (like regular plumbing) corrode aluminum. anaerobic sealants (for example, Loctite 574) or special gasket pastes (Permatex Aviation).

Can I drive without a thermostat?

Theoretically, yes, but with serious consequences: without a thermostat, the coolant is constantly circulating in a large circle, and the engine does not warm up to operating temperature.

Consequences of driving without a thermostat:

• ⚠️ Increased engine wear - cold oil does not provide proper lubrication, which accelerates the wear of piston rings and liners.
• ⚠️ Increased fuel consumption The ECU (if any) enriches the mixture by thinking that the engine is cold.
• ⚠️ Corrosion of internal surfaces In off-shore water systems, constant contact with cold salt water accelerates the formation of rust.
• ⚠️ Power reduction - unheated engine produces 10-15% less hp, especially noticeably on the two-stroke engines.

The exception is temporary operation without a thermostat in an emergency (for example, if it is jammed in a closed position, and far from the shore), in this case, move at minimum speeds, constantly monitoring the temperature with your hand (touching the pipes).

⚠️ Attention: On motors with closed cooling circuit (e.g., Honda BF150 or Mercury Verado) driving without a thermostat will cause the oil to become supercooled and “curled up” – this is guaranteed to be overhauled in 50–100 motor hours.

FAQ: Frequent questions about outboard motor thermostats

How often should I change the thermostat?

The life of the thermostat 3-5 years or 300-500 motor-hoursHowever, in practice, it can fail sooner because of:

• 🔹 Use ordinary water instead of antifreeze (scaling is formed).
• 🔹 Oil entering the cooling system (destroys the wax element).
• 🔹 Operating the motor in salt water without washing.

It is recommended to check the thermostat every season when preparing the engine for the summer.

Can I clean the thermostat or just replace it?

If the thermostat is stuck due to deposits (for example, scale), you can try to clean it.

1. Place the thermostat in a solution of citric acid (100 g per 1 liter of water) for 1-2 hours.
2. Rinse with running water and check the mobility of the valve.
3. If the valve is moving smoothly, set it back. If the backlash is still there or the wax element is damaged, just replace it.

Cleaning helps 30-40% of the time, but it’s a temporary solution.

Why does a new thermostat break down quickly?

If the new thermostat fails after a few hours of operation, the reasons may be as follows:

• 🔹 A substandard Buy only proven brands (Sierra, Globe, Quicksilver).
• 🔹 Clogged cooling system - wash the radiator and the pipes.
• 🔹 Faulty pump. If the impeller is worn, the water pressure is not enough for the normal operation of the thermostat.
• 🔹 Overheating of the engine for another reason Check the temperature sensor and fan (on forced-cooled motors).

Which thermostat is better, the original or the analogue?

Original thermostats (Mercury, Yamaha, Honda) Guaranteedly fit the parameters, but cost 2-3 times more.Sierra, Globe) cheaper but:

• 🔹 Check the opening temperature (should match the original 2C).
• 🔹 Pay attention to the material of the case (brass is preferable to aluminum).
• 🔹 Avoid “no-name” brands – they often use poor-quality wax that loses its properties after a year.

For engines older than 10 years, the original thermostats may not be available – in this case, choose the ones with the best reviews.

Can I install a thermostat with a different opening temperature?

Installation of a thermostat with a lower opening temperature (e.g., 70°C instead 82°C) will lead to:

• 🔹 Underheating of the engine and increased fuel consumption.
• 🔹 Accelerated wear of piston rings due to poor lubrication.

Higher temperature thermostat (90°C+) dangerous overheating, especially in hot weather or with prolonged operation at high speeds, allowed deviation of ± 3 ° C from the original parameters.