Test the thermostat in a pot of water with a thermometer: it should begin to open within a few degrees of its stamped rating, and reach full lift about 10 to 15 C above it, with the jiggle valve at the top when refitted. A thermostat that opens late, opens only part way or stays open is a fault, and a stuck-open unit costs fuel and heater performance without ever overheating. Test the radiator cap on a cooling system pressure tester: it must hold its rated pressure for a minute without bleeding down, and the vacuum relief must open when the system cools. A cap that releases at 0.3 bar instead of 0.9 lowers the boiling point by roughly 20 C, which is enough to boil a healthy engine on a hot day under load.
What good looks like on the bench
| Check | Pass and fail |
|---|---|
| Opening temperature | Within about 3 C of the stamped rating. Later than that delays warm-up control and overheats under load. |
| Full-open lift | 8 to 10 mm at 10 to 15 C above the rating. Half lift means restricted flow at peak demand. |
| Closing behavior | Closes fully as the water cools. A valve that stays partly open overcools in winter. |
| Jiggle valve | Present, free and fitted at the top. Fitted at the bottom it traps air and the engine overheats with a full radiator. |
| Cap holding pressure | Holds the rated pressure for one minute without decay. Any steady drop is a failed seal or spring. |
| Vacuum relief | Opens under vacuum and reseals. A stuck relief collapses hoses and can crack a radiator tank. |
How to confirm it
- Let the engine cool completely and release the system pressure
Open the cap only when the engine is cold, or with a rag over it and the pressure released slowly. Scalding from a pressurized cooling system is the most common injury in this job, and it happens in the second before people expect it.
- Drain enough coolant to remove the thermostat housing
Catch the coolant in a clean container and keep it, because it can go back in if it is the right mix and clean. Note the volume that comes out so the refill matches it.
- Read the rating stamped on the old thermostat and the new one
The opening temperature is stamped on the flange, typically 71, 82 or 88 C for diesel engines. A thermostat of the wrong rating opens too early in cold weather or too late under load, and both are faults that look like something else.
- Suspend it in water with a thermometer and heat slowly
Hang the thermostat on a wire so it does not touch the bottom of the pot, and heat gently while watching both the thermometer and the valve. Note the temperature at which it starts to move and the temperature at which it is fully open.
- Measure the full-open lift against the specification
A valve that opens but only a few millimeters has lost its lift, often because the bridge is bent or the wax capsule is failing. Full lift is usually 8 to 10 mm, and a thermostat that reaches half of that restricts flow at the moment the engine needs it most.
- Pressure-test the cap on the same tester used for the system
Fit the cap to the tester, pump to its rated pressure and hold for a minute. It should hold without decay, then release at the rating. Test the vacuum relief by pulling a vacuum and confirming the valve opens, because a cap that cannot relieve vacuum collapses hoses as the engine cools.
What to do about it
Replace a thermostat that fails any of the three bench checks. Fit a new gasket or O-ring every time, clean the housing face, and confirm the jiggle valve sits at the highest point. Refill slowly, run the engine to operating temperature with the heater open and check the level again after it cools, because a cooling system that is 2 liters short will overheat a machine that has nothing wrong with it.
Replace a cap that will not hold its rated pressure, and use the rating on the original cap rather than a higher one. A higher-rated cap raises the boiling point, which sounds useful and instead finds the weakest hose or the oldest radiator tank. If the cap is correct and the system still pressurizes within minutes of a cold start, test for combustion gas.
Where the thermostat and cap both test good, stop replacing parts and measure: coolant temperature at the head and at the radiator inlet, fan speed, and the temperature drop across the core. A blocked core and a slipping fan produce the same gauge reading as a stuck thermostat, and neither is fixed by a new one.
Thermostats, caps, housings, gaskets, hoses and the specified coolant are supplied by Guangzhou Shenyue Machinery Technology Co., Ltd. from Guangzhou, with 3,000+ part types on the shelf and 100,000+ part numbers available to order, and every line declared as genuine, OEM-quality or aftermarket. The five brands for which the company holds authorized dealer status are Garrett, Mitsubishi Heavy Industries, TONGMYUNG, NIKURA and Marusan, and the Mitsubishi authorization covers MHI engine parts only. Kobelco is not among them, and Kobelco parts are supplied through original channels or as quality alternatives.
Parts you may need, by grade
| Part | Grades commonly quoted |
|---|---|
| Thermostats | Genuine; OEM-quality to the same opening rating and lift. Two thermostats that look identical can be 15 C apart in rating. |
| Radiator caps | Genuine; OEM-quality at the original pressure rating. A cap is a calibrated spring, not a plug. |
| Gaskets, O-rings and hoses | Genuine; OEM-quality elastomer to the original specification, since a plain gasket at that temperature takes a set and weeps. |
Need the parts this covers?
Send the machine model, serial number and the part numbers you need. We confirm fitment before payment and state the grade of every line — genuine, OEM-quality or aftermarket.
Request a quoteFrequently asked
Can I run the machine without a thermostat to get through the season?
You can, and it will run cold: poor combustion, more fuel, no heater, and in some engines a controller that never sees operating temperature and holds the engine in a warm-up strategy. It also removes the restriction the system was designed around, so flow through the radiator can drop. It is not a repair.
How do I know if a thermostat is stuck open rather than stuck closed?
Stuck closed overheats within minutes of a cold start and the top hose stays cold while the head is hot. Stuck open runs cold, takes a long time to reach temperature and the heater stays lukewarm. Both are faults, and only one of them shows on the temperature gauge as a problem.
Does the cap pressure matter as much as people say?
Yes. Coolant boils at about 120 C at 0.9 bar and at about 105 C at atmospheric. Losing cap pressure costs roughly 15 to 20 C of boiling margin, which is the difference between a hot day under load and a boil-over.