A leak-down test measures how much of a regulated air supply a cylinder holds at top dead center on the compression stroke, expressed as a percentage of leakage, and unlike a compression test it identifies where the loss is going. Warm the engine, remove the injectors or glow plugs, and bring the cylinder to true TDC with both valves closed; the balance point of the rocker arms is a more reliable indicator on a worn engine than the crank mark alone. Lock the crank so the piston cannot be pushed down, connect a regulated 100 psi supply through the leak-down tool and read the second gauge. Under 10 percent is healthy on most diesel engines and 20 percent or more is a real leak. Then listen: air at the intake is an intake valve, at the exhaust an exhaust valve, at the oil filler or dipstick tube the rings or piston, and bubbles in the radiator a head gasket or a crack.
Where the air escapes, and what it means
| Listening point | Meaning and next check |
|---|---|
| Air at the intake opening | Intake valve or seat. Confirm with valve lash and a visual inspection of the seat once the head is off; a tight lash holds the valve open and mimics a burned seat. |
| Air at the exhaust opening | Exhaust valve or seat, the most common valve failure on a diesel because of heat and ash. Check lash first, then the seat and the valve face. |
| Air at the oil filler or dipstick tube | Rings, piston or bore. A reading that improves when oil is added is ring sealing; one that does not may be a hole in a piston or a badly scuffed bore. |
| Bubbles in the radiator or coolant pushed out of the neck | Head gasket, cracked head or a liner seal. Confirm with a block test for combustion gas and check for bubbles between two adjacent cylinders. |
| Air at the adjacent cylinder's injector hole | A gasket breach between two cylinders. This is the one result that condemns the gasket outright, and it means the head comes off. |
How to confirm it
- Warm the engine to operating temperature and then disable the fuel system
Leak-down figures taken on a cold engine are inflated by piston and ring clearances that have not closed up. Warm the engine first, then disable injection so the engine cannot fire while it is being barred over.
- Remove the injectors or glow plugs and keep them in cylinder order
Every cylinder needs an open path for the air. Label each injector as it comes out, because the leak-down result and the injector condition together often explain a misfire that neither would explain alone.
- Bring each cylinder to true TDC on compression
Use the crank mark to get close, then confirm with the rocker arms: at TDC on compression both valves are closed and the rockers sit on the base circle. On a worn engine the mark can be several degrees out, and a few degrees off TDC lets the piston move down when pressure is applied.
- Lock the crankshaft
With air pressure in the cylinder, an unlocked crank will turn. Lock it at the flywheel or the front pulley according to the service procedure. This is a safety step as much as an accuracy one: a rotating crank with a bar in the way breaks fingers.
- Calibrate the tool before the first reading
Connect the tool to the regulated supply with the cylinder hose closed and set the regulator so the first gauge reads 100 psi, then confirm the second gauge reads zero leakage. A tool that does not zero will report a percentage that means nothing.
- Read the leakage and record it per cylinder
Open the cylinder valve and read the second gauge. Record the number and the listening result for each cylinder in firing order rather than in the order you happened to test them, because the pattern across the engine is what tells the story.
- Listen at four points and, where needed, add oil
Listen at the intake, the exhaust, the oil filler and the radiator. If the loss is at the crankcase, add a few cubic centimeters of oil to the cylinder and repeat: a reading that improves is rings, and one that does not is a valve or a gasket.
What to do about it
Leak at a valve: check valve lash before condemning anything. A valve held slightly open by tight lash reads as a leaking seat and costs nothing to correct. If the lash is right and the leak is real, the head comes off for a seat and valve job, and the decision on one cylinder or all of them follows the condition of the rest.
Leak at the rings: a uniform 15 to 20 percent across every cylinder on a high-hour engine is wear, not a fault, and the decision is economic rather than technical. A single cylinder far worse than the others points at a scored bore, a broken ring or a damaged piston, and that is a repair rather than a rebuild decision.
Leak into the coolant: stop. Combustion gas pressurizing the cooling system will push coolant out and overheat the engine, and continued running turns a gasket job into a head job. Confirm with a block test before the head comes off, so the job is planned once.
Gaskets, valve train components, piston and liner kits, injector seals and the tooling for this test 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 |
|---|---|
| Head gaskets and gasket sets | Genuine; OEM-quality composite or MLS to the original type. Fitting the wrong gasket type for the liner protrusion is a repeat failure waiting to happen. |
| Valves, seats and guides | Genuine; OEM-quality valves and inserts; guide material matched to the original, since the wrong bronze wears or scuffs. |
| Piston rings and liner kits | Genuine; OEM-quality kits with the correct ring set for the bore and the application. |
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
What leak-down percentage is acceptable?
On most diesel engines under 10 percent is healthy and above 20 percent needs attention, but the builder's figure is the one that counts and it is usually quoted for a specific test pressure. Compare cylinders with each other as well: a uniform 15 percent across six cylinders means something different from one cylinder at 30 percent.
Can I run a leak-down test on a cold engine?
You can, and the numbers will be worse than the engine deserves because the rings have not expanded and the bores are not at running clearance. Cold results are useful only for finding a gross leak, such as a burned valve, and should not be compared with a specification.
Why does leak-down disagree with my compression test?
They measure different things. A compression test turns the engine and is sensitive to cranking speed and cam timing; leak-down is static and sensitive to valve lash and piston position. A cylinder with good compression and poor leak-down usually has a valve held open by lash or a leak that only appears when the piston is stationary at TDC.