Boom drift means oil is escaping from the cylinder chamber that is supposed to be holding the load, and there are only three places it can go. It can pass the piston seals inside the cylinder and flow to the other side, it can leak across the control valve spool back to tank, or it can escape through a load-holding or relief cartridge that is no longer sealing. The diagnostic task is to separate these three, because the repair for each is completely different in cost: a seal kit, a valve section, or a cartridge. The separation is done by isolation, not by guesswork. Cap the cylinder port safely and the cylinder is tested alone; the drift that remains with the cylinder isolated belongs to the valve side. Two observations narrow the field before any wrench is turned: drift that speeds up as the oil warms points at worn sealing surfaces, and drift that affects only one circuit points away from anything shared, such as pump pressure or a common relief.
What you will notice
| Symptom | What it usually points to |
|---|---|
| The boom sinks measurably with the lever in neutral | A few millimeters over minutes is leakage, not settling. Measure the drop against a mark on the stick with a tape, because drift rates are compared later against the service manual's limit. |
| The drift gets faster as the oil warms up | Thin hot oil leaks more through the same worn surface. Drift that is modest cold and pronounced hot is wear, not adjustment. |
| Only the boom drifts; stick and bucket hold their position | A fault in one cylinder or one valve section affects one circuit. A problem common to all circuits would be felt everywhere and points elsewhere entirely. |
| No external oil is visible anywhere | Drift with dry cylinders and dry hoses is internal leakage by definition. The oil is going somewhere; it is simply staying inside the circuit. |
Most likely causes
| Cause | How to confirm it |
|---|---|
| Piston seal bypass inside the boom cylinder | Worn or damaged piston seals let oil flow from the loaded side to the unloaded side inside the barrel. The cylinder holds nothing, and externally it looks perfect. |
| Control valve spool leakage | A worn spool or bore in the boom section of the main control valve lets oil slip past to tank with the lever in neutral. Clearances here are small, and wear shows as drift long before it shows as poor control. |
| A load-holding or relief cartridge that no longer seals | Where the circuit has a load check, counterbalance or port relief cartridge, a contaminated or worn seat provides a direct path to tank. Cartridges fail silently and are cheap compared with a valve section. |
| A cracked cylinder barrel or piston | Rare, but it happens after overload or a previous rod failure. A crack bypasses the seals entirely and no seal kit will fix it, which is why the isolation test matters before parts are ordered. |
The drift pattern, and what it points at
| What you observe | Where to test first |
|---|---|
| Drift only hot, cylinder looks new externally | Piston seal bypass; run the cylinder isolation and bypass test. |
| Drift identical cold and hot, steady rate | Valve spool leakage or a cartridge seat; test the valve side first. |
| Drift started immediately after a hose or valve repair | Disturbed contamination on a cartridge seat; inspect what was touched last. |
| Both boom cylinders drift at the same rate | A common element: the valve section or relief, not two coincidental cylinder failures. |
| Drift with a knock or a jerky sink | Mechanical damage inside the cylinder; stop and strip rather than test further. |
How to confirm it
- Measure the drift rate before touching anything
Raise the boom to a marked position, warm the oil to normal operating temperature, release the lever and time the drop over five minutes against a tape. A number on paper beats an impression, and the manual's drift limit is a number, not an impression either.
- Make the machine safe before any test
Lower the attachment to the ground or support the boom mechanically before a hose is loosened. Relieve residual pressure according to the manual. A drifting boom is the symptom being tested, and it does not stop being dangerous while you test it.
- Isolate the cylinder from the valve
Cap or plug the cylinder port on the loaded side using fittings rated for full system pressure, then reload the boom. If the drift stops with the cylinder isolated, the cylinder is innocent and the leakage is on the valve side. If it continues, the cylinder is leaking internally.
- Run the cylinder bypass test to confirm
With the cylinder suspect, confirm with the direct bypass test rather than condemning it on one result. The bypass test quantifies the leakage and separates seal wear from a cracked barrel or piston.
- On the valve side, test the cheapest element first
Pull the load check or port relief cartridge and inspect the seat under good light. A cartridge is minutes to check and a fraction of the price of a valve section. Swapping cartridges between sections is a legitimate test when the cartridges are identical.
- Verify the repair by repeating the original measurement
After the seal kit, cartridge or valve work, warm the machine and repeat the five-minute drift test in the same position. A repair verified with the same measurement that found the fault is closed; one verified by feel is not.
What to do about it
Cylinder reseal: strip the cylinder on a clean bench, inspect the barrel and piston for scoring and cracks before fitting the kit, and replace the wear rings if their clearance is at the limit. A seal kit fitted into a scored barrel is a comeback waiting for its first hot day.
Cartridge replacement: where a load check or relief cartridge seat is damaged, replace the cartridge rather than attempt to lap the seat in the field, and flush the section before fitting the new one. The contamination that marked the old seat is still in the oil.
Valve section service: spool and bore wear beyond the manual's limit means a replacement section or a valve assembly, and this is the moment to check the others rather than repeat the labor in a month.
Parts for any of these repairs — cylinder seal kits and wear rings, load check and relief cartridges, valve sections and test fittings — 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 the grade of 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 |
|---|---|
| Cylinder seal kits | Genuine; OEM-quality; aftermarket kit matched to the cylinder number, never to dimensions alone. |
| Load check and relief cartridges | Genuine; OEM-quality; remanufactured exchange where supported. |
| Valve sections and spools | Genuine; remanufactured exchange with test evidence. |
| Test gauges and fittings | Rated above full system pressure, from a known tool line. |
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
Is a small amount of boom drift normal?
Yes. Every hydraulic circuit has a permitted internal leakage, and the service manual states the drift limit for your model as a measured drop over a stated time at a stated oil temperature. Compare against that number, not against zero.
Can I keep working with a drifting boom?
Within the manual's limit, with awareness. Beyond it, no: the leakage generating the drift is also generating heat, and the wear accelerates itself. A drifting boom over a trench or a truck is also a load-control problem, not just a maintenance one.
The boom drifts only when fully loaded. Is that a different fault?
Same fault, visible earlier. Internal leakage is driven by pressure difference, and a heavy load raises that difference. A cylinder that holds empty and sinks loaded is leaking; the load is simply making the measurement easier.