Divide the complaint by frequency and by when it appears. A low-frequency shake at idle that smooths as soon as engine speed rises is almost always a collapsed engine mount or a cylinder that is not contributing. A high-frequency buzz that grows with engine speed and is felt through the frame rather than the cab floor points at the crankshaft damper, a bent fan or a loose pulley. Vibration that appears only under load and disappears at idle is a misfire or an injection delivery problem, not a mount. Confirm mounts by prying each one and looking for rubber separation and metal-to-metal contact. Confirm the damper by inspecting the elastomer band and the timing marks. Confirm firing balance with an injector cut-out test before concluding anything about the rotating assembly.
What you will notice
| Symptom | What it usually points to |
|---|---|
| Shake at idle that disappears above 1200 rpm | Mount collapse or a single cylinder not firing. Both are worst at low speed where each combustion event is felt individually. |
| Buzz through the frame that rises with engine speed | Rotating imbalance: damper, fan, pulley or accessory drive. It tracks engine speed exactly and does not care about load. |
| Vibration under load only, smooth at idle | Combustion problem on one cylinder. Load raises cylinder pressure and exposes a weak injector or low compression. |
| Steering wheel and cab floor shake at a specific rpm | Resonance with a worn mount. Note the exact rpm; a resonance band that was not there before is a mount or a damper losing its tuning. |
| Visible movement of the engine on shutdown | Failed mount allowing the engine to rock. Watch the engine while someone stops it and note which side moves. |
Most likely causes
| Cause | How to confirm it |
|---|---|
| Collapsed or separated engine mounts | Rubber bonds to steel plates, and when the bond fails the mount carries load metal to metal. Oil and hydraulic fluid on the rubber accelerates the failure, so a leak above a mount is a warning about the mount as well as about the leak. |
| Failed crankshaft damper | The elastomer band between hub and inertia ring cracks and slips. The damper stops absorbing torsional pulses, vibration rises across the whole speed range and the crankshaft is loaded in a way it was not designed for. |
| One cylinder not contributing | A weak injector, low compression or a valve fault makes the engine run on fewer cylinders. The result is a shake at firing frequency, worst at idle and light load. |
| Idle speed set too low | Below the specified idle, every combustion event is felt. A machine that shakes only at idle and is smooth at the correct idle speed has a setting problem, not a mechanical one. |
| Misaligned engine-to-pump coupling | After mount replacement or pump work, a coupling out of alignment transmits a once-per-revolution pulse that feels like an engine fault and is often diagnosed as one. |
| Bent fan blade or loose pulley | A fan blade that has lost material or a pulley running out of true produces a fast buzz that is easy to mistake for a bearing noise until the belt is removed. |
Narrowing it down by behavior
| Behavior | Test that decides it |
|---|---|
| Worst at idle, smooth at working speed | Injector cut-out test, then check idle setting. |
| Rises steadily with engine speed | Run without the fan belt to separate accessories from the engine. |
| Appears under load only | Cut out each cylinder under load and watch for change. |
| New after mount or pump work | Check coupling alignment and mount preload. |
| Engine visibly rocks on shutdown | Pry each mount and inspect the rubber bond. |
How to confirm it
- Confirm the frequency instead of describing the shake
Note whether the vibration follows engine rpm, track speed or hydraulic demand. Rev the engine in neutral and watch whether the symptom tracks it. A fault that tracks engine speed belongs to the engine or its driven accessories; one that tracks travel speed belongs to the undercarriage or driveline.
- Cut out cylinders one at a time
Use the diagnostic cut-out function or crack each high-pressure line in turn, at idle and then under load. If the vibration changes markedly when one cylinder is dropped, that cylinder was already weak and the fault is combustion, not balance.
- Pry and inspect every engine mount
With the engine off, pry each mount gently and look for separation between rubber and steel, cracks, or a mount sitting metal to metal. Then check for oil contamination on the rubber, which is a failure in progress even before the bond lets go.
- Check the damper band and the timing marks
Inspect the elastomer ring for cracks, bulging or rubber dust and confirm the index marks still line up. A damper that has slipped has also changed the timing reference, so any injection timing check made afterwards is meaningless until it is replaced.
- Run the engine briefly without the fan belt
Remove the belt and run the engine for under a minute at idle. If the buzz disappears, the fault is in the fan, the alternator, the tensioner or a pulley, and the engine itself is not the source. Keep the run short because the water pump is not turning.
- Check coupling alignment after any recent work
If the vibration began after mount or pump replacement, measure coupling alignment and confirm the mounts are seated and correctly preloaded. A mount changed on one side only leaves the engine sitting at an angle that the coupling cannot absorb.
What to do about it
Replace engine mounts in pairs, front or rear as a set, and never shim a collapsed mount back to height. A mount that has separated has lost its bond, and packing it only hides the movement while the coupling and the hoses take the strain. Clean up any oil leak above the mount at the same time, or the new rubber will fail the same way.
Replace a suspect damper rather than inspecting it repeatedly. A damper is a wear item with a service life, and a slipped or cracked one cannot be repaired. On most industrial engines it is also the cheapest way to protect the crankshaft, because torsional vibration left uncontrolled cracks crank webs that no one can inspect from outside.
For a misfire, test before replacing. Compare injector return flow, run a compression or leak-down test, and check valve lash before ordering injectors. The cylinder that changes the vibration when cut out is the one to investigate, and the test costs an hour against the price of a set of injectors.
Restore the correct idle speed and check the pump coupling after the engine is smooth. Vibration that comes from a misaligned coupling is often the last fault standing after a repair, and it will destroy the coupling and the pump input shaft if it is left. Guangzhou Shenyue Machinery Technology Co., Ltd. supplies engine mounts, dampers, couplings, injectors and gasket sets from Guangzhou, with 3,000+ part types on the shelf and 100,000+ part numbers available to order, each quoted as genuine, OEM-quality or aftermarket.
Parts you may need, by grade
| Part | Grades commonly quoted |
|---|---|
| Engine mount | Genuine; OEM-quality. Replace in pairs and confirm the hardness grade matches the position. |
| Crankshaft damper | Genuine only. A damper is tuned to the engine and an incorrect one is worse than a worn one. |
| Engine-to-pump coupling | Genuine; OEM-quality. Confirm the flex disc grade and alignment specification. |
| Injector or nozzle assembly | Genuine; OEM-quality; remanufactured matched set. Test before replacing. |
| Fan blade and pulley | Genuine; OEM-quality. Balance matters; do not fit a blade that has been repaired. |
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 a bad engine mount damage the hydraulic pump?
Yes, indirectly. A collapsed mount tilts the engine and loads the coupling, and that misalignment puts bending stress into the pump input shaft and its bearing. Vibration that appears to come from the pump after a mount failure is often exactly that.
Is some engine vibration normal on an excavator?
A four-cylinder industrial diesel always has a characteristic shake at idle, and every machine has a speed band where the cab resonates. What is not normal is a change from the machine's own baseline, or a shake you can feel through the frame at working speed.
How long does a crankshaft damper last?
It is a rubber part and it ages with heat and hours rather than failing at a fixed interval. Inspect the band at every major service after the first few thousand hours, and replace it whenever the rubber is cracked, glazed or has thrown dust.