Watch the engine while an assistant applies load in gear against the brakes - movement beyond a small amount means a failed mount.

With the handbrake on and the footbrake applied, selecting drive or a gear and raising the revs briefly loads the mounts in one direction.

Repeating in reverse loads them the other way, which reveals a mount that has failed in only one direction.

A sound mount allows a small, controlled movement; a failed one allows the engine to lift or shift noticeably.

Visually, look for cracked, separated or collapsed rubber, for the engine sitting visibly low, and for fluid weeping from a hydraulic mount.

Do this with proper care - keep clear of the engine bay, use the brakes firmly and apply load only briefly.

Because the symptoms - clunks on take-up, idle vibration, notchy gearchange, booming - all sound like driveline or gearbox faults.

A clunk as drive is taken up is the engine rocking on a failed mount before the driveline loads, and it is indistinguishable by ear from driveline backlash.

Vibration at idle that disappears as revs rise is the classic failed-mount signature, and it gets blamed on the engine running roughly.

A notchy or stiff gearchange happens because the gearbox has moved relative to the linkage, and the linkage gets adjusted instead.

Booming at a particular speed is a resonance the mount was designed to damp and no longer does.

Checking mounts takes a minute and belongs at the start of the diagnosis for any of these complaints.

A mount containing fluid in chambers connected by a passage, which gives frequency-dependent damping that plain rubber cannot.

As the mount deflects, fluid is forced between chambers through a restriction, and that flow absorbs energy.

The design is tuned so it is soft to small, high-frequency vibrations and firm against the large, low-frequency shake of an engine at idle.

That is why they are used on larger engines and on vehicles where refinement matters, including many modern diesels.

They fail by leaking, and a wet or oily mount is a definite replacement rather than a judgement call.

A plain rubber mount cannot be substituted for a hydraulic one without losing the damping the vehicle was designed around.

Usually - they are the same age and have shared the same loads, and a single new mount next to worn ones takes a disproportionate share.

Most vehicles have three or four mounts sharing the engine and gearbox mass.

When one has collapsed, the others have been carrying more load for some time.

A single new mount is also stiffer than its worn neighbours, so it carries more than its share and wears faster.

The labour frequently overlaps - the engine has to be supported either way - so doing the set is cheaper than two visits.

The exception is a mount that has failed from a specific cause such as oil contamination, where the others may be genuinely sound.

It lets the engine and gearbox move, which changes driveshaft angles and loads exhaust mountings, hoses and wiring.

Excess movement alters the angles at which driveshafts and propshafts operate, producing vibration and accelerating joint wear.

The exhaust is bolted to the engine at one end and the body at the other, so engine movement is taken by the flexible section and the mountings - both of which then fail.

Coolant hoses, fuel lines and wiring harnesses cross the same gap and can chafe or be pulled.

Gear linkages and cables are stretched and compressed, giving selection problems that look like a gearbox fault.

So a cheap mount left too long generates a list of much more expensive symptoms, which is the main argument for checking them routinely.

Oil contamination, heat, and excessive torque loading - and oil is the most common by some margin.

Rubber degrades rapidly in contact with engine oil, so an oil leak above a mount will destroy it long before age would.

That means fitting a new mount without curing the leak simply consumes another mount.

Heat hardens rubber and makes it crack, so mounts close to a turbocharger or exhaust have a harder life.

Harsh clutch use, high torque and towing all load the mounts more heavily, and commercial vehicles on heavy duty consume them faster.

Poor fitting contributes too: mounts should be tightened with the engine at its normal resting position, not while it is jacked or hanging.

With an engine support beam or a hoist taking the weight from above, or a jack with a spreader plate under the sump - never a jack directly on the sump.

The weight must be taken off the mount being removed, and taken safely, because a moving engine is very heavy.

An engine support bar spanning the wings and lifting from a lifting eye is the controlled method and leaves the underside clear.

A trolley jack with a broad wooden spreader under the sump works where the sump is steel and the load is spread - an aluminium sump or a bare jack pad will crack it.

Raise only enough to unload the mount; lifting further strains hoses, the exhaust and the remaining mounts.

Fit and finally tighten with the engine back at its natural resting position, so the rubber is not pre-loaded.