Engine and gearbox mounts: the vibration that appears at idle

An engine mount has two jobs that pull in opposite directions, and everything about it is a compromise between them.

It must be stiff enough to hold a heavy engine still — against its own weight, against the torque reaction that tries to twist it out of the car every time you accelerate, and against the shove of braking and cornering.

And it must be soft enough to stop the engine's vibration reaching you. The source puts this well: bolt an engine solidly to the body and every combustion pulse goes straight into the shell, which is unbearable to sit in and eventually cracks things.

So a mount is rubber, and rubber is a wearing part with a life.

The symptom that names it

The corpus files vibration felt through the car under engine misfire, which is the commonest cause — but there is a second one, and it has a signature that separates the two immediately:

A misfire's vibration is worse under load and follows engine speed. A worn mount's vibration is worst at idle and improves as the revs rise.

That is because an idling engine's shaking is at its lowest frequency and largest amplitude — exactly what a mount is there to absorb — and because at higher speed the pulses come too fast for the body to respond to. So a car that shakes the mirrors at a standstill and smooths out at 2,000 rpm is describing its mounts, not its cylinders.

The rest of the list:

That last row is worth its own note, because it sends people to the wrong part. A mount that has failed puts its movement into everything attached to the engine — the exhaust flex joint, the driveshafts, the gear linkage, the coolant hoses. So a broken mount can present as an exhaust rattle, as difficulty selecting gears, or as a driveshaft that fails early, and replacing any of those fixes nothing.

Three kinds, and only one of them looks broken when it is

Plain rubber. A block of rubber bonded between two metal plates. It fails visibly: the rubber cracks, perishes, or separates from the metal, and you can see it and often move it by hand.

Hydraulic. The same idea with a fluid-filled chamber inside, which damps a much wider range of frequencies — this is what makes a modern car smooth at idle. It fails invisibly. The fluid leaks out, the mount becomes an ordinary and rather soft rubber block, and it looks perfect. The only external sign is sometimes an oily weep from the mount itself, which is regularly mistaken for a leak from the engine above it.

Active. A hydraulic mount whose stiffness is switched, by vacuum or by a solenoid, so it can be soft at idle and firm when the engine is pulling. When the control fails it stays in one state — usually the wrong one — so the fault presents as vibration at idle with no visible damage at all, and the cause is a split vacuum hose rather than the mount.

The consequence for diagnosis is the important part: on any car built in the last twenty years, "the mounts look fine" is not a finding.

Testing them

Watch the engine move. Bonnet open, handbrake firmly on, the car in gear against the brake — or, on an automatic, in Drive against the brake — and have somebody apply a little throttle. The engine will lift and twist; that is normal. What you are looking for is how far, and whether it returns cleanly or thumps against something. Then reverse: the engine should rock the other way by a similar amount.

Do it with the wheels chocked, from beside the car rather than in front of it, and with nothing of yours near the belts.

Then look properly. Cracks through the rubber, rubber separated from its metal plate, a mount sitting visibly lower than it should, and any oily film on or under a hydraulic mount.

And prise gently. A bar between the mount's bracket and the body should show a small amount of controlled give, not free movement.

Why they fail, and why they fail in sequence

Rubber ages whether the car moves or not — heat, oil, ozone and time all harden and crack it — so mounts are as much a function of years as of kilometres. Oil contamination from a leak above accelerates it enormously, which is one more reason to fix a weeping cam cover gasket.

And there is a pattern worth knowing: a failed mount overloads the ones that are left. An engine held by four mounts distributes its torque reaction across all of them; lose one and the others take a share they were not sized for, and they follow within a year or two. Which is why the usual advice is to replace them as a set, or at least to inspect all of them the day one is found bad — and why a car that has had one mount replaced recently and is vibrating again probably needs the others rather than a warranty claim.

The torque-reaction mount is the one that goes first on most front-wheel-drive cars — the small link, often at the top or bottom, that stops the engine rotating under load. It is the smallest, it takes the largest cyclic force, and it is usually the cheapest part in the set.

Distinguishing it from everything else that shakes

Before ordering mounts, rule out the two things that produce a similar complaint:

• A misfire. It gets worse under load, not better, and it will usually store a code — P0300 and its per-cylinder neighbours. Diagnosing engine noises has the wider method.
• A wheel or tyre imbalance. That appears at a particular road speed and is unaffected by engine revs, which is the opposite dependency. It is not there at a standstill at all.

And the test that separates mounts from everything upstream costs nothing: put the car in neutral and rev it gently while somebody watches. If the vibration is there in neutral, it is the engine or its mounts. If it needs the car to be moving, it is not.

Video guides

Video, for the parts of this that are easier watched than read.