A hybrid stops and starts the petrol engine dozens of times on one trip. Every time it joins in, there is a torque jolt. That jolt should be taken by the engine mount and the upper torque rod. Rubber cracks with years and mileage. A knock is left exactly when the petrol engine appears or disappears on the gauge.
What a worn mount sounds like
- One thump or a short shudder when pulling away from electric driving as the petrol engine joins in
- The same when it stops at the lights — you feel it in the floor and the steering wheel
- At a steady speed, when the engine is stopped, the knock disappears
- When cold it is often louder: the rubber is harder and the engine joins more sharply
If the car drives calmly and the thump returns only when the petrol engine starts or stops, the HV pack is not doing it. The battery does not knock in the body.
What this is not
A whine that rises with speed and is still heard when the engine is stopped is the MG2 bearing. A hum at the brake pedal when stopped is brake actuator. A vibration present for the whole drive, independent of the petrol engine switching on, points rather to the e-CVT transmission than to the rubber under the engine.
In the workshop you see it from below and from above: torn rubber, oil in the mount on the gearbox side, a broken rod. Stopped in READY, with the brake pressed, a sound mount holds the unit in place. A cracked one lets the engine move several centimetres when torque comes on or goes off.
Why not to wait until it “settles itself”
An engine on a torn mount tugs at coolant hoses, the earth strap and the wire by the throttle. Next to it are the inverter pipes and the covers of the orange cables. The knock will not flatten the battery overnight, but a torn hose or a battered bracket will raise a message that looks like a hybrid-system fault. The source stays mechanical.
In a Prius, Auris and CT the upper rod and the right mount usually give way first. In the larger Lexus RX and NX there is also a mount by the transmission — the symptom is the same, the access is different. Before anyone prices the pack, it is worth lifting the car and looking at the rubber.
After replacement
New rubber will not change fuel use in a way you can catch on one fill-up. It changes whether the engine start is soft. If the knock remains, we look for the second mount or for a part the first one has already battered: a cover, a bracket, a hose. The HV pack itself does not damp that thump.
A knock that matches the petrol engine switching on is a mount. A tone that matches wheel speed is the transmission. Those are two different workshops in one car.
A knock at every engine start? Let us look at the mounts before a hose pays for it.
Visit Green Serwis in Krakow — we specialise in Toyota and Lexus hybrids.
Book a visitSummary
Frequent engine stops are normal in a hybrid. A thump in the body on every such cycle is not. The mount and the torque rod are meant to catch that moment. Cracked rubber imitates “something hybrid” and actually sits in the mounting of the power unit. We diagnose the battery when the symptom remains in electric driving as well. The offer is on the page for hybrid service.
