This sounds like a fairly simple case of the engine setting up a harmonic vibration through the car. It’s not an uncommon thing and diesels are worse than petrol engines purely because they vibrate more, particularly at idle. By revving the engine slightly, you’re changing the frequency of the engine’s vibrations and moving them out of synch with what’s called the 'natural frequency’ of the rest of the car.

Why does it manifest at the steering column? Because the column is a long, flexible (in a micro sense) shaft that is far more prone to picking up vibrations than a short, more solid part of the car. Why is it happening now all of a sudden? Probably because there’s some wear in the car somewhere. Your call to change the engine mounts was a good one as these can wear and cause this very problem. But, equally, the wear could be in the exhaust system or one of the points where it attaches to the car, a heat shield, the torque converter, or maybe even a piece of the car’s sheet metal that has come loose and is moving slightly.

The quickest fix is to bump the idle one or two hundred rpm and see if that moves the engine out of the vibration zone at idle.

Let’s get something straight right from the off: The Holden Calais and Holden Captiva are not close family members. While the Calais was an Australian-engineered and built car, the Captiva is a rebadged Daewoo and not a very good one at that. So don’t be tricked into thinking that because they both have a Holden badge and a V6 engine, that they’re peas from a pod.

The reality is the Captiva has an absolutely dreadful reputation in the trade for poor build quality and sketchy reliability. Most mechanics wouldn’t travel 500 metres to look at one, let alone 500 kilometres. I’d take a 231,000km Calais over any Captiva, old or new, any day.

Holden actually sold a lot of Captivas back in the day, mainly on price alone. But many who bought them then stood and watched as they things fell apart and soon began to cost plenty in repair bills. Doubtless there will be some out there who own and love their Captiva, but the reality is that these cars are best left well alone.

There’s a variety of ways a turbocharged engine can lose boost. You might have a problem with the intercooler or even something simple like a dirty air filter. Don’t rule out a fuel supply problem which is causing the computer to reduce boost to account for a lack of fuel.

But one thing that has been noted in these cars is a cracked or broken boost pipe. If that happens, the boost will be lost to the atmosphere before it can get to the engine. Black smoke from the exhaust is a common accompaniment to this problem.