Any vehicle of this age can start to experience wear in the turbocharger. These units live a very hard life, spinning around at something like 250,000pm and operating in a very hot environment. Cracks can occur in the housings, the bearings can wear and even the blades of the turbocharger itself can break or become bent. All of these things will affect performance. And, unfortunately, it sometimes doesn’t matter how well the vehicle has been maintained. Turbos have a finite lifespan and that’s all there is to it.

But don’t simply assume that a lack of performance is due to turbocharger failure or wear. Sometimes on this model, the Exhaust Gas Recirculation (EGR) valve, which is designed to re-burn some of the engine’s exhaust to reduce tailpipe emissions, can become stuck open, reducing boost pressure and, therefore, performance.

Many workshops specialise in turbo-diesels these days, and should be able to help with a diagnosis.

Vibrations through a car at varying speeds or throttle position can be an indication of wheel imbalance, poor wheel alignment, worn suspension parts, a damaged tyre, bent wheel rim, a driveshaft or CV joint problem, engine misfire, gearbox of differential issue, over-tight brake, worn brake drum or rotor and perhaps a million other things.

If the problem is worse since the new engine was fitted, I’d be looking at things like engine mounts which can wear and even fail, leading to a vibration or wobble throughout the whole car.

But if the bull-bar is wobbling in sympathy, don’t rule out a loose bolt(s) in the bull-bar mounts which are allowing it to flop about and send a shiver through the rest of the car. Even a blob of mud stuck in the inside of a wheel rim can be enough to imbalance that wheel and produce the same effect when the particular harmonics of slowing down enter the equation.

You could start with the engine mounts and work your way backwards along the driveline, looking for worn couplings, U-bolts and universal joints. If you have a Go-Pro camera, maybe mounting that under the car and reviewing the footage afterward might give you a clue as to what’s causing the wobble. Having the wheels and tyres balanced would be another relatively inexpensive way of ruling out one possibility.

It’s very difficult to diagnose problems over the phone, just as it is via this website. But, I’d be taking the vehicle straight back to the mechanic in question, because whatever is wrong is going to be fairly major.

In the majority of cases, an engine with a broken crankshaft won’t run. In fact, it won’t even turn over. Sometimes, it will still run, though, and the symptoms you’ve described are spot on for that. Either way, a broken crankshaft is one example of what’s called catastrophic failure and a new crankshaft (at the very least) is the usual result.

But those same symptoms can also be the result of a loose harmonic balancer. And here’s the clue: The harmonic balancer has to be removed to replace the front crankshaft seal (which was done the day before the problems started). If the balancer is not reinstalled correctly, it can start to wobble, tear up the new seal and make all sorts of noises and vibrations. The bad news is that often, the wobbling balancer destroys the end (called the snout) of the crankshaft in the process, so even though the crank is not broken or snapped per se, it may still need to be replaced. It does seem like a fantastic coincidence that the balancer started wobbling the day after it was removed and refitted. That said, if the crankshaft has failed, then a coincidence is precisely what it might be.