You haven’t told me which engine is fitted to your car, but if it’s the turbo-diesel engine, then the cause may be a pretty simple one. This engine has a habit of splitting the plastic trunking that takes the pressurised intake air from the turbocharger to the engine’s inlet manifold. If that happens, the air is free to escape to the atmosphere, and doesn’t make it’s way into the engine.

That’s why you’ll hear a noise like air escaping (because that’s precisely what it is) as well as a lack of power, because the engine is not receiving all the boost from the turbo. You’ll probably also notice that the car is blowing more black smoke than it should.

The fix is a new piece of plastic pipe that should be well within the abilities of any workshop to fit.

Your mechanic is right in deciding that a leak between the engine and gearbox will need the two components to be split to access the problem. Whatever it turns out to be. It could be a broken torque converter which would be expensive, or a simple oil seal which may cost just a few dollars. Either way, though, the labour involved in fixing even that cheap oil seal is going to make this an expensive operation.

Sometimes, you just have to let it go and cut your losses on a car that is starting to become worn out. To be honest, the Holden Captiva was not the most reliable or durable car ever made and many owners have discovered the hard way that this Holden badged car was actually a Daewoo, and not a good Daewoo at that.

Any remedy can only be suggested once you know what’s causing the problem. The first step, then, is to have the car scanned to see if there’s a problem the on-board computer has identified and logged. Once you have that information, you are a lot closer to knowing what to replace.

Logic suggests that an engine that runs roughly when it’s cold might be running lean. If that’s the case, it could be caused by many things including worn injectors, a lazy fuel pump or even a temperature sensor that is not telling the computer that controls these things that the coolant temperature is low and the engine needs more fuel to compensate. Then, once the engine has warmed up, the fuel-air mixture is correct and the engine settles down and runs properly. That said, 30km is a long time for the engine to take to warm up.

You could spend lots of time and money replacing the wrong bits here, so a scan is definitely the way to proceed.