If the engine is not getting hot and the temperature gauge is reading normal, it’s difficult to see how the coolant could be boiling. Perhaps what you’re seeing is the coolant in the overflow bottle bubbling, which can appear as though it’s boiling. But the reality might be that you have a leaking cylinder head gasket.

When that happens, gasses from the combustion process can escape into the engine’s coolant system and appear as bubbles in the overflow system. And to the observer, that can look very much like boiling coolant.

Either way, you need to find a good workshop that can check this stuff and figure out what’s really going on. Only then can you diagnose the root problem and tackle it.

If you're selling or trading-in the car, then a quality, tested second-hand engine from a wrecked iX35 is probably your best bet. This will be the cheaper option and even though you could spend a lot of money having the current engine reconditioned or a brand-new engine fitted, you'll find that won't affect the resale value as much as the extra outlay.

That said, it all depends on what's wrong with the current engine. If it's completely trashed, then a replacement is the go. But if your engine has simply stopped working for a potentially simple reason, then it might be worthwhile fixing it and maybe saving even more money. Either way, making your car a proper running vehicle again will definitely justify the cost as it's virtually worthless in a non-running state.

As for other brands of engines, the only viable choice is the same engine that make and model came with from the factory. Trying to fit anything else is a whole new car of very expensive worms.

If a timing belt breaks, one of two things are likely. Either the broken belt will allow the valves and pistons of the engine to collide, instantly turning the engine to scrap. (This is what happens in what's known as an interference engine design.)

The other alternative is that the belt snaps, but no intrinsic metallic parts collide (this is the case in a non-interference engine). If that's the case, the car won't run, but if a new belt is fitted and the engine retimed, it should work perfectly afterwards.

But even in an interference engine, if the tensioner gets slack but doesn't actually allow the timing belt to jump ship or break, then there's a chance that you've avoided a catastrophic failure. The bottom line is that if your engine suffered a problem with the timing belt, but works properly after being fixed, then all should be good in the long run.