This has been an ongoing problem for many owners of many different makes and models of cars fitted with Diesel Particulate Filters (DPFs). The process of dumping fuel into the engine is designed to get the exhaust system hot enough for the DPF regeneration (self-cleaning) to take place. But a car that travels 500km per week really shouldn’t need this as the exhaust system should be getting hot enough in the normal course of things.

Either way, if you constantly have excess fuel being dumped into the engine, some of it will find its way into the sump (as you’ve discovered). That’s when the check engine light signals the problem, but in the meantime, you’re also driving around with engine oil diluted by the extra fuel. And that’s when engine wear can occur because diluted oil isn’t able to protect the engine’s metal parts as well as it should.

It could be that the car’s sensors are giving the computer the wrong information on how hot it’s getting, and/or how dirty the DPF is. Perhaps the DPF itself is damaged to the point where it won’t regenerate fully. A diesel specialist would be a good place to start, as this problem is hardly unknown these days.

There are several things that could be causing a loss of power on really hot days, but a prime suspect would be an engine that is getting too hot. When that happens, the car will often go into limp-home mode which is usually signalled by a loss of power and a check-engine light illuminating (the precise symptoms you’re seeing).

The air conditioner consumes quite a bit of engine power when its working, and this extra load might be the tipping point for the car starting to overheat. When you switch it off, the engine can relax a little and the temperature falls. So the advice would be to have the car’s cooling system checked. It could be low on coolant or have a faulty pump, hose or thermostat. While you’re at it, have the car electronically scanned to see if any fault codes have been recorded, as this will also help your mechanic diagnose the issue.

It’s amazing where rainwater will get to under the influence of a pressure difference between the inside and outside of the car, not to mention a 100km/h gale trying to blow everything inside the car. As such, you only need a tiny tear or crease in a door rubber for water to enter the car. And if it enters somewhere near the driver’s seat, that’s where it will stay when it hits the carpet.

Check the rubber seal in the door opening for any signs of damage or a small chunk missing. Even a piece of leaf debris stuck to the rubber can allow a gap big enough for water to enter. Make sure, too, that the door itself is properly aligned on its hinges and is making contact with the whole of the door rubber when it’s shut.