Mazda Reviews

This could be something to do with the engine’s idle-air control valve which plays an important role in making sure the engine gets the correct amount of air. This amount varies according to (among other things) the engine temperature. So, in a cold-start situation, the valve helps ensure the ratio of fuel and air entering the engine is correct to account for the lack of heat in the engine.

But you could also be looking at a dud temperature sensor that is sending the wrong information to the engine’s computer and causing the same problem. It could be as simple as a dirty intake system or airflow meter that needs cleaning.

But don’t forget the basics, either; a vacuum leak in any of the many rubber hoses on an engine of this age can lead to a lean mixture and a high idle as a result.

A trip to a workshop with a scanning tool should reveal more about what’s going on.

You might be looking at two distinct issues here. The high idle could be caused by any number of things including a dirty intake system, or something electronic. It could simply be that the idle was set too high the last time the vehicle was serviced.

This may be contributing to the feeling of the engine shuddering when you switch it off, but in reality, a lot of diesel engines exhibit this. It’s all to do with the heavy-duty nature of the diesel engine design. Because of the high combustion chamber pressures inherent in the diesel’s design, the whole thing needs to be more physically robust than a petrol engine of the same size and capacity. That means everything from the crankshaft to the con-rods, pistons and even the cylinder head and crankcase need to be made stronger. And that means more material and that means more mass.

This is why diesels generally don’t rev as high as petrol engines (there’s much more reciprocating mass) and why they can feel a bit lumpy when you load them up and even at idle. So, when you switch one off, it can feel like it’s trying to jump out of the engine bay. However, if this sensation has become more pronounced over the years, it might be wise to check the engine and transmission mounts for wear. Any wear or looseness here can contribute massively to vibrations from the driveline being passed through to the cabin.

The first thing to do is drive the car yourself and see if there’s a pattern to what’s happening. Until then, you have even less idea of what’s going on and a diagnosis is all but impossible at that point. You might find that the car is cutting out after a period of driving (maybe caused by a dirty fuel filter) or when the engine reaches a certain temperature (possibly a sick crank angle sensor).

I’d also recommend an electronic scan to see if any fault codes show up. These codes can be very specific and will often lead you straight to the root of the problem.