Mazda Sedan Range

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.

A car like this shouldn’t frighten too many mechanics, largely because this is a fairly simple machine using well-understood technologies. In a situation like this one, most workshops will go back to first principles. That is: Air, fuel and spark.

On the surface, it sounds like a fuelling problem that is causing the engine to run out of fuel when it needs it most (when accelerating). So a check of the fuel filters, the fuel pump delivery pressure and volume and the injectors would be a good place to start. However, stumbling under load can also be caused by poor ignition, so the spark plugs, leads and coil(s) and charging system should also be checked out. Don’t forget things like a dirty air filter which also cause a case of the stumbles.

If all that checks out, then you need to start looking for a dud sensor somewhere in the engine bay. An electronic scan is a must for any electronically-injected car.

Most mechanics will do a complete check up and tune of the engine as part of tracking this problem and it’s a good start, because a new set of spark plugs and plug leads might just fix this problem. But that won’t always be the case. As such, you could be looking at a dud sensor somewhere in the engine bay that is telling the on-board computer to shut things down or, at least, not providing the computer with enough information to keep it all running. For this reason, an electronic scan of the vehicle should also be carried out at the same time to see what fault codes are cropping up.

A prime suspect for this behaviour would be what’s called the engine’s stepper motor which is a small electric motor that controls the throttle body and, therefore, the car’s idle speed. If this motor is maladjusted or faulty, it could cause the engine to either idle roughly, too slowly, or stall altogether. But have the scan done before rushing out to spend money on parts that may or may not be the problem.