Why does my 1994 Mazda 929’s engine intermittently rev or surge?

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.