Final answer:
An intermittent vacuum gauge reading during gasoline engine testing is mainly caused by a loose or leaking intake manifold gasket, as it can lead to fluctuating vacuum pressure changes with engine operation.
Step-by-step explanation:
When testing a gasoline engine, an intermittent vacuum gauge reading can be caused by a variety of issues, including a burnt valve, worn piston rings, a loose or leaking intake manifold gasket, and worn valve guides. However, among these options, the scenario that typically leads to intermittent vacuum gauge readings is a loose or leaking intake manifold gasket. This type of leak can cause fluctuations in the vacuum reading as the gasket's seal changes with the engine's operation, leading to sporadic losses of vacuum pressure.
Explanation of Options
- A burnt valve would generally cause a steady low vacuum reading because the leak of compression is consistent.
- Worn piston rings would contribute to poor compression, also giving a steady low reading of the vacuum gauge, rather than an intermittent one.
- Worn valve guides would often result in oil consumption and may cause a steady drop in vacuum but not necessarily an intermittent reading.
Thus, the correct answer to what would cause an intermittent vacuum gauge reading when testing a gasoline engine is c. A loose or leaking intake manifold gasket.