How to tell if broken timing belt damage engine?

Determining if a broken timing belt has damaged the engine depends on the type of engine in your vehicle – interference or non-interference:

1. Interference Engines: In interference engines, the valves extend into areas that the pistons travel into. If the timing belt breaks, the pistons may collide with the open valves, causing potentially serious engine damage such as bent valves, damaged pistons, and even damage to the cylinder head or engine block.
2. Non-Interference Engines: Non-interference engines are designed so that the pistons and valves do not occupy the same space. If the timing belt breaks in a non-interference engine, it will stop the engine but is less likely to cause extensive engine damage.

To check for damage after a timing belt failure:

• Visual Inspection: If the timing belt has snapped, you could begin by visually inspecting any visible parts of the valve train and the pistons, if possible. A borescope inserted into the spark plug holes can give you a look at the top of the pistons for any signs of impact from the valves.
• Compression Test: Perform a compression test on the engine. A lack of compression in one or more cylinders could indicate bent valves. This type of test involves removing the spark plugs, inserting a compression gauge into the spark plug hole, and cranking the engine to measure the compression each cylinder can build.
• Leak-Down Test: A cylinder leak-down test can help confirm if there are issues with the valves sealing properly. This test involves injecting air into the cylinder at the top dead center (where the valves should be closed) and measuring how much air is retained in the cylinder.
• Listen for Noises: If the engine still turns over, listening for abnormal sounds when cranking the engine can also be a preliminary indicator. However, without the belt, the engine will not run, so this test is limited.
• Mechanical Diagnosis: A mechanic can partially dismantle the engine to inspect the valves, pistons, and cylinder head. This method is the most direct and accurate way to assess damage.

If there is suspicion of engine damage due to a broken timing belt, a thorough inspection by a professional mechanic should be performed. This inspection will determine the extent of the damage and the potential repairs needed. In some cases, the cost of repairs can exceed the value of the vehicle, especially if significant engine damage has occurred.