Before digging into the timing belt idea, the vacuum test im referring to is different from the boost leak test you've already ran. A gauge is hooked up to a vacuum source (I use one of the balance tube fittings) while the engine is running. It gives a quick overall health check of the engine and can identify valve and timing issues...you'll need to look up a reference chart cause its kinda like interpreting a small language.
01:54:26 08/25/20
