What is the most reliable indication of worn valve guides?

High oil consumption is regarded as the most reliable indication of worn valve guides because when valve guides wear out, the clearance between the guide and the valve stem increases. This increased clearance allows oil to leak past the valve stem into the combustion chamber or the intake manifold. As a result, the engine may consume more oil than normal, leading to higher oil consumption.

This symptom can be particularly noticeable if you observe blue smoke from the exhaust, which often signifies that oil is burning along with the fuel. While other factors can lead to high oil consumption, worn valve guides specifically contribute to this problem due to their direct impact on oil escape paths in the engine.

Other indications, such as low compression, low oil pressure, and excessive engine noise, can suggest various other issues within the engine but are not as definitive for identifying worn valve guides as high oil consumption is. Each of those conditions can be caused by a range of mechanical problems, which is why they are not as reliable for diagnosing this specific issue.