What is the most likely cause for the low compression of 30 psi in Cylinder 3 compared to higher readings in the other cylinders?

When considering the scenario of low compression in one specific cylinder—Cylinder 3—compared to the others, the most plausible explanation relates to poor valve seating.

Poor valve seating occurs when the valves do not make a proper seal with the valve seats, which can happen due to wear or damage. When this seal is compromised, the combustion chamber is not sufficiently closed off during the compression stroke, allowing compression gases to escape, resulting in significantly lower pressure readings like the 30 psi observed here. This absence of pressure translates to an inefficient combustion process and manifests as a low compression reading.

In contrast, factors such as leaking intake valves and worn piston rings can affect compression as well but typically have different symptoms or would impact more than one cylinder, especially if the issue were widespread. A leaking intake valve, while it can lead to decreased performance, may not singularly cause a severe drop in compression seen here unless it is severely damaged. Furthermore, faulty spark plugs, while they can lead to misfiring and starting issues, do not directly impact the compression themselves; they are more related to ignition problems than compression issues. Therefore, focusing on poor valve seating provides a clear and direct cause for the low compression observed in Cylinder 3 compared to the others.