During a leak down test on a 4 cylinder 20lbs of air was applied to no3 cylinder air came out of no 1 plug hole unknown if either cylinders were at TDC at the time how could this have happened?

Answer 1

Well, there are several possibilities. The most likely is that the head gasket is leaking badly, allowing pressure to vent between cylinders. Another possibility is that the head is cracked, which is how the pressure is traveling from one cylinder to another. The least likely possibility is that the block is cracked, but it would have to be cracked severely for pressure to travel from #3 to #1. Definitely you will have to remove the head, at least to replace the head gasket.

Answer 2

If air from cylinder 3 came out of cylinder 1 plug hole during a leak down test, it suggests a leak between the two cylinders, likely through an open intake or exhaust valve. This could occur if both cylinders were close to bottom dead center (BDC) where the valves are both partially open, allowing air to flow between them. It's important to ensure each cylinder is at top dead center (TDC) to get accurate results during a leak down test.