Practical Sailorvideo
How To Test a Boat Engine
July 13, 2024 · 10 min · 17K views
What this video covers
A compression test on any marine engine — diesel inboard, gasoline Atomic 4, outboard, or generator — costs about $50 in equipment and reveals cylinder health before a costly repair or boat purchase.1:02 Philip Decker, writing for Practical Sailor, explains the procedure and documents it on his own Westerbeke 42B4 diesel, where cylinders 1–3 read roughly 400 psi and cylinder 4 read zero.3:59 That zero led directly to a broken exhaust valve confirmed when the cylinder head came off; the symptom was white smoke from unburnt diesel escaping through the exhaust.6:40 Prior steps — valve adjustment and replacement of all fuel injectors — had not resolved the issue.5:44 The full repair included all valves and valve guides, the head gasket, exhaust manifold gasket, and air inlet manifold gasket.5:59 Normal diesel compression runs 275–400 psi; gasoline engines typically read 125–175 psi; cylinders should stay within 20% of each other.3:45 Tim then demonstrates the test on a two-cylinder 9.9 four-stroke Mercury outboard with hundreds of hours on it, using a $50 Amazon compression tester kit.7:51 Cylinder one reads 158–160 psi; cylinder two reads 150 psi — within 10% variance, indicating a healthy engine.9:34 A zero reading in one cylinder, flanked by normal readings, points to a valve or head gasket problem rather than worn rings, which matters because ring wear requires pulling the entire engine.6:31
More from Practical Sailor
From Practical Sailor — see the channel’s boat, crew, and every analyzed video.
The Sailing Directory Newsletter
One email with the fleet's best finds — the moments, boats, and stories worth your watch time. Enter your own address to get your own copy.