NOCTI Diesel Technology Practice Test

When a cylinder shows low compression and you can hear air escaping from the dipstick tube, that’s a sign of blow-by—the combustion gases are leaking past worn or damaged piston rings into the crankcase. Those gases then escape out through the dipstick area, which is why you hear air there. This situation points to broken or worn piston rings as the cause of the low compression. Piston scuffing can reduce compression, but it doesn’t typically produce air blowing out of the dipstick. A worn valve stem leaks through the valve seat and port rather than into the crankcase. A blown head gasket can cause low compression as well, but it usually presents with other symptoms (coolant loss, oil contamination, white smoke) and wouldn’t specifically cause air to come out of the dipstick.