leak down test results

We have a 78 trans am with a 79 Olds 403 engine. We are burning through oil fairly quickly so we did compression test. All 8 cylinders showed 150+ psi. Did leak down test and all 8 cylinders lose 20-25% but according to the gauge setting it is still in the green section. Since we are going through so much oil (1qt/200 miles) it seems 20% is still too much and we have a problem with the rings. Would it be normal for all the rings to fail at this rate?

Each cylinder produced a very small amount of air coming from the oil filler tube only. Barely enough to move a tissue placed over the opening

The "history" of the engine could be important here. Has it "sat" for an extended period of time?

Compression rings can still be okay, while the oil control ring "gives up". If the tension on the rails has "relaxed", the engine can and will "use" oil, even if compression and leakage appear okay. 25% is a bit much, though.

Jim

The engine did sit for a long time, 5-years while someone was planning a restoration that never happened. Upon further investigation we can see that the top of several pistions are wet, potentially oil.

Does it show any smoke? If so, more under throttle or more under deccel?

The pistons being "wet" can mean valve stem seals, as well. Olds uses the plastic "umbrella" type that rides up and down with the valve. If they're delapidated, oil runs straight down the guide into the chamber. Olds guides are factory "knurled", too, creating a straight path for the oil to follow.

Any "build up" on th eplugs? If only "some" on the side, unevenly dispersed, would indicate a valve guide. Even coating all around the plug would indicate rings.

Jim

Yikes 25%, last engine I did was an 331 Hemi for a classic car, leaked 18% used a qt every 300 miles, ring end gap was .075!!, absolutely no wall tension left.

Randomly smokes only on start up. Nothing on hard accelleration or just cruising. There is oil on some of the plugs evenly across the tip. We changed the valve stem seals along with all springs. Sounds like we're in for a complete rebuild or new engine unless you can think of anything else.

25% is internal assuming no valves are leaking.

The "leak down" test is required buy FAA for a 100 hour inspection on all piston-engine aircraft. They will not accept a "compression test". If the engine "leaks" more than 10%, FAA requires a rebuild. Realize, of course, FAA regs are MUCH more stringent than DOT (there are no "rules" for street cars and cylinder leakage) because of the safety issues involved with an aircraft. We don't like anything past 15% unless it's brand new and the rings haven't "seated".

The smoke on start-up could be the valve guides. As said, Olds guides are 'knurled" from the factory. The factory valve stem seals can be a real "adventure" to properly install. It COULD be running down the guides...

A complete rebuild would be the most prudent course of action.

Jim