Ed Weaver:
We have a 65 A23 with the I 346A Continental. The plane developed an oil leak after the last oil change. I had the plane in the shop this week for other things and asked for this to be checked. The oil is leaking from a seal on the intake on the left side of the plane. This is a problem because, obviously, there should be no oil in the intake.
I asked the A&P to do a compression test on the #1 and #3 cylinders on that side. The compression test was good – in the 70’s. He says that he could hear air leaking from the muffler though, indicating a leakage on the exhaust valves. HMMM….
The only thing that has changed is that I went to Aeroshell 100 on the oil change, due to the plane being based in FLA, I thought that would be sufficient. It had been running the 15W/50. I have put about 15 hours on since this oil change and am going through a quart every 3 hours.
This AP is not the same that did the pre-buy or the Annual. Neither of them indicated an exhaust valve leak. That was OCT 2004, and 65 hours ago. The next step is to get an opinion from Tropic Airpower on Monday. Any thoughts or advice is welcome. The engine is 680 hrs SMOH.
Technical Editor:
Continental exhaust valve leakage is legendary. It often waxes and wanes; no decisions should be made based on a single compression test. Use your engine serial number to enroll in Topcare on the TCM website, and read everything available for your engine, including compression and valve leakage limits. Having said this, if you have confirmed repeated leakage through an exhaust valve, I’m not a fan of continuing to fly it that way regardless of Continental’s advisories. If a valve head fractures or separates, due to localized overheating at a burned edge, things can get pretty nasty pretty fast.
Oil leakage from intake runner flange gaskets is commonplace. It most often starts because people use the intake (and exhaust) tubes as grab bars while they are working around the plane. Once the flange rocks on the gasket, a leak can begin. Tightening the bolts or nuts won’t fix it. If it bothers you, or if it is actually leaning the idle mixture of that cylinder, the gasket has to be replaced. It is normal for oil to be in the intake ports near the valves, where it can find its way out through the flange gasket. The low manifold pressure at idle and part throttle (which we call “vacuum” in a car) tends to suck oil down through the guide (from the rocker box), and out into the intake port around the intake stem. Normally it gets washed away as it forms, by the new fuel being introduced. Some of it finds its way to the flange gasket, and out (if the gasket has been crushed or dimpled), especially just after shutdown.. The exhaust valves seldom have this problem because the exhaust gas pressure pushes the oil in the guides back toward the rockerarms. The oil tends to coke up in the exhaust guides instead, for this reason combined with heat; hence the “sticking valve” issue. Bob Steward’s findings last year highlighted why this has tended to happen on some parallel-valve engines, but not on others.
Unless you are literally finding puddles (as opposed to drips) under your cowl, the oil leakage is unlikely to be a significant factor in the oil consumption numbers. Most oil usage comes from it being burned… which is largely normal in our engines (as opposed to auto engines). Many a person has thrown hundreds (or even thousands) of dollars at oil leaks, and achieved a “dry engine”, only to find that their oil usage didn’t change by more than a half-hour per quart.