Ask Paul: What is causing sticking valve?

Q: In 2004, I bought a Lycoming O-320 D1A, via Van’s, and started flying it in January 2005 in a RV-9A. I broke it in according to the instructions (75% or more until oil consumption stabilized). In spite of having an oil filter I always do oil changes at about 30 hour intervals. All through the life of the engine it has used 100LL and the average power (after break in) can be estimated from the fuel consumptions of 5.9 to 6.1 max gph. Cylinder head temperatures during takeoff remain below 400°F on a calibrated (boiling water) steam gauge and at about 350°F in cruise.

In April 2010, at 676 hours, I had a sticking valve. Curiously this manifested itself not at cold start-up but during takeoff.

My friend, an experienced IA, and reamed all four exhaust valve guides. The grease on the reamer showed blackish material that we thought was lead. The No. 4 cylinder, which always had the lowest exhaust gas temperatures, was a little worse than the others. Until that time, I leaned for best power. Most trips were of 90 minutes duration.

I then changed my habit and leaned maximally to slight roughness and then just enough enrichment. Still 6 gph. Just now, at 1,110 hours, I had a sticking valve again, again at takeoff after a normal cold start.

I have not been using TCP in part because proper mixing during or after filling would be difficult; the tanks have baffles with rather small holes and must be filled slowly.

I talked with the folks at Van’s, who do not have the same problems with their O320s, and who were just as puzzled as I am. The only difference we could think of is that they use more power than I do.

What can I do to prevent a repetition? I fly once a year from Oregon to Tennessee, or to Alaska, and it would be awkward to get the same problem far from home. I would really appreciate you thoughts on this.

JOB FABER

A: I find this inquiry a bit different than most and I must admit it has me scratching my head, along with you and the folks at Van’s.

My first thought in a situation like this is to think poor cooling where we’re not taking the heat away from the cylinder head when we have poor baffles or some other cause like running too lean. From the information you provided, I guess I’ll just have to use the process of elimination and hope that takes us to some place to begin.

Since the O-320-D1A is rated at 160 horsepower@2,700 rpm, I’d expect the fuel consumption to be somewhat greater than the 6 gph you stated, even at a greatly reduced power setting. I’d think you should see something like 8 to 10 gph fuel consumption at 65% (approximately 2,350 rpm) to 75% (approximately 2,450 rpm) power for a starting point. With this lower than what I’d expect fuel consumption, I’d expect to see much higher cylinder head temperatures because of a lean condition.

Have you ever compared your fuel consumption with another similar RV using the same identical engine? Maybe Van’s could furnish some fuel consumption data it has experienced from some of its flying or that other customers have provided.

Let’s get beyond this and see what else we might find. I don’t see anything with your leaning procedures, but I’m still not quite sure I like the 6 gph fuel consumption. Did Van’s have any comments regarding this fuel consumption?

The cylinder head temperatures you provided are excellent, providing the instruments are calibrated and considered accurate and the process you mentioned should meet this requirement. My only question regarding this subject is what type of CHT probe are you using? The best and most accurate is the bayonet type probe versus the spark plug type.

I am somewhat surprised that you’ve experienced this problem during takeoff rather than noticing it during initial start-up. It’s not that it couldn’t happen during takeoff, as you’ve experienced, but it’s not what we’d typically expect. May I ask how you learned the results of this problem? Did the stuck valve cause the pushrod and shroud tube to bend, causing a loss of oil, or was it detected as a rough running engine with a slight loss of power?

I commend you for having your experienced IA ream all four exhaust valve guides following your first problem because if you have one exhaust valve stick, the probability of others sticking remains high and reaming all four guides is the proper approach. The material removed from the guides certainly sounds like burned oil carbon, which we would usually expect to see from an engine that is running hot and has high CHT temperatures. Improper cooling of the cylinder head due to poor baffling or cowling fit is the usual cause of this. This is where I, and I suppose others, really begin scratching our heads as to what the cause is with your engine with the operating temperatures you’ve mentioned.

While I really don’t have any specific answers for you, I will offer some suggestions that you may want to consider. First, if you haven’t already reamed the guides since this most recent occurrence, I’d recommend that your IA comply with Lycoming Service Bulletin 388C, which tells you the condition of the valve to guide clearance. Another thing I would consider is changing the brand of engine oil. While I believe all of the aviation grade oils available today are excellent, you may want to give this some thought.

One last question for you: Has the sticking valve problem always occurred on the same cylinder? If this is the case, then I’d look at the installation and baffling. If it’s been two different cylinders, then I’m back to scratching my head again!

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