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Lycoming O-540 Stuck valve - high oil consumption 

To:Sent: Sunday, May 23, 2004 9:12 PM
Subject: Lycoming 540

Hi Roger,

 I just finished reading the AOPA Pilot April issue about engine compression testing done by Little Flyers.  It was very informative and leads me to the following:

 Last summer the Lycoming 0-540-J3A5D engine in my 1980 Piper Dakota suffered a stuck #2 exhaust valve and bent the push rod and broke the cam follower.  Subsequently I had the engine torn down and all six cylinders were honed and re-ringed.  The engine had approximately 900 hours since a Lycoming overhaul when the stuck valve occured.

 Prior to the exhaust valve incident, the engine used approximately 3 quarts of oil between oil changes (every 50 hours).  After I broke the new rings in, and with almost 200 hours since the engine was worked on, I now use about one quart every 4 to 5 hours. 

Would a thorough compression test, including differential pressure, direct compression and crankcase pressure testing give me an indication of why my engine now uses a much higher rate of oil? 

I've owned the Dakota since 1988 and have flown it over 2,300 hours.  I plan on keeping it for many years to come.  I'm based in Southern California and would consider flying to your shop to have the tests done if you believe it would be of value.

 I look forward to hearing from you.



Roger's reply: 

You did not mention in your e-mail that new oversized pistons were installed when the cylinders were honed.  If this is true, then you may have increased piston to cylinder wall clearances.  These increased clearances could lead to excess oil consumption if using a multigrade oil.  A single grade oil could help reduce the oil consumption. 

 We are curious to know what your break-in procedure was and the oil you used for the break-in period. 

 The Dakota is a great airplane.  Unfortunately, most of the engine installations run hot cht's.  This is partly the reason for the stuck exhaust valve.  We have found that the oil temperature tends to run hot due to the location of the oil cooler and the baffling surrounding it. Modifications can be done to help cool the engine and reduce the oil temp.  These modifications would help reduce the stuck valves from the carbon building up in the valve guide clearances.   

 Please let us know what your break-in procedures were and if you have any further questions.  

Hi Tim,

 No, oversized pistons were not installed since there was nothing wrong with the rings or pistons when the valve stuck.  Only the rings were installed.  Then the cylinders were honed.

      I was using a single grade oil before the stuck valve, but not the Shell 100 Plus.  Since the break in, I use Shell 100 Plus exclusively.  Being in Southern California, multi-grade isn't necessary.

      I utilized the break-in procedure recommended by Lycoming and the engine shop that did the work.  This is my second "engine break-in" and the first one was very successful as I mentioned since I rarely burned even 3 quarts in 50 hours.

      I try very hard to keep my oil temperatures below 234 degrees F on climbout on the hottest days.  Most of the time I keep them well under 220 on hot days and shoot for no more than 215 regularly.  Of course in the winter it's difficult to even get to 200 degrees.

      The baffling was replaced in when the engine was worked on after the stuck valve.  I've checked it and it seems to be making a very good seal.  Though I do agree that the oil cooler could be better located. 

Since I last wrote you, I now have 300 hours since the stuck valve and subsequent rebuild of the engine.  The oil comsumption has been reduced somewhat.  I still wonder if the three compression tests would give me a better indication as to whether it's a ring, a seal or a valve problem.

 I appreciate your time on this,



Hi Bernard,

    I should have written the words oversized rings and not oversized pistons.  My apologies, I was thinking turbocharged engines and not normally aspirated (new cylinders would have not excessively worn by that time).  The excess ring gap and lack of tension from standard rings may cause the engine to have excess oil consumption.

    We have been having a discussion about this.  First, if you can tell us what type of dipstick you have, we can send you a case pressure gauge and adapter.  There are three major dipstick styles Lycoming makes.  The first is the lift-cam in which you lift a lever to remove the dipstick.  The other two are screw in types; let us know the diameter if you have one of these types. 

    Basically, you install the dipstick adapter an route the hose and gauge into the cabin.  Then the case pressure gauge is read while a full power ground run is completed.  

     The case pressure gauge tells us only if the case is building up pressure.  This increased pressure can be from three independent occurrences.  The first occurrence is that the rings may not be sealing properly.  The second occurrence is that the breather may be restricted.  The third is that the crankshaft seal is not sealing and pressurizing the case in flight.  Sometimes the case pressure is negative on a Lycoming due to the rings used. 

    Typically, if the rings leak:

1.  The bottom spark plug is wet with oil

2.  The case pressure is high.

3.  The differential compression is either low at the top, in between, or bottom, of the combustion stroke. 

4.  The direct compression would be under 110 psi.

5.  You may find excess oil coming out of the breather, and/or coming from the exhaust.

    Typically, if the breather is restricted:

1.  The crankcase pressure builds with increased rpm. 

2.  The crankcase has multiple engine oil leaks.

     If the crankshaft seal leaks.

1.  Excess oil coming from the breather.

2.  When the aircraft is flown with a case pressure gauge, the case pressure will vary in climb, cruise, and descent.  We have worked on a number of airplanes (both TCM and Lycoming) with this problem.  This specific problem is very difficult to find. 

    The differential compression (the compression test normally done by most mechanics) will determine leaky valves or rings.  A low differential compression does not always mean leaking rings.  A leaky exhaust valve will produce a hummm or whine in the exhaust which is easily verified by placing a hand over the respective exhaust stack with air on the cylinder.  A ring leak may be found by placing a hand over the dipstick tube with the dipstick out (and air on).  Intake valve leaks are uncommon for most cases and are difficult to find.  Often a leaky valve can be repaired with the cylinder on.   

     The direct compression is a comparison of all cylinders and how they work together (the automotive version).  

1.  A high differential and a low direct compression (below 100) could indicate a sticking valve.

2.  A low differential (below 65) and a high direct compression could indicate a leaking valve.

3.  Both a low differential and direct compression would indicate a major cylinder problem (rings, piston, or valve). 

 It is vary rare that a valve problem would cause excess oil consumption.  Valve problems typically cause engine roughness, hick-ups, stutters, and vibration.  The hick-ups and stutters can come from intake valves sticking. 

  We don't have enough information to tell you what the leak is from.  How much oil is coming out of the breather?  Is it on the belly of the aircraft?  Is there oil coming out of the exhaust? 

 We are pretty sure your engine is running a little too hot temperature wise.  We found that the air outlet on the bottom cowl did not have enough opening to remove all the hot air the engine produces during cooling.  This draft problem reduces the air flow through the oil cooler. On the Dakota we worked on the oil temp would not cool down very good. We were able to get the oil temp below 190 degrees in cruise after a few modifications.  The oil cooler is a bit too small for it' s location as well (the further back the oil cooler is in the cowling the larger the typical oil cooler is).  However, the oil cooler still built up some heat in climbout, but would cool off once the aircraft was leveled off. 


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