Check security of #2 intake runner IO-360 - Leaks

[garytex]

This M20F is new to me, I have 10.5 hrs in it, and a new complex high performance signoff.  It and had an occasional rough #2 left mag.  My mechanic pulled the plugs, all of which have huge gaps, and obviously haven't been gapped since the overhaul.  Oops!, #2 bottom is oily.  Logs show #2 has been a problematic cylinder since overhaul 800 hrs ago.  Within 200 hrs of overhaul, 1 or two at a time, the overhauled cylinders from J & J in Pleasanton, TX have been replaced with ECI IFR (their brand name for a pressure annealing process) head rebuilds on new through hardened steel barrels.  #2 has been replaced or overhauled (can't tell wether it has been the same jug or exchange from the logs) twice.  Borescope shows #2 has a slight shiny spot on the cyl. wall, and has shown highest compression of all the cylinders for several years before and after replacement.  My mechanic, Jerry Stanton at Hill Country Aviation thinks it has been pumping oil (which sealed the compression ring and yielded the higher compressions) for a long time.  Off comes the jug, lots of carbon on the piston sides and rings, and "something doesn't look right"  I'm thinking that when we measure it we'll find no choke, and that's why it has been pumping oil since the word go.  Jerry is a good, smart mechanic, and a great diagnostician in his own right, but he also knows when to seek advise.  He calls Mark at Custom Airmotive in Tulsa, who is an overhauler both Jerry and I think a lot of.  He says "#2 is a stinker on that engine and shows the most problems of any of the jugs."  (oddly enough, as an aside, Aviation Consumer did a statistical analysis of cylinder Service Difficulty Reports years ago, and found that #2 was a statisticaly significant problem child on both Lycoming and Continental engines.  This is wierd, as #2 is on the front of a Lycoming and usually runs cold, and on the back of a Continental, and runs hot).  "Look at the seal surfaces on the intake runner.  It is the longest in the system, and prone to harmonic vibration.  It then leaks and then the cylinder leans out and runs hot.  I also frequently see case cracks around the base of #2 "  We look, sure enough the intake sealing surface gasket shows uneven contact around the circumference, narrowing to about 1/32nd on one side.  We had also noticed some oil on the case there too, and I'm thinking "oh no, please, no crack" (which is the opposite of what Washington D. C. mayors think, I guess).  I had noticed #2 running occasionally hotter EGT, but chalked it up to only one plug firing and a slower flame front still burning in the exhaust manifold.  The previous owner typically ran 50 ROP, and 75%, so a lean cylinder might have been really  toasty.  The cylinder is off to Mark for a good measuring, to see if it is serviceable.  I'd say that it pays to replace those intake tube gaskets if a jug is off, and not reuse the same one over and over.  

Here is the take away.  According to Mark, you can feel slop in the runner when it has vibrated and compressed the runner gasket loose if you shake it.  (of course, more than 3 times is a sin according to Father Murphy).  I have no "feel" to tell you about, as the jug was off before we heard about this, but maybe it is worth a little shake, next time the cowling is off.

Gary

[jetdriven]

Good info.  Just to plug the practice, running LOP will not only tell you if you have an induction leak before ruining a cylinder, but will also keep the piston rings from fouling up with carbon and then wearing out the bore (rings stick and refuse to rotate).  A leaner cylinder (one with an induction leak) will run also cooler while LOP.  Anything larger than a very small leak, the engine will refuse to run smooth LOP which is a cause to investigate.  A dirty injector will also not allow it to run smooth LOP.  So will a bad harness lead, or a bad plug that still bomb tests good.   Anyways, it can tell you a lot.  Now I am going ot go check my intake seals.

Our #2 runs warmest and is about the same as #4. 

[garytex]

The last time i flew it there was no other indication besides the higher egt, and I was at peak and leaner.  Must have had some 1 plug firing effect going on.  I also wasn't paying the best of attention, either, with a recalcitrant GPS, encoder intermittant, really noisy radios from a condenser failing in the right mag and the 11,500 ft stupids going on. Well we'll see how she flys after all the fixing. 

[Swingin]

When I had about 30 hrs on my M20E, I developed a rough idle that would clear out at higher power settings.  Difficulty starting sometimes as well.  Finally I had enough when the vibration during a reduced-power descent became pronounced.  Culprit was that the #2 intake gasket had rotted out and the ensuing vibration caused one of the bolts to back completely off, and I had a massive intake leak as you can imagine.  Interesting to know that it's more widespread.

[garytex]

Another F (IO - 360 A1A) came in to Hill Country Avaition within the last couple of days for annual.  Rough engine throttled back was also a squawk.  Except for one jug that had been recently worked on, all intake tubes were leaking.

[jetdriven]

You would think with an engine monitor you could tell if they were leaking.

[garytex]

Mine showed up as a leaner cylinder.

Another odd thing, Jerry says the flange itself is worn almost off on one side on my #2 tube.

[Swingin]

Arriving for a fuel stop on an XC today, pulled the power back to get to the pattern and it started running rough.  #1 EGT was near zero, but worked fine at higher power settings.  Pulled the cowl and the #1 intake tube was slightly askew with one bolt missing.  Thought it was gone for sure out the cowl flaps, but it was at the bottom of the cowl.  Found a mechanic on field to put it back in and check the security of the other 7.  Many were loose.  He said vibration was a likely culprit, so I'll be balancing my prop soon.

[garytex]

Swingin,  based on the amount of wear I saw on my intake tube flanges, it might pay to take a look at your next chance.  One of mine was worn almost completely off on one side of the flange from working in its socket.  No fix, replace.

In fiddling with one of the leakers today noticed that it can leak without telltale looseness in runner.  So a secure (non shaking by way of hand, but harmonically vibrating) intake tube can still be a leaker.

So if you are a hangar tinkerer how do you check? 

I'm not all that excited about blowing my shop vac dirt backwards thru the intake manifold, which is the old school way to pressurize the intake with high volume low pressure air for the soapy water bubble test.  I didn't think to ask Jerry what he used.  Maybe compressor air.  

I wonder if there might be enough valve overlap to let the relatively lower volume compressor air escape out the exhaust on these engines?  If so, one could probably rotate the prop a little and get everything closed.  Mags and mixture off please.   The 6 cyl continentals sure sound like it at idle.  Booo bowlbow bo-bo bo-bo  Reminds me of a long duration auto cam from the 70's.  The amazing thing is that current technology coughs up smaller engines that make more power, and purr like kittens at idle.  The down side is that a farm boy can't work on them any more.  Thats what I like about aircraft engines, they are still simple and based on the technology I learned as a boy and can understand.  But the other side of the coin is that we are giving up the efficiencies (and the maybe the inscrutable failure modes) of the new technology.