'wet' spark plug = worn valve guide?

[podair]

Hi

at my last maintenance check, the mechanic pulled out all the plugs and the #4 bottom one had a lot of oil on. He says it is a sign that the valve guide might be worn, so it might be best to think about replacing it at some point. Best course of action according to him is replacing the whole cylinder assembly, keeping the old one as a spare when we overhaul, as he had a patchy experience in trying to fix valve guides.

Engine is 15 years old, 1300 smoh, running perfectly. Compressions are all in the high 70s, egt/chts always the same. #4 does have a tendency to foul quickly, but clears easily on run up. It is the only cylinder to show signs of fouling after an extended taxi, so it does get singled out. However LOP mag test (I run peak/slightly lop always) is consistent in flight across cylinders. I am running on Tempest massives. I suggested we try fine wire, he said it will only make it worse. I did have to replace that very plug a couple years ago as it went dead. 

I am tempted to think it is just a consequence of a long taxi to the mechanic's shop and did not lean aggressively enough after landing. Everything else is looking perfect so I am not too keen to pull a cylinder just yet, but if it is only a couple AMUs for extra peace of mind, I am tempted to heed his advice. I have little mechanical knowledge,  he is a great mechanic but quite old school (not a believer in LOP etc). 

Any views? What are the possible consequences of a worn valve guide? Apart from the fouling it has had no impact in engine ops, roughness etc so far. 

1967 Mooney M20F with a IO360-A1A

 

[ArtVandelay]

Can't he do the wobble test to check for worn guides?  Search for lycoming and wobble test. I'm no mechanic, just my 2 cents

[M20F]

Aggressive leaning or lack thereof isn't going to cause oil to get on the plug.  Details on checking valve guides:

http://www.caa.si/fileadmin/user_upload/pageuploads/AD-NOTE/AD-2006/093_sb_SB388C.pdf

Do you have 1/2" or 7/8" valves?  Rebuilding the cylinder (piston, rings, valves, etc.) is reasonably cheap.  If the stem breaks you are minimum buying a new cylinder and at worst overhauling the engine.  Not saying on the basis of one instance of oil to rush to change it but I definitely would troubleshoot it quickly and if it is a valve problem get it sorted before it becomes a really expensive problem.

 

[podair]

no idea, the engine is I believe a late 1970s engine or early 80s , overhauled by a reputable shop in Sweden in 2000. 

So yes, if a a new cylinder assembly gives peace of mind for a couple grand, so be it. 

[Andy95W]

22 minutes ago, M20F said:

Do you have 1/2" or 7/8" valves? 

I think you mean 7/16" exhaust valves.  The Lycoming service instruction to replace 7/16" exhaust valves with 1/2" was issued back in the 1960's.  It is doubtful any are left in service.

[M20F]

18 minutes ago, N1395W said:

I think you mean 7/16" exhaust valves.  The Lycoming service instruction to replace 7/16" exhaust valves with 1/2" was issued back in the 1960's.  It is doubtful any are left in service.

You are correct and I had them on mine (since replaced).

[N201MKTurbo]

Valve guides are one way oil can get into your cylinder. Either way it will probably run for years like that. If it were me I would just take the cylinder to a cylinder shop for an IRAN, replace the piston and rings. It will get you to overhaul for around 1 AMU. Well that is what it would cost me, I've never paid someone to R&R a cylinder, so there is about 5 hours.

[jetdriven]

Do a leak down test and scope it. Oil more likely comes from rings.  If the oil consumption is ok you can monitor it. Otherwise pull the cylinder, hone it and install new rings.  This costs about 500$.  I've done it myself   

There are lots of threads that start with a cylinder that isn't performing as well as it should and quickly spirals from a glazed cylinder or sticky valve to a new cylinder to a complete overhaul. One of my friends went down this path because #4 was running hot and overhauled it to find it still ran hot, it was a new slick magneto. One of two which was replaced because one failed. So one mag was ok but changed any way which precipitated the hot cylinder and the mistaken belief the cam was bad. Don't throw 30 grand at a 500$ problem  

if your mechanic can't diagnose where oil is coming from and can't either re-ring it or ream the valve guide, find a new mechanic. 

[Shadrach]

1 hour ago, N201MKTurbo said:

Valve guides are one way oil can get into your cylinder. Either way it will probably run for years like that. If it were me I would just take the cylinder to a cylinder shop for an IRAN, replace the piston and rings. It will get you to overhaul for around 1 AMU. Well that is what it would cost me, I've never paid someone to R&R a cylinder, so there is about 5 hours.

why would you replace the piston after a hone and rering?

 

[Shadrach]

Your engine is about where mine is. OH'd in 2000 1150SMOH.  If I were you, I would do the wobble test before anything else.  If it passed the wobble test then I would run it for 10hrs and then recheck.

[N201MKTurbo]

14 minutes ago, Shadrach said:

why would you replace the piston after a hone and rering?

 

Rings seat just as much on the top and bottom of the individual rings as they do on the cylinder walls. As a piston wears the ring groves get tapered and the sides of the rings will not seal against the piston groves. Especially if you clean the piston. There is a certain amount of buildup of carbon and other combustion products in the ring groves that will improve the ring seal. When these are removed during cleaning the seal is lost. Last time I bought a Piston it was about $200, cheap insurance.

[Guest]

6 hours ago, M20F said:

Aggressive leaning or lack thereof isn't going to cause oil to get on the plug.  Details on checking valve guides:

http://www.caa.si/fileadmin/user_upload/pageuploads/AD-NOTE/AD-2006/093_sb_SB388C.pdf

Do you have 1/2" or 7/8" valves?  Rebuilding the cylinder (piston, rings, valves, etc.) is reasonably cheap.  If the stem breaks you are minimum buying a new cylinder and at worst overhauling the engine.  Not saying on the basis of one instance of oil to rush to change it but I definitely would troubleshoot it quickly and if it is a valve problem get it sorted before it becomes a really expensive problem.

 

7/8" valve guides might be overkill.  They used to be 7/16" solid stems, now they are 1/2" sodium filled and still break on

occasion.

Clarence

[M20F]

45 minutes ago, M20Doc said:

7/8" valve guides might be overkill.  They used to be 7/16" solid stems, now they are 1/2" sodium filled and still break on

occasion.

Clarence

Yeah my bad, fingers quicker than the brain. 

[philiplane]

That's hilarious.... fine wires will make it worse? It's the other way around...

Anyway, check the magneto before you assume anything about the cylinder condition. Especially if you have Slick mags. Slick mags are notorious for misfires that appear to be the fault of the cylinder when it's actually the plug not getting enough juice. Weak ignition will cause oil consumption on the affected cylinder because the rings will not seal due to low firing pressure. 

Also verify the condition of the spark plug harness with a load tester. A weak wire will also cause problems. Once these are ruled out, revisit the cylinder with a compression check and borescope inspection.

[Shadrach]

On November 10, 2015 at 6:02:02 PM, M20Doc said:

7/8" valve guides might be overkill.  They used to be 7/16" solid stems, now they are 1/2" sodium filled and still break on

occasion.

Clarence

When was "now", 1965?  I'm pretty sure my The IO360 in my F came from the factory in 1967 with 1/2" sodium filled valves.

[DXB]

On 11/11/2015, 10:36:17, philiplane said:

That's hilarious.... fine wires will make it worse? It's the other way around...

Anyway, check the magneto before you assume anything about the cylinder condition. Especially if you have Slick mags. Slick mags are notorious for misfires that appear to be the fault of the cylinder when it's actually the plug not getting enough juice. Weak ignition will cause oil consumption on the affected cylinder because the rings will not seal due to low firing pressure. 

Also verify the condition of the spark plug harness with a load tester. A weak wire will also cause problems. Once these are ruled out, revisit the cylinder with a compression check and borescope inspection.

This is interesting and informative. Can lead fouling secondarily lead to oil fouling?   Per the OP, he states that his #4 bottom fouls easily but is easy to clear-  presumably lead.  Then he finds oil on this one plug.  Could altered spark from lead fouling itself cause rings not to seal, then causing the oil fouling, even with if there are no issues with the mags, harness, and plugs themselves?  I ask because when I first got my plane, I noticed the same correlation between having to clear my #2 bottom at runup after taxi, and then finding oil on that one plug when they were all pulled to clean.  The issue went away when I started leaning aggressively on the ground, suggesting that lead fouling was the primary issue.  Also my recent in flight mag check analysis from Savvy looked fine, suggesting that the ignition system was not the issue.  And my oil comsumption is 10hrs/qt, suggesting I don't have a serious problem with rings leaking oil all the time.  Finally borescope inspect and wobble test on #2 were fine at prebuy, suggesting it wasn't oil leak past the valves.

[ArtVandelay]

If the 1 bottom plug gets lead fouled, then it's more likely not to be able to burn off the oil since it's not firing.

do you have GAMIs?  Maybe that cylinder is running richer than the rest?

[podair]

Exactly the same thoughts and findings as DXB. I ll just keep an eye on it for the next few hours and have it wobble and borescope checked at the next annual which is due very soon anyway.  In flight lean mag tests and oil consumptions and download from the JPM all good. No GAMIs but garmi spread is 0.25 ish so was told not needed and happily go LOP all the way to 50 LOPish but peak seems optimal (ie engine sounds happy) at my usual 8000-11000ft cruising altitudes.

[DXB]

16 hours ago, teejayevans said:

If the 1 bottom plug gets lead fouled, then it's more likely not to be able to burn off the oil since it's not firing.

do you have GAMIs?  Maybe that cylinder is running richer than the rest?

No Gamis, just a carb in my case. Oddly #2, the one likes to foul the bottom plug, runs the leanest pretty consistently in cruise.  I've seen no rhyme reason to what is leanest at idle/taxi when the fouling happens, or at takeoff power either - haven't looked that carefully, but sometimes it's clearly not #2.

[M20F]

Oil on the plug has nothing to do with leaning on the ground or in the air.  The oil may burn off if you lean for sure but that is just getting rid of it, leaning isn't the cause.