Engine gurus, Oil usage problem M20K 231

[Steve Dawson]

I bought this plane in May of 2023. Number 1 cylinder had always had oil fouled plugs. Leak down tests showed ~ 70/80 on all cylinders however #1 had air leaking by the rings. All the other cylinders were very good. (bore scope showed good on all cylinders except #1, It showed no cross hatch and scratches on the wall). Replaced # 1 cylinder with an overhauled cylinder and after a three hour flight the plug is still foiling. And just like before, there was also oil on the belly coming from the vent tube. 

NOTE: when removing the cylinder there was oil in the intake manifold between #1 and 3 cylinders. I believe this is where the oil usage is coming from. What would cause this? 

[N201MKTurbo]

Did you replace or at least clean the piston when you changed the cylinder?

 

[N201MKTurbo]

Oil in the intake manifold must come from worn intake valve guides. The manifold has, well, manifold pressure, which in a NA engine is always less than ambient pressure, and crankcase pressure is often higher than ambient pressure. both of these could cause oil to be pushed past the valve guides.

With a 231, don't discount a bad oil check valve in the turbo. This could let the turbo fill with oil during shutdown. That oil could be blown into the engine on startup. Have someone watch when you start the engine and see if it puts out a big cloud of smoke when you start it. A bad check valve could let a 1/2 cup of oil or so go into the turbo on every engine start cycle.

[Steve Dawson]

The piston has been cleaned with the overhaul and has new rings as well. As far as a worn guide in another cylinder, it could be however the borescope shows that the valves have normal wear on them. It could still be the guide but I'm trying to put as many possibilities in a priority list first. I thought about the turbo check valve as well as a worn or bad seal in the turbo however the oil is only between the 1st and 3rd cylinder manifold.. The MP would be below ambient when idling or when pulling back during a descent as examples. As for any smoke I haven't noticed any but I haven't been able to have anyone look when it's started. 

Thanks Rich,

I was also wondering if the drain on either #1 or #3 maybe clogged. 

[201Steve]

I had terrible oil fouling problems after overhaul because the cylinders were redneck overhauled. Instead of resurfacing the cylinders with a real honing machine, they were simply scuffed up with a dingleberry brush on the end of a drill. Not all cylinder overhauls are created equal. 

[Steve Dawson]

This was done by a well known and respected shop that my mechanic suggested and who other shops used too, so I don't think, or hope they did a poor job.  

[Dialed In]

Have you done a bore scope inspection and compression test on the number one cylinder since overhaul?  What do you have for engine monitoring? How hot is the number one cylinder vs others? Have you performed an inflight mag check?  When do you find the fouled plug?  At the next run up or after the flight it runs rough?  

[Steve Dawson]

Just a quick bore scope, there is oil in the cylinder, the monitor showed normal considering it was an overhauled cylinder (within parameters and I didn't do an inflight mag check.

What would that show that would link it to an oil issue? 

[Steve Dawson]

Curious:for anyone that may know or has experience, how would a lifter issue or am over length push rod on the intake valve effect on the causing a vacuum in the intake tract? 

[Dialed In]

I'm just trying to get a more complete picture.  An inflight mag check would tell you that both ignition systems are firing under load.  A runup mag check might be good but when the pressures and temperatures are higher in flight a maginal plug may be not firing.  I'm pretty green on aircraft piston engines but I do know that when running on one plug the EGT's for that cylinder go way high.  The engine will run like this in cruise but I have heard under full power it can be detrimental.  Also if the plug is not firing it will accumulate carbon and foul.  

I would perform a compression check on the cylinder to see if there is air leaking anywhere it should not be.  Don't assume the cylinder is good just because it was just gone through.  

Turbo's don't produce positive pressure all of the time, just when the drive pressure from the exhaust is greater than the suction from the intake.  It is common to have a vacuum in the intake system at idle.  

How much is your oil usage in quarts/hr?  Is everything else satisfactory? Good power?  If the only complaint is fouling a plug it could be a bad plug, bad wire, or something wrong with the mag.  

 

[A64Pilot]

I would get some numbers as in compression check and if it’s not real good I’d call whoever did the overhaul on the cylinder, ask them.

I thought standard practice for a cylinder overhaul was a new piston?

How can you oversize a cylinder without an oversized piston?

Putting a light hone on a cylinder, fitting new rings on the old piston is I’m sorry to say a Redneck overhaul in my book. How does that restore clearances in what has to be a worn cylinder, based on honing worn off.

Maybe I’m misunderstanding the scope of work that was done.

[Shadrach]

16 hours ago, A64Pilot said:

I would get some numbers as in compression check and if it’s not real good I’d call whoever did the overhaul on the cylinder, ask them.

I thought standard practice for a cylinder overhaul was a new piston?

How can you oversize a cylinder without an oversized piston?

Putting a light hone on a cylinder, fitting new rings on the old piston is I’m sorry to say a Redneck overhaul in my book. How does that restore clearances in what has to be a worn cylinder, based on honing worn off.

Maybe I’m misunderstanding the scope of work that was done.

I don’t believe that cylinders are bored oversized on general principle at overhaul. If the barrels are cleaned to bare metal, measured and honed.

incidentally, I have seen field repaired, dingleberry honed, cylinders establish excellent oil control, compression and service life. I would still want a precision honed barrel if I were buying an OH cylinder, but I would not automatically assume that a field honed cylinder would have poor oil control.

[A64Pilot]

I’m not assuming it, he says it has excessive oil consumption

In order for a cylinder to get to excess oil consumption from wear and be saved by a light hone and new rings means almost all of the wear has to happen in the rings, which is possible but unlikely if the cylinder has enough wear for the cross hatching to be gone.

”Overhauling” anything means different things to different people. To simply put a light hone in a cylinder and fit new rings isn’t in my book an overhaul, it’s a repair.

One thing we don’t know is was the exh valve and guides replaced or not? Which should be done in my opinion to call it an overhaul.

As I have posted before all that’s required to overhaul is disassemble, clean inspect and reassemble if everything meets serviceable limits.

Then there are quality overhauls.

Cylinders aren’t black magic, they are actually simple things. I’d bet money the rings didn’t seat due to sloppy clearances and or guides worn excessively and oil is coming in through there.

The fix will be a good cylinder.

Many cylinders are “freshened up” from prop strike inspections etc, ones that were good when disassembled but the wise decision was made to put a few bucks into one and greatly extend its life, often insurence money. These are the cylinders that can get a light hone and new rings, but the ones that are worn down to a smooth bore it’s unlikely that’s going to save one.

I understand the attempt for cylinders like our angle valve Lycoming ones that aren’t available, but the Conti ones are readily available aren’t they?

I’m afraid that the money that’s been spent on this one may be wasted, unless the shop that did the work will stand behind it

[A64Pilot]

Bore polishing is what being worn down so the cross hatching is gone is called, it most often occurs from extended periods of sitting, the bore rusts when sitting, then the engine is run, the rust is cleaned off, aircraft sits, bore rusts, repeat. It often occurs in one cylinder if the engine stops with that cylinders valve(s) open repeatedly. 

Once the bore is polished it can no longer hold oil on its surface and wear proceeds rapidly, it’s not long until wear is out of limits, and vertical bore scratches are common.

I’ve seen it often in aircraft, but not cars that are stored and can’t explain why, is it because cars have long intake and exhaust systems that hold considerable volume? I’ve got a couple of cars, three actually that have been stored for years at a time, infrequently run and exhibit no bore polishing, one even sat since before WWII and when I pulled the head off of it to replace its valves, the bores were seemingly perfect, not much cross hatching to be honest, but did 1923 engines have deep cross hatching? I don’t know. If it were a motorcycle or aircraft engine it would have been seized I’m sure from rust.

There is a C-140 in the neighborhood that the owner has passed away, it hasn’t flown for a few years and I’ve been told that the engine is gone, all cylinders have rust, and it was hangared.

 

[Shadrach]

1 hour ago, A64Pilot said:

I’m not assuming it, he says it has excessive oil consumption

In order for a cylinder to get to excess oil consumption from wear and be saved by a light hone and new rings means almost all of the wear has to happen in the rings, which is possible but unlikely if the cylinder has enough wear for the cross hatching to be gone.

”Overhauling” anything means different things to different people. To simply put a light hone in a cylinder and fit new rings isn’t in my book an overhaul, it’s a repair.

One thing we don’t know is was the exh valve and guides replaced or not? Which should be done in my opinion to call it an overhaul.

As I have posted before all that’s required to overhaul is disassemble, clean inspect and reassemble if everything meets serviceable limits.

Then there are quality overhauls.

Cylinders aren’t black magic, they are actually simple things. I’d bet money the rings didn’t seat due to sloppy clearances and or guides worn excessively and oil is coming in through there.

The fix will be a good cylinder.

Many cylinders are “freshened up” from prop strike inspections etc, ones that were good when disassembled but the wise decision was made to put a few bucks into one and greatly extend its life, often insurence money. These are the cylinders that can get a light hone and new rings, but the ones that are worn down to a smooth bore it’s unlikely that’s going to save one.

I understand the attempt for cylinders like our angle valve Lycoming ones that aren’t available, but the Conti ones are readily available aren’t they?

I’m afraid that the money that’s been spent on this one may be wasted, unless the shop that did the work will stand behind it

I don’t disagree. But do recall the cylinder that was removed was 70/80 before it was replaced with an overhauled unit. Seems a bit suspect that a cylinder holding 70/80 that was then replaced with an overhauled exchange manifest the same problem. Not saying it couldn’t be he case, but I’m certainly curios about other possibilities.

[A64Pilot]

Yes I think when he said #1 had air leaking past the rings meant it was lower than 70. You can’t get much of a leak and have 70 psi on a leak down test.

I have three cyl on my C-85 that are 68 to 70 and you can’t hear any leak, one is 58 and that one you can hear the leak by listening to the crankcase vent tube for instance.

I need to pull that one and see what it needs because I’m fighting a high oil temp issue that I believe must be from blow-by. That engine was pickled and stored for three years but I think it had an open valve, anyway there is pitting on the top of the cylinder from I’m sure corrosion.

Oil consumption of course isn’t always rings it can and often is worn valve guides, turbos of course add another possibility but then I’d expect all cylinders to show oil ingestion not just one.

But the statement of no cross hatching and scratches running vertically to means the cylinder needed to come off.

[Fly Boomer]

On 12/10/2023 at 12:49 PM, Dialed In said:

Don't assume the cylinder is good just because it was just gone through.  

Best advice ever.

[Dialed In]

@Steve Dawson any headway yet?