Fresh(ish) Continental A-65 Oil Fouling Plugs/Oil Leaking from Airbox

[duffontap]

Hi All--new member here:

I recently purchased a 1940 Luscombe 8A with less than 100 hrs. SMOH, but the overhaul was way back in 2012(!!). The plane flies great, but we've been dealing with oil fouling and sometimes oil leaking from the airbox below the carb. after shutdown. Here are the facts:

1. It seems to be using about a quart/6 hrs.

2. The engine has Millennium cylinders and my IA suspected premature wear in the valve guides, so we pulled the cylinders for inspection. Stem-to-bore clearances are at half the wear limit or less (averaging .0026").

3. Compression test results: 75/80, 75/80, 77/80, 78/80.

4. Crankcase breather is clear.

5. Oil pressure is normal/healthy.

6. Four of the old Champion plugs failed a resistance test, so they will be replaced with Tempest UREM37BYs, which should help with the oil fouling, but won't solve the oil leak problem.

I understand that this is not an uncommon issue with small Continentals, but the oil leaking/fouling/consumption seems excessive for the condition of the engine. I'd love to hear any explanations, theories, or shared experiences other members might have. Cheers, JD

 

[jesse]

I owned an A75 (basically the same thing) for a few years, so I'll give my thoughts. Any chance you do much idling? What RPM do you idle the engine at? Why do I ask?

At idle, oil can easily get past A65 rings into the cylinder. This will foul the bottom plugs. Additionally, with a fresh strong engine, the idling may create a higher than desirable intake vacuum differential which could suck oil into the intake manifold, down the intake tubes, into the carb, and out the airbox.

The above would explain all the problems: #1 oil usage, #2 fouled plugs, #3 oil leaking from airbox

If it were my airplane, and my mechanic agreed (do whatever they say, not what I said), I'd keep flying it, keep monitoring, and would avoid excessive idling (idle at a higher RPM and hold the brakes). As the engine breaks in, the "at idle" intake vacuum differential will decrease, and some of these problems may go away.

 

[Dan Thomas]

At idle, oil can easily get past A65 rings into the cylinder. This will foul the bottom plugs. Additionally, with a fresh strong engine, the idling may create a higher than desirable intake vacuum differential which could suck oil into the intake manifold, down the intake tubes, into the carb, and out the airbox.

Now, how does oil get sucked into the intake when the airflow is into the cylinder, not out of it? The only route is via the valve guides, and their clearances are half of the service limits, so they're OK. And even then the rocker boxes would have to be full enough with oil to flood the valve stems, and that's extremely unlikely with a small continental. Drainage is down both pushrod tubes. Even if one was blocked somehow, the other would drain all the oil and then some.

I'd like to see the engine's plumbing. What is the fuel primer line connected to? Knowing the incompetence of some mechanics, I suspect it's maybe been plumbed into the oil pressure line, and small amounts of oil are getting past the primer nozzle and into the intake spider. That would also make it drip oil out the airbox after shutdown. The oil pressure in these engines is low, the primer line is 1/8" OD copper (tiny inside) and the nozzle present serious restriction. It would pass enough oil to create troubles.

Maybe there are two primer nozzles. There are two ports for them on the spider. One would be plumbed for fuel, and the other? Maybe oil. It sounds far-fetched, but I have seen stranger stuff than that.

Another, more likely possibility is the crankcase breather line plumbed into the intake, like with cars. Some owner or mechanic might have figured to eliminate the oily belly by doing that. As we might see, it is not a good idea at all. Feeding oil mist into an aircraft engine will only foul the plugs real quick. PCV systems do not work on aircraft engines because the manifold vacuum is too low (high MP) most of the time. And without a PCV valve, you risk a backfire igniting the flammable blowby gases in the crankcase. This tends to spread engine parts far and wide, and in an airplane, it would shift the CG aft a whole bunch, to put it mildly.

 

[duffontap]

Thank you for the replies. I'm a student pilot and doing transition training so we're doing a lot of work in the pattern, and that involves power-off approaches and idling along on the taxiways.

The plumbing has been checked and found correct.

I spoke with a guy at an engine shop today and he thinks the high compression (77/80 and 78/80) numbers may be inaccurate--resulting from an excess of oil in the cylinders. His money is on blow-by caused by glazed cylinders, caused by improper break-in procedures. Cylinders are going in for an inspection, and likely a re-honing and new rings.

 

[Jdm]

Good job checking resistance. Btw, Champion seems to have addressed the problem of high resistance with their newer plugs.

Back to the oil issue: What about the scraper rings? I don’t know any specifics about this engine model but could a bad scraper ring cause such an issue?
Also, have you done a borescope on the cylinders? Might find oil pooling or something else that could help narrow it down.

 

[Jdm]

His money is on blow-by caused by glazed cylinders, caused by improper break-in procedures. Cylinders are going in for an inspection, and likely a re-honing and new rings.

Then maybe ask you mechanic what he thinks about switching to mineral oil and running the mess out of it for 20 hours. Might be surprised at the results

 

[jesse]

Now, how does oil get sucked into the intake when the airflow is into the cylinder, not out of it? The only route is via the valve guides, and their clearances are half of the service limits, so they're OK. And even then the rocker boxes would have to be full enough with oil to flood the valve stems, and that's extremely unlikely with a small continental. Drainage is down both pushrod tubes. Even if one was blocked somehow, the other would drain all the oil and then some.

If a cylinder is on an intake stroke, then yes, air will be flowing INTO that cylinder...but, ONLY that cylinder, there are three others. Those cylinders will be somewhere else in their lifecycle, not generating intake vacuum, and instead could be at a higher pressure than the intake pressure, resulting in oil through the valve guides. This is amplified by excessive idling at too low of an RPM.

Here's at least one other person that thinks this is plausible, I suspect you'll find others with some Googling...
http://johnpropst.yolasite.com/resources/Oil out the Airbox reva.pdf

 

[jesse]

Thank you for the replies. I'm a student pilot and doing transition training so we're doing a lot of work in the pattern, and that involves power-off approaches and idling along on the taxiways.

The plumbing has been checked and found correct.

I spoke with a guy at an engine shop today and he thinks the high compression (77/80 and 78/80) numbers may be inaccurate--resulting from an excess of oil in the cylinders. His money is on blow-by caused by glazed cylinders, caused by improper break-in procedures. Cylinders are going in for an inspection, and likely a re-honing and new rings.

Is this advice from a shop that specializes in overhauling engines? Hint: You are getting a sales pitch, not honest qualified advice. This guy hasn't even seen your engine and convinced you that your mechanic messed up the compression checks?

Anyhow...It seems very dramatic to yank the engine apart based on what you've described so far. Does your day-to-day mechanic who has been involved with your ownership from day one feel this is what needs to be done?

Your A65 is not your modern car engine and will always have oil usage. 0.16 quarts per hour of oil usage is pretty damn typical for a Continental A65. If you rebuild it, you might find yourself using about the same quantity of oil. Hell, maybe more...

Take a look at this thread (lots of people with the same or similar engines to you talking about their usage):
https://www.supercub.org/forum/showthread.php?49248-Oil-Consumption-vs-Length-of-Flight

You'll notice a theme of people talking about how their oil usage goes up if they do a lot of pattern work (lots of idling) to about what your usage is. Then you'll notice how they talk about how it goes down when they do cross-countries (less idling).

 

[Dan Thomas]

If a cylinder is on an intake stroke, then yes, air will be flowing INTO that cylinder...but, ONLY that cylinder, there are three others. Those cylinders will be somewhere else in their lifecycle, not generating intake vacuum, and instead could be at a higher pressure than the intake pressure,

The intake system is a unit and is all at the same manifold pressure.

Here's at least one other person that thinks this is plausible, I suspect you'll find others with some Googling...
http://johnpropst.yolasite.com/resources/Oil out the Airbox reva.pdf

If it's leaking oil out the airbox right after a flight, this does not explain it. The rush of air through the induction system does not allow such drainage.

I have lots of hours in three A-65s, including in my own airplane, and never had airbox oiling. I am also a retired commercial pilot and flight instructor and aircraft maintenance engineer. I taught Aircraft Systems in a college-based flight training program, and was Director of Aircraft Maintenance there.

One other possibility for an oily airbox is a leaking crankcase dripping oil onto the intake spider, where it runs down onto the carb and airbox and can find its way inside those leaky airboxes.

Your A65 is not your modern car engine and will always have oil usage. 0.16 quarts per hour of oil usage is pretty damn typical for a Continental A65.

0.16 quarts per hour is about six hours per quart. Eight quarts per 50-hour oil change? That's terrible. I never went through that much oil in ANY engine except the ancient Gipsy Major in the Auster I towed gliders with in the 1970s. Inverted inline engines are famous for oil consumption, and some of it was just plain leakage. It could go through up to a quart an hour, and it had a twelve-quart oil tank to allow for that. It also had almost 700 hours on it, nearly worn out for those old engines. When I tore it down I found the rings and cylinders worn right out. Big clearances. There were no air cleaners of any sort on those engines.

The OP mentions his mechanic saying that the cylinders might be glazed from improper break-in. I have flown many new engines on their initial break-in flights, and they have to be run hard for a couple of hours, keeping the temps within limits, to get the rings and cylinders mated properly. Babying a new engine is a sure way to make it use more oil, but it will also have lower compression.

 

[Jeff767]

Don’t pull those cylinders because you think the might be glazed. Borescope it and see if they are glazed! Your mechanic is recommending open heart surgery when a simple cheap test can determine if needed.

 

[Dana]

Could also be a broken or gunked up oil ring. A solvent flush might fix it.

 

[Dan Thomas]

Lycoming's break-in instructions for field-overhauled engines are here, and are useful for most engines:
https://www.lycoming.com/sites/default/files/attachments/Lycoming%20Reciprocating%20engine%20Break-In%20and%20Oil%20Consumption.pdf

An excerpt:



Now, for an O-320 in a 172, 65 and 75% power must be determined from the cruise charts in the POH:



So to reach 75% at 4000 feet ASL on a standard day, I had to use 2600 RPM. That's nearly full throttle. Fixed-pitch props are pitched to get redline at full throttle in level flight. 65% required 2450 or so. An awful lot of pilots are afraid of RPMs like that. Full throttle for the 30-minute period got me the 2700, but only about 82% power due to the altitude. I'd have to be at sea level to get that, or have a MUCH colder day to do the test flight, to get the DA down to zero.

That 172 sure boogied at 2700 RPM. 120 knots, nearly 140 MPH.

 

[jesse]

The intake system is a unit and is all at the same manifold pressure.

Sigh...Yes...The intake system is all at the same manifold pressure, but that does not mean all the valve guides are at the same pressure as their parent cylinder, because the cylinders do not all maintain the same pressure. If one cylinder is sucking, the guides of other cylinders will be exposed to that suction.

I have lots of hours in three A-65s, including in my own airplane, and never had airbox oiling. I am also a retired commercial pilot and flight instructor and aircraft maintenance engineer. I taught Aircraft Systems in a college-based flight training program, and was Director of Aircraft Maintenance there.

So what? I am a bunch of things too. Notice how I don't feel a need to throw those out randomly in a discussion...