Hi Guys,

I am new to the forum and hope I can get some ideas from owners and A&P

I just started flying(2.7 hours) after a top overhaul on a Lycoming IO360, during the overhaul I replaced with 4 new pistons, new rings all around, new rockers. One new intake valve and all other valves were checked and found within the Lycoming tolerances. However, no valve guides were replaced as the shop claimed they are ok.

Earlier today I took the manifold off for closer look after I found oil on the bottom of the aircraft, which dripped from the throttle body. See pictures attached


Anyone had this before and can provide additional information about this? Would I be correct to assume that valve guides can result in this?

Any ideas are welcome


Many thanks!

Are you sure it’s oil and not some soupy old avgas? It looks pretty clean for oil. Do all the intake pipes look like that or just one? If it’s just one you know what cylinder it’s coming from. I doubt it’s broken in yet with that low of of time since overhaul so it could certainly be some blow by but that sure is much cleaner then I’m use to seeing.
If it was a worn guide I don’t think you would see pooling in the intake. Even with super wore out rings you get blow by but not leaking into the intake and leaking out. You could have a cracked intake tube inside the oil sump. Was it running poorly and that’s why your investigating?

PAMR MX wrote:Are you sure it’s oil and not some soupy old avgas? It looks pretty clean for oil. Do all the intake pipes look like that or just one? If it’s just one you know what cylinder it’s coming from. I doubt it’s broken in yet with that low of of time since overhaul so it could certainly be some blow by but that sure is much cleaner then I’m used to seeing.
If it was a worn guide I don’t think you would see pooling in the intake. Even with super wore out rings you get blow by but not leaking into the intake and leaking out. You could have a cracked intake tube inside the oil sump. Was it running poorly and that’s why your investigating?

Hi PAMR,

Thanks for your time replying to me.

I got the local A&P to check it as well to confirm its indeed a mixture of oil and fuel, based on his comments I am here seeking additional information to confirm his suggestions.

Having no real experience or exposure to it, I am not sure how much fuel/oil is normal in the induction system of an injected Lycoming. The reason I took a look at the induction was due to the puddle I found on the ground 18 hours after I last flew the aircraft.

Otherwise, the engine runs beautifully..!

If this fuel injected engine is on a high wing airplane with fuel up in the wings you may be getting a small amount of fuel flow through your throttle body, up to the spiders and down the injector lines into the intakes when the airplane is parked. This would also explain the puddle under the airplane, it will drain out the sniffle valve once it gets into the intake.

I would confirm your mixture control is going all the way to the stop, and make it SOP to turn the fuel selector to off when parked.

PilotPeat wrote:If this fuel injected engine is on a high wing airplane with fuel up in the wings you may be getting a small amount of fuel flow through your throttle body, up to the spiders and down the injector lines into the intakes when the airplane is parked. This would also explain the puddle under the airplane, it will drain out the sniffle valve once it gets into the intake.

I would confirm your mixture control is going all the way to the stop, and make it SOP to turn the fuel selector to off when parked.

Thanks PilotPeat,

The aircraft is a small Pitts S1, while I am not sure how much pressure still exist after engine stop, I did close the fuel selector and will report tomorrow.

In the meantime, I opened again the throttle body for a good clean, full of oil, unfortunately, which I don't believe is normal - see picture below

Have you pulled any of the intake tubes to see if it is coming from a specific cylinder, as opposed to a cracked sump?

This article talks about valve guides pulling oil into the intake after a fresh overhaul.

http://johnpropst.yolasite.com/resource ... 20reva.pdf

What about a plugged cylinder head oil drain tube?
If the valve cover were to fill over the valve stem height it could drain past a valve stem.

Long shot but simple enough to check