Continental C-90 question...

[cargoflyr69]

I have a C-90 that is attached to an Aeronca Champ. In the last 15 hours or so the oil consumption has shot up significantly. it is around a quart every 4-5 hours. previously it only sipped oil maybe a quart per 20-25 hours.

I changed the oil myself and within 3 hours it had gone to from honey fresh color to chocolate syrup color. not good i think. I had a compression check done and all 4 are in the mid 70's except that on #1 you can hear air escaping past the rings at the bottom of the compression stroke. the other 3 dont do this.

My mechanic thinks that my oil problems have to do with a bad crankcase prop seal, and a bad #1 cylinder. he wants to replace both. he says the good news is I can get a new cylinder assembly for around $650

My question is how the prop seal has anything to do with the problem, and if the whole clyinder is really bad and not just needing new rings. I've only owned the plane for 200 hours, but the engine was toppped 600 hours ago and that cylinder was replaced then. so, i should think it should be fine at 600 hours and maybe just needing a ring job vs. all new hardware. Am I wrong?

thanks for any constructive thoughts or discussion! I want to get my motor running right for the most value of my dollar and i dont want to have to throw more money at it in another 200 hours.

 

[avbug]

Cargoflyer69,

Doesn't sound to me like enough information is given, yet. Is your propeller shaft seal leaking? Are you finding evidence of oil in your exhaust and exhaust stains on the airplane? What do your spark plugs look like? Are you finding evidence of oil at the breather?

Oil color doesn't mean anything.

Are you running spectrometric oil analysis with every oil change? You should be. If you haven't been, you absolutely need to start. It will cost you a few bucks, and may save you many.

When you say you can hear air escaping through the rings, how do you know...where are you hearing this? If you're listening through the oil cap or breather, are you seeing evidence of oil loss in these places? Is the engine dry, yet you're still seeing the oil loss? How close to full has the engine been when losing the oil?

Was your compression test done hot or cold, and how many times? Were you using calibrated equipment? Did the equipment use the right orfice?

Replacing rings...you're making a lot of assumption there; namely that the ring are at fault. Throwing rings at an unknown problem may fix nothing, may make the problem worse, or you may get lucky and tackle it. So far as ring condition, you're not going to know what you've got until you get the jug off.

Where has the oil been going? Has your engine been smoking? Are you seeing it in the exhaust stack or plugs for the #1 cylinder?

More information is needed.

 

[cargoflyr69]

Sorry, indeed more info is needed.

The engine is dry. I solvent-washed the engine after the last oil change and there are no visible leaks, including the prop seal and breather tube. Filler cap is secure. I like to run with 3 qts of 15w50 in the kidney tank, anything over 3.5 seems to blow by. The plugs on 2,3,and 4 are looking fine, just ashy-grey matter. on #1 the top plug is ashy and bottom is slighly oil stained, but not shiny. Mag check is fine, so it doesnt seem oil-fouled. both stacks are ashy color, but on the struts and belly there is some darker (oil?) stains that have appeared in the last 10 hours. The compression test was done warm and cold. I flew it around the pattern first then we did the check and went to lunch and did it again when we got back an hour later. It was the cold check that we heard air at the bottom of the stroke. I dont remember if the oil cap was off at the time, but you could hear a slight "hiss" this was using compressed air through the spark plug hole "orifice." bear in mind this is the only clyinder that did this.

I agree that the only way to know about the rings is to pull the jug, and that is the plan this weekend. I just want to sound a little more educated and know what the possible problems are here and what the outcome may be.

Still questioning is the prop seal. if it is bad, wouldn't you see oil leaking from it?

 

[avbug]

The engine is dry. I solvent-washed the engine after the last oil change and there are no visible leaks, including the prop seal and breather tube. Filler cap is secure.

Very good. This is important information...it's not leaking out, it's going through the exhaust. If the crank seal isn't leaking, I'm not sure why the drive to replace it. it's not like swapping a jug provides an opportunity that would make it any advantage. If you've washed the engine and it stays dry...you've made an important insight.

I like to run with 3 qts of 15w50 in the kidney tank, anything over 3.5 seems to blow by.

Also very good. You're right...one low is usually a good idea. The problem here is that if you're getting excess blowby due to worn rings, you're going to see a pressurized case, and ought to be seeing oil on the outside of the engine ...often around the oil cap. You might be looking at a damaged oil control ring...but that oil could be coming from some place else. Valve guides spring to mind.

If you're running it at three, what is your yardstick for determining when to add oil?

The plugs on 2,3,and 4 are looking fine, just ashy-grey matter. on #1 the top plug is ashy and bottom is slighly oil stained, but not shiny. Mag check is fine, so it doesnt seem oil-fouled.

Also good information. Your engine is running well, the oil isn't fowling plugs yet, but you are noticing a difference on the lower #1 plug. Based on what you've said, that sounds right.

both stacks are ashy color, but on the struts and belly there is some darker (oil?) stains that have appeared in the last 10 hours.

If the oil stains are wet, then you may have still been seeing oil drain from some other place, such as a garlock seal on the accessory pad. if the oil is more of a dark grungy exhaust stain that varies from a dark to light or chocolatey brown, then you're blowing it out the exhaust stack most likely. Going back to washing the engine...did you wash everything and inspect from top to bottom for leaks or oil trails...including behind the accessories and behind the engine? Stuff leaking down behind the aft cylinders close to the baffles can easily escape detection.

The compression test was done warm and cold. I flew it around the pattern first then we did the check and went to lunch and did it again when we got back an hour later. It was the cold check that we heard air at the bottom of the stroke. I dont remember if the oil cap was off at the time, but you could hear a slight "hiss" this was using compressed air through the spark plug hole "orifice." bear in mind this is the only clyinder that did this.

I'm not sure if I understand this one. That you did the compression test more than once, and warm and cold is good. You're looking for consistency there, and you got it.

When you say you heard hissing near the spark plug orfice...one spark plug should have been in place (usually the lower one), and the other spark plug hole should be filled with the differential pressure adaptor. You shouldn't have any spark plug holes in the cylinder from which you can hear anything...when you're doing the test, both plug holes are...plugged. Accordingly, if you hear a hiss in the oil tank via the oil cap filler port, then you're indicating ring blowby...if you hear it through the carburetor and induction you're showing an intake valve leak, and if you hear it through the exhaust stack, then of course you have a potential exhaust valve leak.

I dont know that with compressions consistant with the other jugs and no obvious signs of a problem, I'd be in too big a hurry to go pulling and replacing jugs. There's still something missing here.

Why does your mechanic believe the forward prop shaft seal needs replacing? What are his reasons?

What has been the pattern of oil useage? You indicated that it's risen from one quart every 20-25 hours to one every 4 hours. Do you keep a log on that? How are you tracking the averages (and how often are you changing oil)?

Have you detected any other changes, such as roughness, temperature changes, more throttle travel necessary to produce the same power, unusual mixture settings, increased fouling or apparent icing in flight or on the ground? Have you changed the way you operate the engine, such as doing more pattern work lately vs. cross country continuous power settings? Are you seeing anything on your firewall...did you wash it, too? Did you wash the belly? How wet is it...exhaust stains or wet or potentially wet oil?

You're not into a dangerous oil consumption condition here, though the change is certainly enough to get your attention. You might have just a broken ring (has your mechanic tried putting a dowl to the cylinder wall when the engien is running and putting the other end against his ear, to listen)?

When you get to changing rings, you may end up needing to swap the cylinder anyway; it will certainly need to be honed. You won't really know until you pull that jug, but I'm still not sure why the jug needs to be pulled yet...seems something is still missing.

When you move the prop, do you hear any unusual sounds from the #1 cylinder? Try an automotive stethescope (or a wooden down held against the cylinder with the other end in your ear). See if you can hear a clicking sound as the cylinder is moved onto the power stroke and then wiggled backand forth at mid stroke.

If you do elect to pull the jug, pay attention to the inside of the intake and exhaust ports on the head...see if they're wet. If so, you're looking at valve guide leakage, rather than rings, which could mean anyting from worn guides to stretched or bent valve stems.

While oil analysis isn't the be-all and end-all any more than a differential compression check is, throw that engine on an oil analysis program now. It won't tell you much until it's been on through several oil changes and you can see trends...but it's a darn handy thing to have at times like this.

You've verified that the sump plug isn't leaking of course, right?

 

[skydivinguy]

Run it at 1200 RPM and let a tablespoon of Ajax go into the induction system. It will reseat the rings. Hope that helps!

 

[cargoflyr69]

Run it at 1200 RPM and let a tablespoon of Ajax go into the induction system. It will reseat the rings. Hope that helps!

You must be kidding....

I've heard of this being done on old-school cycles but I don't think i'll do that on my plane

I'd rather pull the jug and have my mechanic help me hone the cylinder with a proper tool instead of hoping the Ajax would do it for me while putting on a new ring kit at the same time.

I think there is still some diagnosing to be done before we tear her apart

 

[cargoflyr69]

avbug,

I did not think of the pressurization of the crankcase, but that would explain a loss of oil, but i'm pretty sure the breather is clean. yes the belly stains are dark, but the inside of the stacks are just ashy grey... indeed it could be valves too. we will have to look more at this tomorrow.

thanks for your insight. I really don't know what else to tell you until i get a chance to see my mech. tomorrow. I will ask again what the rationale is for the crank seal replacement is.


Oh, I usually keep oil at 3 and add half quart at 2.5... I have been changing oil at 25 hours because it has a screen only. I will be putting on a STC spin-on adaptor next annual.

 

[avbug]

Sounds like you're doing right by your engine. I wish more owners and operators were as contientious about their aircraft and engines. Do keep us posted.

 

[belchfire]

A few of things.

First, you don't check compression at the bottom of the compression stroke, or the power stroke- just at the TDC Compression. Now I can't say for certain without a valve timing diagram, but I bet that there is a valve opening towards the bottom of the power stroke. If you don't hear air escaping in the other three cylenders (out say the exhaust) at BDC on the power stroke you have bigger problems than the rings!

A leaking crank seal may not show up on the engine. Is there oil on the face of the prop? That's where you would see it. There and if it is really leaking badly near where the prop tips pass in front of the wing and struts and on the windshield. (note-the face of the prop is the part you face from the pilot seat in a tractor configuration SE airplane.)

If the breather tube and belly behind it are clean you are not pressurizing the cranckcase. That is a near certianty. Does the oil breather have an air-oil seperator...some have been installed!

Based on my experience with the TMC engines you are at 600 hours approaching 1/3 of the life of the engine-the cylenders particularly. I would expect to see an increase in the oil consumption-though not that much.

As the oil gets time on it your highly engineered oils like the 15W-50 start to break down. Towards oil change time you can see significant changes in the rate of oil consumption. Given that I would keep running it, run an oil analysis at the next change and run a little log sheet to see exactly when during your 25 hour interval you have to add oil and how much.

As for just doing a ring job-that is largely a matter of down time because you would have to inspect that cylender for wear and then order the appropriate parts. If you get the whole kit it is a matter of swapping assemblies. If the bore of your cylender is within limits and has no damage there is nothing wrong with doing the rings and gaskets, though some mechanics are leary of doing it.

One question. Has the engine ever set still for an extended period? That cylender may have picked up some rust if it did.

 

[PreRVSM]

"I want to get my motor running right for the most value of my dollar "

The lowest cost would be to just fly it for another 10-20 hour and see if it self heals.
The really question to ask is, is it us safe and is it airworthy as is.
If you look up the max oil use per hour it will really scare you. Your cruise range will be limited by your oil tank not your gas tank.