Liquid oil in the exhaust.... and other oddities
 

Hey y'all;
I need some help, and it seems I've diluted my AOS topic enough that this specific issue hasn't attracted any attention. Either that or nobody has an opinion.... which is hard to imagine, haha.

In my "AOS, what breaks first" thread, I've described the situation surrounding this specific question, but to nutshell: I've got oil in the exhaust. I still seem to have good compression in all cylinders, and no obvious signs of intermix. This could be simple catastrophic AOS failure, but there's still one issue nagging at me, keeping me from diving in:

Note the photos below. Large cam plug on the front of intake cam on the left bank is MISSING. Both small cam plugs on the back of the same bank are canted/ crooked. I can't account for this. It's almost as if they've been pressurized and blown out. I've been trying to imagine ways that crankcase pressure could reach those levels, and none of my imaginings are pleasant. None of these areas show obvious signs of oil dripping from those holes, but there is signs of weeping, and the foam sound insulator opposite the missing plug is saturated.

Can y'all please help me with your experience here?

What could have caused this?

Thanks.

https://uploads.tapatalk-cdn.com/201...48fdf1acec.jpghttps://uploads.tapatalk-cdn.com/201...b532479675.jpghttps://uploads.tapatalk-cdn.com/201...5bdaa5b36e.jpg

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I can’t speak to the left side but once had a cam plug fall out of the right side of the motor and it pumped quarts of oil out in a matter of seconds. Odd that you can run without them. That being said if they popes out it’s more than likely a pressure problem which points to the AOS. Id replace the AOS and the plugs (MAKE SURE they are well hammered in, not like your photo) and then send it.

Are you sure the engine didn't spin the wrong way as a result of the car spinning and then traveling backwards?

Something caused a lot of pressure to build up.

No, I'm not certain. The car died when I spun. That may be from what you describe. It happened pretty fast, as it was precipitated by the oil left by our friend in the Ford. :: sigh::
Don't get me wrong.... I blame only myself for the spin, but the conditions leading to it have to be acknowledged too, haha.

So, if the motor spun backwards, what's the biggest fear here?




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Bent valves....

Right, but that's assuming cam timing changed, which could've happened forward or backward, yeah? (Or is there another way to bend those valves that I'm not considering? )
But I'm still pushing healthy compression numbers in all cylinders.

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As far as what fails in the AOS, it seems to be the rubber diaphragm that splits.

You may have escaped damage - I'm just postulating what blew out the plugs...

In the end this was an atypical AOS failure. We have all seen smoke, exhaust oil fire not so much.

Rotating the engine in reverse, even by hand, is sufficient to whack the valves; we have had people bring us cars that they had done exactly that while attempting an IMS retrofit, and even though the cams were still in time, they had kinked a couple of valves and every piston had bark marks from contact.

!!!??!!!

I believe you, JFP, because I have no experience otherwise. But I'm struggling to wrap my head around the HOW.

The location of the cam (and therefore the entire valvetrain) is fixed, in relationship to the crank (and therefore the pistons). I'm trying to imagine how direction of rotation can affect this? Teach me.


Hahahaha.... I'm rarely "typical" in anything, Max. Surely you've understood that about me by now, haha. :cheers:

I think it's time to break the borescope out.

slack in chains.

yup...on it tonight.
BUT: a bent valve isn't consistent with good compression numbers. Jus' sayin'.

seriously?
Is this something you've seen? or are you postulating?
Shouldn't the chain tensioners be working regardless of direction of rotation?

The chain tensioners function off oil pressure, when the engine is turning backwards, the oil pump is sucking the oil out of them..………..

it can happen in older engines due to stretched chains. recall that on our cars we get chain slap on start up due to low oil pressure and the tensioners, like lifters, require oil to inflate, so without oil pressure there is some slack in chains.

While the pic attached is of a 5 chain engine and yours is a 3 chain the mechanicals similarities should be evident.

On these engines the chains have a "driven"(under tension) and a slack side that the tensioners control. The vario cams can move ~23 degrees in relation to the crank and you see how short they are. How many degrees backward does the crank move before all the slack is pulled out of the crank to IMS chain and the camshaft drive chains on the slack side? JFP might know but it would be considerable. Obviously enough to interfere with piston travel.


http://986forum.com/forums/uploads02/51541453414.jpg

aaaahhhhhhh..... Bach. ($5 to anyone who recognizes that reference, haha)

Okay. thanks to Radium King, JFP and 911Monty, My eyes have been opened. YEah, I can see it now... makes total sense. that ALSO leads me to understand what MaxD was getting at, about the engine turning backwards....

y'all are a wealth of knowledge here.

now, excuse me while I back-up a little bit, as I take a long look at the way Porsche engineers designed that entire system, and shake my head in wonder. (as in: I wonder why in hell they still had jobs after that mess?) :confused:

Of course, I've seen worse, because I've owned Jaguars, RangeRovers and Maseratis. :rolleyes:

In your previous thread, Post #12, I asked if you got the clutch released before going backwards in your spin when I asked about the scavenge pumps. Have you checked their condition? The sources for pressure in these engines come from compression/combustion, thermal/ water, and mechanical/ oil pump. Since you don't have water in the oil there is really only 2 sources you need to evaluate for pressure in the valve covers. The AOS is a source of vacuum not pressure. It is basically a regulator between the intake vacuum and the crankcase.

Well that's just a mash-up of where it could have come from. ;)

haha, good man.
Where am I sending your $5? :cheers:

Thanks Monty. I had to go back and look, because I didn't remember you asking about the motor spinning backwards. Shonuff: you asked. I may not have seen it, as it is a "PS" at the bottom, which I might've dismissed as a sig-line. Either way: I appreciate that you were looking ahead for me. :-)

No, I haven't yet inspected the scavenge pumps. Any second now. haha.

Seriously, I haven't really had any time to do much of anything yet. I've got too much else distracting me right now. When I find a minute to work on the porsche, it's usually just that: a minute.

Hopefully after my business-related fires, my son's wedding, my daughter's Missionary-departure (we're Mormons over here :-) ) and my wife's various and sundry honey-do's... (that's Wife's, not Wives'... even though we're Mormons over here, haha).....

I'll get to it sooner or later.

Ha sounds familiar. My daughter served the Cebu mission in Philippines. We dropped her off the MTC and the next day was the BYU vs Texas game that was delayed due to lightning and flooding. Shoulda seen it coming, 3 days after arriving in the Philippines was the massive earthquake and then of course typhoon Yolanda tried to drown them all. And we thought the hardest thing was for her to have to wash clothes in the river. HA! We're thankful she made it home. Hope yours goes better.

Borescope:
See the photo below. Honestly, with 150k miles on it (only about 3000 of those by me) I wouldn't have expected to see somebody's sharpie scribbles on my piston top still.

Otherwise, absolutely nothing odd in the initial view. https://uploads.tapatalk-cdn.com/201...5562f20253.jpg

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Scavenge pumps;
Left side was full of oil when it came out. Turns smoothly, seems to be in good working order. The key was oriented correctly when it came out, so no concerns in my mind.

Ditto the right side, except that it was NOT full of oil. Not "dry", per se, but didn't drip at all when I unseated it.

What's "normal" ?



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So to clarify, the left (bank 2?) cam plugs have been blown out and the scavenge pump is possibly not moving oil? Is this also the bank that the camshaft plug may have blown out on? These are not positive displacement pumps. They are rotor. gear pumps. Possible pump damage or blockage downstream at de aerators to sump? Bank 1 (right) could be dry due to AOS directly attached there and most/all oil going to intake?

Ok so just thinking here and it's later than clock indicates.... I referred to the AOS in the basic term as a pressure regulator which is correct, it is attempting to regulate the crankcase pressure at idle to about neg 4.4" wc. Since the plug is missing on bank 2, the AOS "sees" this as positive pressure in the crankcase and opens in an attempt to draw it down. Since it can't with the hole open then AOS is fully open and ingesting massive quantities of oil to the intake tract. Might be worth getting all plugs back in place and test run.

EDIT: The correct term for the AOS is a diaphragm operated back-pressure regulator.

great thread !!

Very interesting comments from 911 :)

Okay, I like this working theory (thanks, by the way). The only question it leaves unanswered for me immediately, is probably born from not having a full-scope view of the AOS system in it's entirety: if it's wide-open, as you've postulated, why is it ingesting oil and not air? Do I have a drain-back that's blocked somewhere, keeping the "intake" of the AOS fed with oil?

Okay, I like this working theory (thanks, by the way). The only question it leaves unanswered for me immediately, is probably born from not having a full-scope view of the AOS system in it's entirety: if it's wide-open, as you've postulated, why is it ingesting oil and not air? Do I have a drain-back that's blocked somewhere, keeping the "intake" of the AOS fed with oil?


I don't know which is bank 1 or 2. Someone could clarify for me right now: If I'm facing the same direction as the car, the left side of the motor is the one I'm talking about. Is that Bank 2? Probably, I guess, as it's offset-to-the-rear. ??

Yes, the Left side has the missing large cam plug, the smaller cam plugs that appear to have been blown-out, and the scavenge pump that drained oil when I pulled it out.

This could get long so let's answer this first so I don't get timed out. From rear of car looking forward on a LHD car. Bank 1 cylinders 1,2,3 are on right, passenger side. Bank 2 cylinders 4,5,6 are left, drivers side.

Short answer is the AOS is ingesting both. If you look closely at the AOS the bellows at the bottom of the de-mister section is directly attached to bank 1 right over the camshaft drive chain. It is attached to bank 2 through the long convoluted tube that crosses over the engine. Due to pressure drop across that tube the AOS will preferentially draw from bank 1. The million dollar question is why did the plugs blow out of bank 2? Still no answer to did the engine spin backwards, but I'm going to say probably. My theory basically is that one cylinder had recently fired but did not get to exhaust valve open. When the piston reversed, the compression ring lost seal on the piston ring-land and dumped as blowby into the crank case. Excess pressure in the crankcase popped the plugs before it could bleed through that long tube to the AOS which had no where to dump due to loss of vacuum in the intake tract when engine quit running. Pressure then relieved through the first weak point. I don't have the dimensions of the plug that popped out but I'd estimate about 3" diameter? Doing the math that puts it ~7 square inches. A cylinder head pressure of only 2 psi would exert 14 lbs of force against that plug.

Sorry, I didn't mean to leave this hanging. the truth is that I don't know. It's quite possible. Everything happened very quickly. This wasn't a slow spin from powering through a corner, that I was trying to save. This was a very fast LOOP, caused by a previously oiled-down corner on the track. The fact that the motor died as a consequence of the spin could suggest that I didn't get the clutch in fast enough, and so yes, it could have turned backwards.
I realize that's an unsatisfactory answer, and probably shows my novice-level at the track... since this was, in point of fact, my FIRST spin (unless you count the day in 2006, when I took one of the school mustangs out in the snow with a Journalist who'd come to see the then-new track facilities, haha). I have to say though, unequivocally, that the spin in the Porsche was far preferable to the equivalent on a motorcycle. :rolleyes:

this all makes sense to me. What we'll hope then, is that I didn't break a ring-land in the process.

I'll try to run a leeakdown test before I start putting things back together.

No problemo. The oil on the track may have caused the spin but it's possible due to the lack of traction may minimized engine reverse rotation, and that's a good thing. When you do assemble and get ready to fire up, even though the AOS may be undamaged, I'd recommend disconnecting the tube and plug the port to the intake. Then take a hose and run the tube to a bucket. No sense in taking unnecessary risk.

I'm hoping to get back at it this week.
With this working theory, above, I want to do a leakdown test before i begin reassembly, just to be sure I didn't break a ring land or something.

My problem, or question that I'd appreciate some wisdom on, is this: I've always done leakdown tests on a hot motor, with the understanding that a cold motor won't seal anyway. So... I can't warm this motor up.... it isn't running right now. How useful IS a leakdown on a cold motor? And what sort of numbers are acceptable for a motor at 150k miles?

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Under 5% is preferable. But I'd say that anything less than 10% is probably ok on a well worn engine. This engine had 90K street miles when I installed it in my car. Then I had added 70 track hours of racing on top of those miles! That's got to be equal to 150K or more street miles.

If you see anything like 15% or 25% leak down, its time to worry. With low compression and poor leak down numbers in cyls 4 and 5, I knew that this engine was on its way out. It finally failed 11 track hours after this test.

The fact that your compression numbers are good brings hope that the leak down numbers will also be good. Best of luck.

http://986forum.com/forums/uploads02...1524264917.jpg

That's what I'm hoping, too.
Thanks for the response. Any thoughts on cold vs hot test?


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I am new to the site and new to Porsche.

If I understand this event, the vehicle spun and the engine may have turned backwards.

Image one of the cylinders just fired the plug and all the valves are closed while combustion is taking place. There is pressure on the piston to push it down, however the weight of the vehicle (going backwards) does not allow the piston to go down.

The piston is forced up against combustion pressure and as the crankshaft begins to rotate a few degrees backwards, the intake valve opens,(motor is turning the wrong way). The cylinder is above max combustion pressure and this pressure is released into the intake manifold. The throttle valve is shut. The pressure takes the path of least resistance, travels past the AOS into the crankcase. The pressure expands, inside the crankcase, just enough to blow out one cam plug and unseat the others.

Any takers on this "small bang" theory?

Plausible.
Either way, it seems that combustion gases pressurized the crankcase.

I'ma do the Leakdown. I think I'll find that things are fine (I probably just jinxed it). Then I'll feel confident buying new parts to reassemble (I'm going to do fresh coils, plugs, probably a baffle in the sump, etc etc).

Thanks again for everyone's help.

I wonder how many times I have posted this link. On a Boxster, 911monty is quite right.