Not AOS - What Now ?

[Spock]

Help !!

I could use some direction with this ..

Boxster S, 2000, 75K. Originally, I had the usual problem of blue smoke from the exhaust when driving hard, and then lifting off. The AOS was replaced, but the problem persisted. It was suggested that there could be oil pockets still which would burn off. After over 1000 miles, the problem persists as bad as ever. As a long shot, perhaps the replacement AOS was faulty, so it's been replaced again. No joy. Compression has been checked and is OK, but all 6 spark plugs show some oil covering, though. Oil level is OK. AOS drain is OK. Nothing showing on the diagnostics. Pretty well everything external that could be checked has been. I'm now being told that further investigation requires engine out and strip. That almost makes it a write-off at today's prices.

I would really appreciate some other ideas / suggestions / tests we could work on.

Thanks.

[logray]

A leak down test should reveal where the problem is. If the air is leaking through the valves, valve guides or seals then the air will be heard through the intake or exhaust manifolds. If the air is leaking through the oil filler tube then the piston rings are worn and have likely lost ring seal.

If it is just valves and valve guides, you might just need a top end rebuild (valve job). The purple/blue smoke would be from the oil leaking past the seals and/or through the guides. A valve job does not always warrant a new engine.

OR

If the air on the leak down test is through the oil filler tube (crankcase), then the smoke and oil contamination on the plugs could be due loss of piston ring seal, which would require a bottom end job as well...

In which case you are pretty much better off sourcing another engine due to the cost of total engine teardown and rebuild.

Another way to test is by use of a vacuum gauge on the intake, if at hot idle the gauge reads below 17inches HG and/or on quick blips of throttle goes 0 and then only back to 20 or 21 that is an indication of worn rings. The vacuum gauge can also diagnose valve problems.

Edited September 29, 2011 by logray

[wvicary]

Has the engine ever been overheated? How much oil are you consuming over 1000 miles?

[logray]

It could also be due to overfilling of oil, which is of course very simple to check and correct.

[Spock]

Thanks for the initial thoughts. It hardly loses any oil between changes (about 6000ml). Hasn't overheated, as far as I know - I've had the car just over 4 years. We have checked the oil level, and it's OK.

[wvicary]

Even though you have put on 1000 miles you might not have gotten the exhaust system up to temp to burn off the residual oil. I've had to take these vehicles on a 50+ km drive at high RPM's on the highway to clear them up.

[Spock]

If there was residue in the exhaust, it would surely burn off whenever the exhaust got hot, but I just get a 'lot' of smoke on gear change/lift off at 5000+ RPM. Below that, it runs normally. Also, there's no sign of oil deposits around the exhaust outlet.

[wvicary]

If you have it showing signs of smoke on over run then it points to a problem in the valve train with a worn guide/valve stem or damaged stem seal.

[Spock]

We've disconnected the AOS breather into the throttle body, ran that into a container, and bunged the throttle body hole. There was no smoke, indicating no residual oil. As the symptoms are very similar to an AOS failure, it has been suggested that the drain channel for the AOS in the engine block could have become blocked. Revving the engine once or twice is OK, as the oil in the AOS would slowly drain away, but doing it repeatedly fills the AOS quicker than it can drain, and we get the smoke. Does this seem feasible ? Does anybody know if it's possible to clear the channel from 'outside' ie without taking the engine out ?

Thanks all for your comments.

[logray]

This is interesting, I have heard of this before.

I would have a few questions though. What is blocking it, is it from dirty oil, an exterior leak (air/dirt leaking into the system)? And say it were blocked, if you were to "unblock" it, where would that grime and material go next, and could it potentially damage the engine's interior due to oil passageway blockage. For example even though that material may just go into the sump, it is possible in the stock filter housing design for some oil (and debris) to bypass the oil filter. In which case I would not want to risk trying to unstick that material without an engine teardown, at least not without getting additional advice first.

You may want to consider the larger motorsports AOS version for your Boxster, this may have extra capacity at higher flow rates and could solve your smoking issue, since I've read about this solving similar smoking issues on cars that are tracked heavily.

Along with the Motorsports AOS installation or even prior to spending that money I would still perform a leakdown test as high RPM then off the gas and smoke out the tailpipe is an indication of valve train issues as was mentioned earlier a few times.

Edited September 30, 2011 by logray

[logray]

Also what were your compression numbers?

[Spock]

Also what were your compression numbers?

I don't have the numbers, but the guys were happy that they didn't indicate any problems. They've now checked the AOS drain, and think that isn't the problem, as it seems to be clear. They're now at a loss as to where to go. There doesn't seem to be any good reason to take the engine out and strip it down just on a 'fishing' trip. What else can cause AOS failure symptoms but not be due to a failed AOS ?

[logray]

What is the crank case vacuum? Here is information regarding how to definitively test for a bad AOS:

What are the leak down results? A leak down test will indicate whether or not there are issues with the valve train or piston rings.

What are the intake manifold vacuum gauge results? That in conjunction with the leak down can provide very valuable evidence as to where the issue might lie.

Did you install the larger Motorsports AOS or are you still running the stock AOS?

[Dale_K]

Arent' there some screen filters in the evacuation "pumps" on the ends of the cams? If those were clogged (maybe with debris from the tensioners) would they allow oil to accumulate in the cam areas? I'm just guessing here. I like the idea of measuring crankcase vacuum and comparing it with a known good car. Seems like you might be able to to adapt a plain vacuum gauge to the dipstick tube but I admit I've never tried it.

[logray]

...Seems like you might be able to to adapt a plain vacuum gauge to the dipstick tube but I admit I've never tried it.

You would need a very fine grain gauge. 4-7 inches of water is equal to 0.294 - 0.514 inches of mercury. Very difficult to see on a standard gauge.

edit: it might work though, with very excessive vacuum if it reads above 1 in hg might indicate bad AOS.

Edited October 3, 2011 by logray

[logray]

Arent' there some screen filters in the evacuation "pumps" on the ends of the cams?...

Here are the guts of an oil scavenge pump. Just a gear and the housing. No filters.

[Spock]

The problem was, initially, a faulty AOS (I think). But the problem of smoke at high RPM after throttle closure has persisted after replacement. Another AOS unit has since been tried, but made no difference.

" If you have it showing signs of smoke on over run then it points to a problem in the valve train with a worn guide/valve stem or damaged stem seal. "

The problem started suddenly, so doesn't suggest wear. Also, the smoke is a large 'puff' and then stops. Wouldn't it continue smoking on over-run if it was worn components ? Blocking off the AOS stopped the problem.

Is there a work-around? What would happen if we blocked off the AOS entry to the throttle body, and fitted an oil breather after the AOS ? No oil vapour can get into the inlet, and the crankcase can breathe to atmosphere. Is there any liklihood of engine seal damage ? I believe this sort of arrangement has been used on track cars.

[logray]

Here's the link to the Motorsports AOS:

http://www.planet-9.com/reviews/cayman-performance-modifications/p183-with-porsche-motorsports-oil-seperator.html

[Spock]

Here's the link to the Motorsports AOS:

Thanks for that. This seems a little overkill for a standard road car. Would my idea work: " What would happen if we blocked off the AOS entry to the throttle body, and fitted an oil breather after the AOS ? No oil vapour can get into the inlet, and the crankcase can breathe to atmosphere. Is there any liklihood of engine seal damage ? " But I'd still like to know what's causing it.