hey guys,

Trying to diagnose possible AOS failure and was looking for a little extra insight.

2004 S, 104k miles, no major issues to date.

Car has always had the occasional but REALLY impressive 'Boxster belch' on start up although in my case seems to be independent of whether the car has sat or not. Worse when driven lightly, sitting out in sun. Totally unaffected by auto cross and track days, never does it except for start up.

Recently has been a little more frequent, maybe 2x a week driven 4-6 times per week. Car does not run rough except maybe a bit at idle.

Finally did the oil cap test and even when rotated to the 'lift off' position I can't get the cap off. Engine off, comes right off.

So the insight I'm looking for is, does this sound still mostly consistent with an AOS failure despite not being exacerbated by aggressive driving? Or more suggestive of another source causing excessive crankcase vacuum?

Thanks a bunch,

 

Yes. If ya can't get the cap off. That's a good (bad) sign the AOS if further failing.

 

The belch is normal after parking it cold, and occurs even when the AOS is working just fine. If you park it hot, do you see any smoke the next time you start it?

 

I'd say yes, occasionally, but not when it gets driven hard then restarted shortly thereafter (I.e. AX, HPDE). Only if it sits most of the day or longer.

 

So many factors contribute to the propensity of the engine to emit smoke upon cold start that I won't list them all, even if I knew them all.

One factor I will mention is the efficiency or lack of same of the AOS. If this fails to remove the oil vapor from the crankcase fumes as they pass through on their way to the intake manifold oil collects on the intake walls.

If the engine is running this oil is burned without any real indication oil is being burned. Mainly, one won't see any smoke. Oil consumption can be higher than one might like, but this is almost always attributed to some shortcoming in the engine, the rings/pistons are blamed.

If you had stopped your post with just the observation regarding the propensity for the engine to belch smoke at startup I'd say the behavior was normal.

However, with the smoking to be accompanied by rough running, and the inability to remove the oil filler tube cap with the engine running, the AOS has to be highly suspect.

I'm always reluctant to just flat out say replace the AOS (or anything else) based on my layman's diagnosis and I won't this time. But you need to get the car checked out.

If you continue to drive the car sooner rather than later the AOS is going to fail -- my opinion but based on my experience after going through 3 AOS failures. By "fail" the AOS will deteriorate to the point the engine will smoke like crazy off and on as it is running eventually -- within minutes -- getting the point it smokes continuously, and at some point in this smoking progression run rougher than all heck.

At this point you do not want to attempt to nurse the car any further than the side of the road to await a flat bed tow truck for the smoking means almost all oil vapor is being pulled out of the engine crankcase and routed to the combustion chambers. If the volume of oil is too great the engine can suffer hydraulic lock with disastrous results. There is also the concern about the engine's volume of oil dropping dangerously low even if one manages to keep the engine running a while given its behavior.

Have to mention that if the AOS failure progresses to this point, the engine is still at risk of suffering a hydraulic lock as the intake walls will be coated with oil.

After the engine is shut off the oil will run down into one or more cylinders and upon the next attempt to start the engine can lock up from this oil if its volume is greater than the cylinder's combustion chamber at top dead center. The 1st AOS failure my Boxster suffered this lock up occurred at the dealer when the engine was cranked.

If the AOS failure is severe enough the engine is smoking all the time before you give up and call a tow truck my advice, just to be safe, would be to remove the plugs before attempting to spin the engine and to after the AOS is replaced and the oil level topped up to crank the engine some with the plugs removed to expel any oil accumulation.

Afterwards the plugs get installed -- I'd use new plugs as I do not like to reuse old plugs. The engine will almost certainly smoke something fierce initially but the smoking should taper off rather quickly. However, it can as the engine warms up as the car driven it can smoke again as the residual oil makes its way from the intake into the chambers.