Fly911

My 2000 Boxster has an unstable idle. It is running at 750rpm (too low?), and is then dipping down to 600rpm for one second, before it recovers and go up to 800rpm. Then it settles at 750 again. This repeats itself every 4-5 seconds. The engine has never stalled, but it's clearly not right.

Any ideas what is wrong, or what I can do to fix it?

Macster

IIRC idle speed is 740 rpm +/- 20 rpm. The dropping down to 600 is not normal.

My WAG is AOS. This is the symptom my 02 exhibited the 1st time the AOS failed at around 78K miles.

With engine idling unscrew the oil filler tube cap and see if you can remove it against the pressure difference inside and outside the crankcase.

When I tried this towards the end of my diagnosis I was unable to remove the cap against the pressure difference, the AOS was subjecting the crankcase to huge vacuum.

I called the service manager and described the symptoms and he told me "AOS" and "do not drive the car".

This is important: The less you run the engine even start it to self-diagnosis the more risk you run the AOS will allow the engine to ingest too much oil and subject the engine to hydraulic lock. If this occurs it can ruin an otherwise perfectly good engine. My 02 has had 2 AOSs replaced, at 78K miles and over 100K miles later, and suffered nothing more serious than a ride on the back of a flat bed tow truck both times.

Sincerely,

Macster.

StanThePorscheFan

Any smoke on start up?

Fly911

One out of 20 starts it has a little puff of white smoke for two seconds, but that is pretty normal, I've seen that on even brand new Porsches. I haven't had the time to do any trouble shooting yet, I will do that in the week end.
What would the cause for AOS (I assume you mean Air Oil Separator) clogging be? I run Royal Purple 15W50 oil, oil cunsumption has been one bar on the oil level gauge on the dash over 13k miles, so there is hardly any consumption. The car has now 101k miles on it. My typical run time is 20 minutes, hardly ever shorter, and often longer, so the oil temperature should be up to 185 deg.F, and possible water condensate should vapor off. So I believe oil/water "butter" is out of the question.
Does the Boxster have an ISV like my 993? My 993 had similar problems, that disapeared after cleaning the ISV.

Macster

Smoking 1 out of 20 starts is about par. Some smoke a bit more frequently, some a bit less (and there are some that smoke almost every time but there's always an exception that proves the rule).

The AOS doesn't clog, at least that is one failure mode it hasn't exhibited yet.

I don't know the failure mode of the 1st AOS but I cut the 2nd failed AOS apart and found a tiny hole in the rather thin rubberized diaphram that was controlled by and controls the vacuum submitted to the crankcase.

My guess is the hole/crack/tear -- maybe the rubber/canvas separate -- at first is small enough that it can vary in how much vacuum it will accept or deny and cycle in and out of some equilibrium state into a state that either represents some noticable vacuum leak -- this at idle speed -- or none and this accounts for the hunting idle.

After some time -- miles/hours who knows? -- the tear or hole or whatever enlarges and then more symptoms appear. Excessive and constant vacuum at the oil filler tube opening, excessive, persistent and heavy smoking to name a couple.

The varying idle may be aided by crankcase fumes heavily laden with oil vapor -- due to imcomplete processing by the AOS due to its malfunctioning state -- which deposit oil on the throttle body butterfly valve. When I took this apart the 1st AOS failure the butterfly valve was wet enough with oil to form a drop of dirty oil at its bottom edge.

The AOS goes bad, if it is bad, due to time and miles. There is nothing you can do but be aware of the possibility of it going bad, what the symptoms are and how to recognize them and if they point to the AOS have the AOS replaced or replace it as soon as possible and with as little engine running -- none is best -- until then.

I do not think Royal Purple a recommended oil for these engines. I believe I have somewhere some info which suggests you could use a better oil but I don't know where it is. However, I don't think the immediate problem is due to using Royal Purple and I don't believe an oil change will cure the symptom.

If the car is a 2000 or newer and has E-Gas (should be the same thing) it does not have a ISV (idle speed valve). Idle speed is controlled by the E-Gas control system and the Ecu.

Sincerely,

Macster.

Fly911

Macster, thank you! I will do some troublke shooting in the week end, check the crank case vacuum and check the throttle valve assembly.

Fly911

The idle problem is fixed. I did a much needed cleaning of the K&N air filter, I cleaned the MAF sensor, and dismantled the E-gas throttle body and cleaned the gears and electronics. I don't think I have an AOS problem, there is no vacuum in the crank casse, the oil cap comes off easy with the engine running. And there was no sign of oil in the throttle body or in the intake plenum. So now the engine idle is smooth as ever.

Macster

With no signs of oil on the throttle body or in the intake and with no other AOS failure signs I agree with you the AOS is not the cause of the idle problems.

Either you didn't mention earlier or I didn't see it, but had I known you have an aftermaket air filter system installed I would have told you aftermarket air filters cause more trouble than they're worth in my opinion and recommend you return to the stock air filter system. The stock air filter is simply the best air filter.

As for the E-Gas module had you asked I would have advised against touching it unless symptoms pointed to it requiring touching. In fact, I didn't think one could really get in to it. There are no servicable items inside -- according to the Porsche tech write up on E-gas. The E-Gas control module's pretty much a "black box" and sealed against the elements. And it has pretty good self-diagnostics and while it can't fix itself should it detect a problem it can let you know there is a problem.

Sincerely,

Macster.

Fly911

The E-gas is not a black box, it's silver (lol). It has a black lid that comes off easily. With the lid off, the electronics and gears are accessible. It probably didn't need to get cleaned, but I didn't want to rule it out while I already had it off.

The min reason for the K&N filter is to get a little more sound out of the engine. The Boxster is way too muffled. I should probably get a cone filter (with an isolated box around it, to avoid hot engine air), and get rid of the muffler box upstream the MAF. And a new exhaust of course... I don't care if it gives me a HP increase or not, it's the sound that gives you the feeling of going fast, not necessary the speed itself.