kunkstyle

Searched the forums all night with mixed results. Long story short:

2000 996 C2 (manual) has been idling rough the last few weeks/month. It only occurs when the engine is warm. Fluctuates between 780-620 rpm at idle. No performance issues while driving.

Not sure what it could be. No CEL (had 2 regarding the MAF about 6 months ago. Cleaning that and the TB later today to see if that helps). No smoke on startup, no oil leaks (except one spark plug tube, that's getting fixed on monday).

I had noticed that a lot of people testing for a failing AOS remove the oil filler cap. Not sure what I should see when I do that. I removed it, and the idle increased. I could hear a vacuum through the filler tube, but I would assume that's what is supposed to happen.

Hopefully somebody can point me in the right direction. I've got a trip planned for tuesday and need the 996 to get me there and back

Macster

With cap removed you should hear air rushing into opening. Normal. Engine controller will react and engine idle speed should change/stabilize.

The test is to see if you can remove the cap with the engine idling.

My 02 Boxster developed a idle problem -- not a rough idle per se more like an unstable idle or hunting idle in that the idle would increase mildly then just a mildly drop back down to near normal.

Cleaned MAF -- wasn't dirty -- and cleaned TB. Dirty. Oily. Even had a drop of oil hanging from throttle valve. Oil at hose connection behind TB. Should have clued me in, but didn't realize significance at time.

Turned out to be -- of course -- AOS. No check engine light. No other symptoms until I had spent several hours checking things, starting engine, running it at idle, etc. Then as time went by the engine began smoking and running rougher.

One of the last things I did before I packed it up on truck for dealer to look at was to with engine idling to unscrew the oil filler tube cap. While I could unscrew the cap just fine, I was not able to remove it against the pressure difference.

If you can remove your engine's oil filler tube cap with engine running this is *not* proof AOS ok but possibly just a sign it is not yet bad enough.

As always, when an AOS is suspect I advise caution and suggest you get the AOS eliminated as possible suspect as soon as possible. Severe engine damage can occur if AOS ignored. My 02 Boxster has gone through 2 of these with no apparent side effects but I did not continue to attempt self-diagnosis for too long.

Oh, the two failures resulted in different symptoms. The 2nd failure started out with check engine light and two error codes -- engine controller at rich threshold. I cleared error codes and continued on for nearly 2K miles having to clear codes as I brought car into motel parking lot at end of long day's driving.

After reaching final destination next AM started engine and smoking ensued. Continuous. Heavy. Picked up phone and arranged flat bed truck to pick up car and haul it to dealer 90 miles away to have AOS replaced.

Sincerely,

Macster.

kunkstyle

Thanks so much for the help.

I cleaned the MAF/TB today. The TB was fairly clean (no oil dripping down). A bit of build up on the bottom but nothing out of the ordinary.

It seems to have helped a bit. It's hunting (I guess that's the correct term, not 'rough idling') from about 780-700 now. But as it's still hunting, I'm assuming that's not the problem, but does it make any sense that it would help even a bit?

I'll ask my indy when it's in for the spark plug tubes on monday. I'm taking it out on a hunting trip and it'll be about 2500 kms (1500 mi) before I can get it back in the city . Should I be concerned about taking it this far? If it is the start of an AOS, how long does it usually take to get severe?

Bad timing for car troubles

kunkstyle

Another update:

Found a small oil leak on the driver's side, about 8" behind the tire, seems to be in the area of the exhaust manifold.

Hopefully my indy has some time and parts tomorrow....

fasteddie99

Oil leak could be your oil separator bellows.

Macster

Bingo!

To OP I think time to change the AOS. While the bellows might be available separately the extra cost in changing out the AOS I think worth the cost and would it if I were in the same situation.

Sincerely,

Macster.

kunkstyle

Had 'er down at the shop twice now. He says there were no signs of AOS failure, and found no air leaking through the bellows, and the cause for the odd idle was probably the spark plug seal leak that had gone from 1 bad cylinder to 3 (and the source of the oil leak). So he changed those and (as I was planning on anyways), and it got better for about 1000 kms. Then it started acting up again.

Went in last week for a coolant resevior change (found coolant on the coil plugs below the tank, which explains why I was losing level...), and had them change the MAF as that was showing signs of failure. The MAF has pretty much taken care of it. Although on occasion it still fluctuates 25-50 rpm.

I've been paying so much attention to this in the last few months, I now forget if this was normal before, or if I'm just watching it too close and noticing the fluctuation now?

Bobbyg

I have the same the same problem now on my 986 2.7L. The most suggested cause is always the AOS.

Not quite sure how accessible it is on a very well concealed engine :-).

Porsche Group

Hello mates i have my 996 C4 model 2000 suddenly when the car heat the warning oil light comes on at low idle and a white smoke comes out of the car but when i drive the car the oil warning goes off any help please?

Porsche Group

The white smoke comes once i start the car and dont stop for a while

Macster

A possible problem with oil supply or pressure is not something to mess with. The engine is at risk of damage, possibly total failure.

The possible explanations for what is going on are too many to cover.

Obviously a check of the oil level and coolant level is called for. Be sure to check the coolant level with the engine cold.

If you find either down or either up too high that's not good. I say "high" because there is the possibility of an oil/coolant intermix problem.

Really this is not a symptom you can address by some simple tweak of something on your driveway.

My best advice to you is do not run the engine anymore and to flat bed the car to a qualified shop and have an experienced/trained Porsche tech go over the car/engine and make a diagnosis.

Porsche Group

No intermix between oil/water nor water/oil just after washing the engine but the rough idle came stable after removing oil cap and the oil warning lamp goes away

Macster

Well, the engine wash can explain a lot. In some cases this can have coils triggering misfires. If the misfires are bad enough to cause the engine RPMs to drop low enough this can have the oil warning light on.

You must be sure the oil level is correct. Not too full but not too low either. (I like to run the oil level at the max line -- when measured with the oil hot, up to temperature -- or very close to it.) The oil wants to be reasonably fresh and of the right type and viscosity. None of that 5w-30 stuff!

If the oil level is good and if the coolant level is good and if there no signs of more serious problems you might (might) "get away" with just running the engine enough to get it nice and warm and then run it longer to keep it warm to dry out the engine and the coils and the wiring.

If at any time the engine starts running poorly though, making noises, acting up in any way, shut off the engine first and ask questions later.

The smoking should go away. But if it doesn't this can be a sign of a failing AOS (air/oil separator). If the AOS is suspect then the warning about not running the engine, not driving the car, and instead getting it flat bedded to a qualified shop for a professional automotive tech with good Porsche experience diagnosis applies.

Be sure once the engine is warmed and still not showing any signs of acting up the oil filler tube cap doesn't leak. See if you can move it around on the tube -- don't get all animal with the thing -- and "cause" an air leak. My experience is the cap will age/wear and leak at least at idle. (I've had to replace the oil filler tube cap on my 2002 Boxster twice now. I get around 100K miles per cap.)

Alan Herrod

I have a 1998 996 tip 3.4. Manual throttle. I have a tick over / low rev problem. The revs vary about 150 on low throttle openings ( below 2200) Feels like old style hunting.. Also an intermittent starting problem. Starts first time sometimes . Others it turns over for up to 10 seconds and eventually starts. seems to make no difference hot or cold . HELP !!!!

DBJoe996

Fuel, air and spark make an engine run. You are going to have to start checking some things. Vacuum leaks are common, Check the vacuum lines. Check the oil filler tube for cracks. Make sure the oil filler cap O-ring is not cracked and is on tight. Clean the MAF sensor. Check the intake pipes and make sure the clamps are tight. Remove the TB and clean it out. Good idea to replace the TB O-ring when reinstalling. Remove the Idle Control Valve from the TB and clean it out completely. Cycle it with a 9V battery and make sure it moves the valve inside smoothly. Back to the MAF sensor, unplug it and run the engine and see if the engine runs better or worse. Any codes pop up? Disconnect the negative battery lead for a few minutes to reset the DME/ECU so it defaults to baseline values, then drive the car to reset adaptations. There is more, but that is a start.