Crank-case pressure vrs Idle speed

[Guest]

Hello All,
I had my 996 C2 back from its major service about a month ago - a few oil leaks (vario-cam solenoid, spark-plug tubes & oil-seperator/breather) were sorted out at the same time. Since then it's really just had its feet-up.
I've just been out to give it a run and when back on the drive noticed that when the oil filler cap is removed the engine idle speed is significantly affected, just holding my hand over the filler pipe is enough to alter how the engine runs - is this normal? (had quick scan of FAQ but did'nt notice anything).

Also noticed a small amount of smoking from the right exhaust pipe at tickover, which dissapears after about 10seconds, but then comes back a few minutes later and stays for the same period - no evidence of water usage or contamination. Should I be concerned?

Thanks.
Peter.

1998 996 C2 Tiptronic S
Prev.
1997 993 C2S
2001 Boxster S
1990 964 C2
1977 911 SC

 

[Ian W]

The crankcase breather system on the engine keeps the inside of the crankcase under partial vacuum. When you remove the oil filler cap, you are letting air into the crankcase which then finds its way, via the breather system, into the inlet mainfold. This makes the car immediately run lean (the air going in via the oil filler tube isn't being metered by the air mass sensor).

As you take the cap off, you'll notice the revs rise momentarily as the extra air gets in and the engine will miss a couple of beats as it goes too lean. The elecronics will very quickly compensate and the idle will settle down. This takes the whole closed-loop emissions system out of calibration and can end up throwing a 'Check Engine' light as the system is effectively having to compensate for an air leak. Once the cap is back on the system will re-calibrate back to normal but you may need to reset the engine control unit if it's alredy registered a fault condition.

In short, it isn't designed to run without the cap on so don't mess!!

Ian W

PS. Leaky oil separators (usually caused by a split 'bellows' pipe) also cause exactly the same condition as removing the cap. Air is sucked in through the hole in the belows and the engine eventually ends up running like a dog because the system cannot re-calibrate around a significant air leak.

 

[Guest]

Thanks Ian.
I'm aware the engine is'nt design to run 'cap-off' but the crank-pressure + the exhaust smoke, made me think I could have a cylinder liner or piston-ring issue. Also one of the spark-plugs from the recent service was also very black so this added to my feeling all may not be well ....

If its just 3.4l paranoia I'll be more than happy ....

/Peter

 

[Ian W]

I don't think there's anything to worry about. The various oil leaks you had fixed are known problems on the 3.4 and not caused by high crankcase pressure - just by seals that have been improved and re-specced as the years have gone by. The seals you have now are far better than the originals!

The behaviour at tickover is exactly what would be expected when removing the oil cap (for the reasons stated above) so again it points to things being 'normal'.

The occasional puff of oil smoke I'm not sure about. If it was a damaged bore, it would be happening constantly so it's unlikely to be that. If there were leaks related to a breather that have just been fixed, it's more likely the breather system clearing itself.

Drive it like you stole it and enjoy!!