Oil in Airbox: Normal?
>I was re-fixing the straps on the big black airbox at the back of the
>engine today. I lifted out the large orange air filter, & saw a small amount
>of oil sitting at the base of the airbox?!?!? Is this a normal thing? or
>should I be eating rice for the next six months?!?!?
Both of my 928's exhibit this symptom and was told that it's normal if there is a SMALL amount of oil. If you have a tendency to rev her up for a prolonged period of time, then you might want to look into an oil baffle offered by DEVEK.
RE ; OIL in intake --- J R comments -" Oil in the intake is completely unrelated to such matters. Oil in the
intake is the result of a) excessive crankcase pressure, " ....
WHERE DOES THIS PRESSURE COME FROM ????...The pressure in the crankcase is from gasses on top of the piston getting on the bottom of the piston -" Blowby " going past the rings on the piston caused by worn rings, worn cylinder bore, broken rings, or big scratches in the bore all of which are good to know about when considering purchasing a used 928 . The 78/79 engine has the breather hose vent from the top of the oil separator, were you add oil, into the lower air cleaner housing just below the air cleaner element. There is no PCV valve on 78/79 just an open hose to allow crankcase pressure to find it's way into the aircleaner where it would get pulled into the engine intake and eventually burned . The oil separator has a mesh/wire "filter" which is supposed to take oil particles out of suspension in the air and drain back to the oil pan . An old dirty air cleaner element might increase the flow of air/oil from the crankcase as there would be more of a
J R continues:
> .... " There is a hose (typically 1/2 inch ID or so) that runs from the crankcase
> (typically one or both valve covers) to the air box (or something between
> the airbox and the throttle body). This is where the oil in your
> intake is
> coming from. THIS HOSE is where the oil is coming
As Ed Ruiz says YMMV ------- the 928 of any year does not have a PCV valve; does not have "valve covers" for that matter. The 86 and > 4 cam engines do have two vent hoses connected to 90 degree fittings on the cylinder head covers both on right side . These vent into the intake near the throttle body but do not incorporate a Positive Crankcase Valve The 85 and newer also modified the oil strainer/filler incorporated an extended fill spout and added a louvered baffle plate to try to keep liquid oil from being thrown up from the spinning crankshaft. They also eliminated the oil return line running down into the front of the oil pan, it seems that under high RPM use oil flows up this tube into the separator.
J R continues:
>An engine stuffing enough oil atop the valve train to make a fundamental
>difference in the PCV valve's behavior (to me) would be an engine that's
>well on it's way to expiring. I'd expect many other symptoms (like really
>poor idle, stumbling, etc.). "....
The top of the cylinder heads, camshaft carrier, etc. are lubricated with oil. There are large drain channels cast into the engine block to allow this oil to return to the oil pan . This also allows " blow by " to circulate from the oil pan to the camshaft housing area to be picked up by the breathers in the camshaft cover for 85 and newer . Doing a leak down test which is very easy would indicate if pressure is bypassing the rings and getting into the crankcase/cylinder head oil passages and eventually the intake. This is a very long way to explain that excessive blowby leaking past the rings will fill the intake with oil -- much oil, the engine may run reasonably well but it is worn .
79 928 5spd
80 928 5spd
68 911 5spd
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