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Old 09-08-2002, 03:42 PM
Posts: n/a
Blowby and intake air

I have searched the forums for discussions of Blowby, and found lots of discussion, but nothing that provides measurable data, such as: How much compression loss is usual in vehicles with enough blowby to accumulate oil in the air cleaner? Is there any pressure or pressure/volume relationships that can be measured? For example, the fuel system has a liters/time period test. IF we use a cylinder pressure test with a leakdown rate ( such as 1 bar/30 seconds at 10 bar test load), would that be part of understanding the problem, and defining a service limit?

My 1985 190E 2.3 8v, has compression of 140 to 160 psi ( appprox. 9.5 -10.5 bar ) service manual for 9.0 compression engines gives 8.5 as service limit. Yet this engine has oil all over the air cleaner and intake system from the crankase ventilation system. It appears from removing the valve cover that the engine was maltreated by previous owners, for there is significant burned on oil in the cover, and the connector tube from the oil mist separator is rock hard. I have followed and inspected the hoses, and removed/cleaned the distribution block on the intake manifold, which has a restrictor in the vent side so the idle control valve has the larger opening.

Since there are oscillating and rotating masses in a piston engine, there is bound to be movement of air, especially under the pistons, as well as the leakage of combustion pressures past the rings and through the ring end gaps. What I understand from other discussion threads is that the hose size in the air cleaner is based on an effort to balance the varying volume of displaced air from the engine so the vacuum value is low enough that the system doesn't pull oil out of the system. I currently have 15 - 16 steady inches of vacuum in the manifold at idle and see numbers in the low 20's when dropping back to idle. Even at these numbers, I get oil in the air cleaner. Has there ever been something done like a check valve in the air cleaner hose? How about changing the orifice size in the distribution block? I see later systems ( 87, I think) changed this setup, and removed ( from what I read in the service library) the orifice.

Since all these leaks bypass or otherwise affect the movement of the airflow plate, I suspect there are many drivability problems associated with failures in this emissions control system.

This same oil problem fills the smaller vacuum hose to the fuel pressure regulator, causing it to decay, and making another vacuum leak
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