oily deposit in Plenum

[red griff 500]

Is it usual to have an oily film on the inside of the plenum - upstream of the throttle flap valve it is perfectly clean and downstream the walls are covered. The rocker cover breather is connected upstream of the valve and there are no deposits around its outlet into the plenum - the breather is not blocked at all. I did notice that the little atmospheric breather at the back of the nearside rocker cover was blocked, but I've now cleaned it).

The oily deposit was also apparent on the air by-pass stepper motor cone.

Anyone know what causes this?

[GreenV8S]

That's all quite normal. As far as I know it is caused by a combination of oil from the crank case breather, and hot exhaust gases entering the intake manifold under low speed part throttle conditions.

[red griff 500]

phew!!