I chased this in a 420 and almost resolved it, so here are a couple of things to look at:

- the gasket that runs across the top of the cowl fits against the hood. If this is not sealing well, the blower will pull engine fumes into the cabin.

- make sure that the breather hose from the valve cover is connected to the air cleaner. Be sure that all breather hoses are sound and well-connected

- look very carefully for any valve cover leak

- see if there is any sign of a transmission seal that's leaking and getting fluid on the exhaust pipe

- same for the rear engine seal

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe