mercruiser 350 positive crankcase ventilation / flame arrestor issue

[skypig]

The saga continues...I got my fuel pump in my Mercruiser 350 and ran my boat up. I noticed that the flame arrestor where the pcv tubes come in was causing the boat to run rough at idle. When I took the flame arrestor off the engine ran 50-75 rpms better at idle. I cleaned the flame arrestor out with simple green. If I take the pcv tubes off the flame arrestor the boat runs great. It appears the fumes from the crankcase (It looks like steam) are choking the motor out. I am thinking I have oil blowing by the rings and am going to have to buy a new motor. I haven't done a compression check or leakdown test. Could it be an exhaust issue like a bad flapper? I think I am going to put some longer 1/2 in heater hose on the valve covers to vent it out of the engine compartment to get through the summer. Any ideas? Thanks, Pat

Edited August 5, 2010 by skypig

[fkboatman]

The saga continues...I got my fuel pump in my Mercruiser 350 and ran my boat up. I noticed that the flame arrestor where the pcv tubes come in was causing the boat to run rough at idle. When I took the flame arrestor off the engine ran 50-75 rpms better at idle. I cleaned the flame arrestor out with simple green. If I take the pcv tubes off the flame arrestor the boat runs great. It appears the fumes from the crankcase (It looks like steam) are choking the motor out. I am thinking I have oil blowing by the rings and am going to have to by a new motor. I haven't done a compression check or leakdown test. Any ideas? Thanks, Pat

[Woodski]

You certainly should not have enough blowby to choke the engine down. If it looks like steam, I would suggest a look under the valve covers to see if there is any water in the oil, you can also check the dipstick color, a chocolate milk color is bad news. A leak down test is needed to see if you might have a bad cylinder head gasket allowing water to get into the crankcase or combustion chamber. Other entry points should be checked also, like the integrity of the exhaust manifolds. If the steam is actually blueish in color, then blowby is the culprit. Leak down test results and understanding where you hear the escaping air will pinpoint where the problem lies.

[skypig]

The oil looks fine. I just think it is blowing by the rings. The thing is it is not a huge amount... however it will kill the motor slowly if I leave them attached at the flame arrestor. I routed 1/2 water hose through the bilge and under the vent and it runs fine. I am pretty sure the motor is shot. I just want to use the boat for the summer. I was hoping someone had a similar occurrence.