I have a 60 Cadillac convertible with a 390ci motor, and noticed the last few times out in it a lot of oil vapor coming form the road draft tube, so I did a compression check and sure enough one cylinder has only 25psi, I squerted some marvel mystery oil in that cylinder and it came up to around 40psi, I'm getting no smoke out the exhaust so I'm thinking a stuck ring??? the motor only has about 5000 miles on a rebuild (new rings, bearings etc.) Funny thing is when I first built the car about 6 years ago it did the exact same thing with another motor, dead cylinder, blowby out the road draft tube, I pulled that motor and yanked the head and saw nothing, no score no burnt valve. Any suggestions????
What number cylinder is it? Is there a chance that you have one out-of-round or tapered cylinder? Did you have the original motor rebuilt or a completely new block and heads? Is it running hot? Water in the oil or anything like that? Might just be the wrong size piston in that one bore.....
If it were me, I think I would severly THRASH, ABUSE, and get that motor good and HOT before I pulled it apart, you might scare that ring into compliance. And, all in all, you may as well thrash it... Consider that MAYBE you are being too soft on it in it's break in... That is not good for seating rings.
It's #3 and it is an original stock bore block, the bores were cleaned and honed to brake the glaze, and the heads are redone originals as well, runs cool and there is no coolant loss, just looks like I'm spraying for west nile out of the draft tube!
Having the pressure come up with the oil points to a ring issue. I'd give it a MM soak in that hole for a couple of days then take it out for a hard run. Give it another week of general driving before deciding to tear it down. Bob
A wet compression increase of 15 psi. Indicates valves, not rings. If the heads do not have modern hard seats installed in them and you are using pump gas. You may have valve seat erosion or warped/burned valve. Also possibility of head gasket leak. Possibly bad cam lobe. Any way you go. The heads probably have to come off. Check for warps/cracks and valve seal.
I yanked the valve cover and the valve train is moving as it should, so the cam is fine, I need to hook up my compressor to it and see if I get any air out of the carbs or exhaust, when both valves are closed, this I believe will rule out valve issues if there are no leaks. From past experience head gasket problems with only loss of compression involve blow out between two cylinders and both adjacent cylinders have over 150psi, so I'm ***uming it is not a head gasket, and there is no coolent loss or oil/ water mixing.
After letter her soak with a concoction of MMO, and seafoam, I took it out got it real hot and ran the snot out of it, and still nothing, so I tried the dribble of water down the carb while running, still nothing, I am fairly sure it is the rings, because before I fired it up I looked in the spark plug hole and every bit of liquid was gone, (and it was full to the top) so it looks like I have one more winter project, thank goodness I got my garage built before it got too cold!
a long awaited update I pulled the motor out and tore the one head off, and found....nothing, dropped the pan and it was full of ring and piston pieces, pulled out the piston and all the rings were broken and 2/3rds of the ring lands on the piston were gone, good thing is it did not hurt the bore. I have a spare 390, and was thinking if it is the same bore just yanking a piston out of that one and throw her back together.
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