Problem with compression
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Joined: Sep 2005
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Problem with compression
I am looking for some answers on why and how I lost compression (80lbs) on my 351W that has only 54000 miles on it. This is in a F250 pickup 4wheel drive. I just got done rebuilding it, put in new rings and bearings, had the block, heads, and pistons checked. Everything checked out ok. I checked the compression and is only at 90lbs. I put oil in the cylinders and it comes up to 158lbs. Is there something I am missing or I need to look for? Help
Steve
Steve
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Joined: Mar 2003
Posts: 14,413
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From: Sarnia, Ontario, CANADA!
Hi and welcome to FTE! Be sure to read this to help you around this site: https://www.ford-trucks.com/forums/s...d.php?t=300691 There are also State Chapters located near the bottom of the main forum page.
I moved your thread to the 351 forum. You should get some more help here.
See ya on the boards!
-Matt
I moved your thread to the 351 forum. You should get some more help here.
See ya on the boards!
-Matt
Hotshot
Joined: May 2004
Posts: 12,170
Likes: 5
From: The hills of No. Calif.
You just got done rebuilding it? Are you saying its got 54K since the rebuild, or its got 54K total and you just rebuilt it and now the compression's low? How many miles since the rebuild?
If its just been rebuilt, a couple of things come to mind. First of all, we know from the results of the oil trick that the problem centers around the rings and their sealing in the cylinders. It looks from your post as though all cylinders are low, not just one. What kind of rings did you use? Cast or moly? Cast rings are best if the engine isn't freshly bored out. If it's stock bore and you do a home hone job, and then put moly rings in it, they often won't seat properly casing smoking, blow-by and loss of compression. It could also be a result of a home hone job incorrectly done.
How about the rings? Were they installed on the piston correctly, not upside-down? Depending on the ring, most have some kind of mark on them indicating which side faces up, and sometimes the mark and the instructions that go along with the rings can be a little confusing. How about the ring end-gap? You did check them, right? They should be somewhere around .016 thou. for the top rings and about .010 for the second ring.
If it was just one cylinder on a fresh rebuild, I would suspect a broken ring. These are the main things that I can come up with and unfortunately, there's no easy fix. You can keep running it in the hope that the rings will eventually seat, but you may wind having to pull the motor and go back in.
If its just been rebuilt, a couple of things come to mind. First of all, we know from the results of the oil trick that the problem centers around the rings and their sealing in the cylinders. It looks from your post as though all cylinders are low, not just one. What kind of rings did you use? Cast or moly? Cast rings are best if the engine isn't freshly bored out. If it's stock bore and you do a home hone job, and then put moly rings in it, they often won't seat properly casing smoking, blow-by and loss of compression. It could also be a result of a home hone job incorrectly done.
How about the rings? Were they installed on the piston correctly, not upside-down? Depending on the ring, most have some kind of mark on them indicating which side faces up, and sometimes the mark and the instructions that go along with the rings can be a little confusing. How about the ring end-gap? You did check them, right? They should be somewhere around .016 thou. for the top rings and about .010 for the second ring.
If it was just one cylinder on a fresh rebuild, I would suspect a broken ring. These are the main things that I can come up with and unfortunately, there's no easy fix. You can keep running it in the hope that the rings will eventually seat, but you may wind having to pull the motor and go back in.
