Pressure
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Joined: Dec 2002
Posts: 2
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From: Tulsa USA
Pressure
[updated:LAST EDITED ON 05-Dec-02 AT 03:36 PM (EST)]I have a 78,, 300 6 cyl. I have just replaced the #6 piston, all rings, head gasket, etc...., Still having to much pressure, blowing oil out of oil cap, pvc, dip stick. All this money and work,, still not working right. Did a compression check, a few are in the low 100's, others are in 80 and 90's. Any suggestions?
Posting Guru
Joined: Jun 2002
Posts: 1,146
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From: Colorado
Pressure
I have an 85 Bronco, 300 I6. When it started blowing oil I checked compression. 5 cylinders had over 100lbs. compression, one at20 lbs. When I took it apart, all 6 had broken rings. I suggest you check your piston ring grooves too. I didn't check during the first rebuild, ended up tearing it all down again about 13,000 miles later. I had to replace all 6 pistons. Again, all 6 cylinders had broken rings. Some rings were in 4 or more pieces. Luckily I suffered no cylinder wall damage.
Hotshot
Joined: Oct 2002
Posts: 18,488
Likes: 22
From: The Big, Oregon
Pressure
Most likely, the walls are washboarded. What this means is that they have ripples around the circumference of the walls, causing the rings to loose contact. The cylinders should have at least been cleaned up with a berry brush, and ideally should have been properly honed or bored as needed. The glaze in the walls will not allow the rings to seat properly, and contribute to the blowby. The best thing you can do right now is to tear it all down again, wrap rags around the crank throws, get a cylinder hone from your local parts store, and check the bores with it. If the cylinders are striped, that means the are washboarded. If they aren't, and don't appear to be excessively tapered, you can just run a berry brush through them to break the glaze, and if you haven't run the motor very much, you can re-install the pistons and rings. If you have driven the truck more than about 3 miles, you really should get new rings. If the block is severely washboarded, or appears to be tapered, it should be removed and sent to the machine shop, and the appropriate oversize pistons and rings installed.
Jared
Jared
