Question about Blowby
Senior User
Joined: Aug 2002
Posts: 327
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From: Belleville USA
Question about Blowby
Hello, this is my first post to the engine forum, but I thought I should ask the experts. Can rebuilt head that have not fully seated seals cause blowby? I bought a '71 F100 with a 360 and the guy said he work on the heads. Now the blued smoke has stopped coming from the tailpipe and I can't see oil vapor shooting out of the oil filler cap anymore, so I am assuming the blowby is gone. I'm not an FE engine technician by any means of the word, but the more I read this forum, the more I learn. I appreciate the guys at FTE. I think I should send them a donation or something? Thanks again for your commments.
Senior User
Joined: Mar 2002
Posts: 288
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From: Farmington MN U. S. A.
Question about Blowby
[updated:LAST EDITED ON 03-Oct-02 AT 10:47 PM (EST)]I dont know if poor seated seals is considered "blow by" but as far as I understood, replaceing bad seals would stop oil up in the valvetrain from leaking into the combustion chamber.(Thus no blue smoke) I always understood "blow by" to be gases getting past the piston rings. However, I am young and have been wrong before. My 2 cents! 
Elder User
Joined: May 2002
Posts: 799
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Question about Blowby
Blowby is normally associated with a small fraction of the combustion that gases *blow by* the rings, through the ring gap or just past the rings in other zones. Older engines with more wear, or with rings stuck in their respective grooves, or with bad wear patterns in the bore would have more blow-by.
The valve seals are another kind of leak. Usually blue smoke on startup is an indicator of bad intake valve stem seals. My startup blue smoke slowly got better when I started using Delo 400 15W-40. It may have just been that I was putting more miles on it and flexing the seals -- it sat for a while before I bought it.
I think you can also get blue smoke from a bad auto trans modulator.
1972 F250 Custom Sport Camper Special 4x2 360 2v C6 3.73
Soon to have a 104" wheelbase as part of a Broncification project
My FTE Website -- http://www.clubfte.com/users/mlf72f250/index.html
My Galleries -- https://www.ford-trucks.com/user_gallery/displaygallery.php?userid=1108
The valve seals are another kind of leak. Usually blue smoke on startup is an indicator of bad intake valve stem seals. My startup blue smoke slowly got better when I started using Delo 400 15W-40. It may have just been that I was putting more miles on it and flexing the seals -- it sat for a while before I bought it.
I think you can also get blue smoke from a bad auto trans modulator.
1972 F250 Custom Sport Camper Special 4x2 360 2v C6 3.73
Soon to have a 104" wheelbase as part of a Broncification project
My FTE Website -- http://www.clubfte.com/users/mlf72f250/index.html
My Galleries -- https://www.ford-trucks.com/user_gallery/displaygallery.php?userid=1108
Senior User
Joined: Aug 2002
Posts: 327
Likes: 1
From: Belleville USA
Question about Blowby
OK, now you two have taken me to another question. The blue smoke I spoke of earlier has stopped, so I assume valve seals have seated properly.
But, oil blows out of the valve cover oil filler hole through the PVC valve. I have tried to watch it happen but I cannot physically see it coming out, but it is on the underside of my hood. Now, there was a time (when the blue smoke was coming out at startup) that I could see the oil vapor (smoke) coming from the oil filler hole in the valve cover. Maybe I should wash the oil off the underside of the hood and see if that is just residual (but it is wet so I don't think it's residual) What praytell is causing this oil to blow out of the oil filler hole?
But, oil blows out of the valve cover oil filler hole through the PVC valve. I have tried to watch it happen but I cannot physically see it coming out, but it is on the underside of my hood. Now, there was a time (when the blue smoke was coming out at startup) that I could see the oil vapor (smoke) coming from the oil filler hole in the valve cover. Maybe I should wash the oil off the underside of the hood and see if that is just residual (but it is wet so I don't think it's residual) What praytell is causing this oil to blow out of the oil filler hole?
Elder User
Joined: May 2002
Posts: 799
Likes: 0
Question about Blowby
When everthing is working normally, there should be a slight draw through the PCV valve that keeps the vapors from wafting out of every opening. In fact, depending on the system, there might even be a slight vacuum in the crank-case. At high load things are different. There, you often have greater blow-by, there is also less vacuum (because the throtle is wide open) to clear it from the crank-case. In this case, oil and vapors will tend to push out.
I have an oily film covering almost all of my engine (and frame and rear axle, shocks, etc.). When I get some time I'm going to clean it and go after the possible leaks one by one until things stay dry.
Good luck!
Well, I'm off to weld up a crack in my frame -- it never ends...
--Matt
1972 F250 Custom Sport Camper Special 4x2 360 2v C6 3.73
Soon to have a 104" wheelbase as part of a Broncification project
My FTE Website -- http://www.clubfte.com/users/mlf72f250/index.html
My Galleries -- https://www.ford-trucks.com/user_gallery/displaygallery.php?userid=1108
I have an oily film covering almost all of my engine (and frame and rear axle, shocks, etc.). When I get some time I'm going to clean it and go after the possible leaks one by one until things stay dry.
Good luck!
Well, I'm off to weld up a crack in my frame -- it never ends...
--Matt
1972 F250 Custom Sport Camper Special 4x2 360 2v C6 3.73
Soon to have a 104" wheelbase as part of a Broncification project
My FTE Website -- http://www.clubfte.com/users/mlf72f250/index.html
My Galleries -- https://www.ford-trucks.com/user_gallery/displaygallery.php?userid=1108
