Blow By
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Elder User
Joined: Nov 2009
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Blow By
I sold a 91 7.3 that i heard run before i pulled it and everything was working properly. He called me today and said that he got it in his truck and started it but it had excessive blow by. Compression tested good on all eight, so i am eliminating ring or piston failure. What else cause this. He said it was so bad that it was coming out the rear main. Did he hook something up wrong?
Freshman User
Joined: Sep 2009
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The only thing I can think of is if he somehow plugged off the CDR valve (or a bad CDR valve). That is what is supposed to take the blowby gasses and put them back into the intake to burn them again. If that's plugged those gasses have to come out somewhere! In my experience most of these high compression IDI's have some blowby, especially at idle, then at about 1000 rpms or so it usually will disappear, on a good motor anyway. My 2 cents worth for ya...
Posting Guru
Joined: Jul 2004
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From: West Valley Utah
How does the guy know the blow by is excessive? My dads 87' has 73,000 original and very well cared for miles. It does have a banks turbo and exhaust system. At idle there is quite a bit of visible blow by. By quite a bit I mean it's visible and coming out under pressure. Normal for these engines but what I consider quite a bit.
I'd like to know how he saw blow by coming out the rear main. If that was so, it seems like oil would be blowing all over at the same time.
I'd like to know how he saw blow by coming out the rear main. If that was so, it seems like oil would be blowing all over at the same time.
Cargo Master
Joined: Oct 2008
Posts: 3,330
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From: Eaton, CO
This is one reason I'm hesitant to purchase an engine I never saw run and take someone's word for it. I'm not implying you were dishonest, but I know people around here are when trying to sell their junk. I can usually tell on the phone whether or not they're full of ****, or it will take me a few minutes after meeting the person and observing their junk for sale.
Tell the guy to go over the installation with a fine tooth comb to check and see if everything was done properly. I'm confused about the RMS mention as well.
Block the guy's phone number and email. I HATE it when folks contact me after I sell them a vehicle. This collector did so after I sold him an immaculate '69 Mercedes diesel with 103k miles on it. He called to inform me it acted funny around a left corner at 45mph one day.
Tell the guy to go over the installation with a fine tooth comb to check and see if everything was done properly. I'm confused about the RMS mention as well.
Block the guy's phone number and email. I HATE it when folks contact me after I sell them a vehicle. This collector did so after I sold him an immaculate '69 Mercedes diesel with 103k miles on it. He called to inform me it acted funny around a left corner at 45mph one day.
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Elder User
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He says it gets considerably worse when you give it throttle. They let it run for two hours and no change, but have not driven it yet. The problem i have is he used a backyard mechanic(literally.) This guy has no shop and only the tools he can carry on his truck. Changed the motor in a hay field and has never swapped a diesel motor before. He does drive a 91 IDI though.
Senior User
Joined: Jun 2010
Posts: 361
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From: Western Mass
Have more faith in the back yard mechanic. I did and engine swap with NO prior automotive experience on a concrete slab behind my neighbors barn, using basic hand tools and a borrowed engine lift. not saying this guys any good i'm just saying we're not all terrible
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Elder User
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He called me back today and asked how much oil these trucks hold. When i told him he said "oh s**t i put 4 gallons in." Could all the extra oil cause to much pressure and be causing the blow by
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From: West Valley Utah
With 4 gallons in it I'm surprised oil isn't coming out in more places. Yes with that much oil the blow by is going to look way way worse.
Cargo Master
Joined: Oct 2008
Posts: 3,330
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From: Eaton, CO
LOL!
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