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 Blowby/low compression problem 24 valve 5.9 cummin
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wjbusguy
Active Member

United States
24 Posts

Posted - 05/31/2013 :  2:23:05 PM  Show Profile  Visit wjbusguy's Homepage  Reply with Quote
I am working on a 2000 bluebird/freightliner school bus with a 24 valve motor in it. The story goes like this, the bus had about 125,000 miles on it and it starts blowing oil out of the crankcase breather, that comes off the front cover. I took it to the local cummins dealer and they tell me it has excessive blowby on one cylinder, cant recall which one it was now. The problem got worse and it started missing on that cylinder. I ran it through the rest of the school year, (substitute bus) then pulled it out and tore it down. I couldnt really find a problem with the cylinder, it didnt look any different than the rest of them. The bore was maybe a little loose so I took it the the machine shop and the guy decided it could stand a bore job so he bored it out 20 thousands and put new pistons, rings, ect, new turbo, and rebuilt the head. He was thinking maybe it got hot and took the tension out of the rings or something, and maybe thats why it had the blowby issue. I put it back together and stuck it in the bus and it ran great for the first 500 miles or so then it started blowing oil out of the breather again and eventually got bad enough that it started missing again. I finished the year out with it again and took it back to the cummins shop and they checked the compression and said it has a low cylinder again, number 2, cant remember which one was the problem the other time. I pulled the head today and took the piston out, expecting to find a broken ring or something that I might have done when I put it together but the rings all look good, the bore looks good and the head and head gasket look fine on initial examination. I didnt mike out the bore, piston, or rings the first time because I trusted the machinist to do all that but checking it out today, the bore measures 4.0355, the piston measures 4.030, giving me 5 and one half thousands clearance and the spec is 4 and a half to 6 and a half thousands so the bore and the piston are within the spec. The top ring spec is supposed to be ten to fourteen thousands and my ring measures 16 thousands so its a little loose. Havent checked the middle ring yet. The piston is sooted up down to the top ring and below the top ring and on down, the piston is still nice and shiny and doesnt show any sign of combustion leaking past the top ring. I am a little puzzled by this thing. The cummins dealer said they put air in that cylinder and it seemed to be leaking past the rings and blowing air out of the crankcase breather but so far I cant find a problem with anything. Any ideas on what to look at??

Mike
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