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How much blow-by is normal?

65K views 12 replies 7 participants last post by  bigbomar4 
#1 ·
Hello all again. I have a quick easy question this time. How much blow-by are you guys getting out of the oil fill cap with the engine running? A buddy was wondering how much is normal on his 7.3, because he noticed smoke out of the oil fill cap. I also get a fair amount of smoke, but if you put your hand over it, there isn't a lot of pressure blowing out. Are whiffs of smoke normal on an engine with 60k miles on it? The 7.3 has something like 250k miles on it. Thanks!
 
#2 ·
I think the test is to put the cap on the spout upside down and if it blows off you have excessive blow by. My truck smokes pretty good and i only have 91k on it. I think the 7.3's all have a fair amount of blow by.
 
#3 ·
I asked this question too, because I have a what I thought was a significant amount of blowby, but Scuffy says it's normal for my 119K mile truck. It's enough to see at a stop (I have the CCV mod). It pisses the drive through people off enough to make them close the window on me--LMAO
 
#4 ·
I will go try the test. My guess is that the cap will stay on mine, but maybe not the 7.3. I suppose blow-by is not as much of an "issue" on diesels, is that about right?
 
#6 ·
So if you hold your hand over the oil fill, is it "normal" to feel pressure? Mine smokes out of it, but I cannot actually feel air blowing out. Where as some engines, (like our 1975 diesel massey tractor) have a lot of blow-by, to the point where you can hardly hold your finger over the CCV hose that comes off the valve cover.
 
#7 ·
I would say your just fine. I have seen some road tractors that the ccv dumps out under the engine and they were smoking like a steam locomotive. Like i said before, mine also smokes out the cap and like Jon said its normal for diesels to do just that.
 
#13 ·
Yes blow by is caused by this but not all crankcase pressure is caused by this. If the turbo seals go it can also cause crankcase pressure. This is not nearly as commen but should still be checked before you start pulling engines. Its a quick check just grab the turbo shaft if it moves side to side the bearings are gone replace the turbo and recheck. If the shaft doesnt move side to side then its blow by and not the turbo. DONT FORGET THE SIMPLE CHECKS!!!
 
#12 ·
Ford Spec for blow by is 3" H2O using a gauge and a special tool.

Recommended Procedure:
Note: Do not plug hole on Crankcase Orifice Restrictor Tool 014-00743.

Make sure the engine is up to operating temperature. A cold engine will give higher readings. Remove the ducting to the turbocharger inlet pipe and remove the inlet pipe and elbow that connects to the breather box. Block the outlet at the breather box with the cap provided in Pressure Test Adapter Kit 014-00761 or equivalent. Install a protective screen over the turbocharger inlet.

Screw the Crankcase Orifice Restrictor Tool 014-00743 and Crankcase Pressure Test Adapter or equivalent in the oil fill cap hole. Plumb to the Magnehelic® gauge in the gauge block. Make sure the Magnehelic® gauge has been zeroed.

Start the engine and operate at 3400 rpm. Hold for 30 seconds minimum and take a stabilized reading. Do not block the hole at the top of the restrictor tool.
 
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