Lack of Blowby on 7.3

[IHDiesel445]

I took the oil cap off with the engine running today to check for blowby. There was NOTHING coming out. The engine had only been running for 15 minutes or so. Is that enough time? Or does the engine need to be hot? Could there be something wrong if there is no blowby? I wouldn't think so but, I certainly don't know everything.

Eric

 

[tonkadoctor]

No Blowby is a good thing. You don't want blowby.

Blowby is usually an indication of the condition of the compression rings (top rings) on the pistons and can be caused by bad valves and a few other things like bad oil seals on a turbocharger if you have one.

 

[zigg]

I took the oil cap off with the engine running today to check for blowby. There was NOTHING coming out.

Eric

You gotta realize where the blow-by normally goes.

On the back of the intake manifold, there is a hole. The blowby is sucked up outa the crank area via the valley pan, through the CDR valve(big canister bolted onto the back of the intake) and into the intake manifold.

If you pull off the oil fill cap, the blow-by is still being sucked into the manifold via the valley, so in order to see how much blow-by there really is, you need to remove the CDR valve(only held on by 2 bolts on the back of the intake) and plug the hole in the valley, so the blow-by is now being pushed up/out of the oil fill hole.

The engine needs to be at operating temp, and you'll likely see more blow-by at idle than at higher rpm.

Zigg

 

[Mr_Roboto]

Is your wife too thin and beautiful too?

Jeeez.

 

[Agnem]

I'd check it when it's hot after a good hard pull, and with the air cleaner off. Not to burst your bubble, but the Moosestang has no blow by when cold, but smokes em' out at traffic lights when hot.

 

[Diesel JD]

I think a lot of that no blowby cold and a lot hot has to do with the fast idle being on cold. On a semi healthy motor, the blowby will go away as soon as you get the rpms just a bit above idle. If it is really excessive you won't be able to miss it even with the CDR on and the air cleaner attached. Say if you had real bad rings or a cracked piston. Now these guys are right that to know what you really have you need to at least take the air cleaner off and either plug the valley pan or leave the oil fill cap on and watch what comes out of the valley pan hole. Even better would be to plug every orifice except the oild dipstick tube and use a manometer.

 

[Mr_Roboto]

Once you take the air cleaner off you lose your vacuum so yes you will see more blowby out the filler tube.

In a non throttled engine, the only vacuum is from intake restriction, primarily the air cleaner. So the higher the RPM (air flow) the higher the vacuum level).

 

[typ4]

Dirty air filter??