Curious on blow by

[c1500sbc]

I was checking the all in time on my 383 and making sure it wasn’t adding some at the upper rpm range. When getting the engine up to a higher rpm range it blew the dipstick out and pushed some oil out of the tube onto the headers. This was at idle of course just using my hand to rev the motor up. So afterwards I ran the motor down the road to the rev limiter through 3rd gear and the dipstick wasn’t out.

I’ve never noticed the dipstick being pushed out at all. I’m pretty hard on the motor and I’ve never popped the hood and seen the dipstick out.

Why would that happen at idle, but not under load? I’m going to do a leak down test soon. Motor is pretty fresh so it would be unfortunate to have a ring sealing issue. PVC system is hooked up and working.

[KnightEngines]

With no load there is not much combustion pressure to push the rings against the bores, so they don't seal near as well & there is more blowby.

It is a symptom of not great ring seal tho, they should be able to seal low combustion pressure even without being shoved against the bores - unless you have low tension rings with gas ports.

[mag2555]

The OPs first paragraph is a nightmare to make sense of!

[c1500sbc]

The OPs first paragraph is a nightmare to make sense of!

Ha! Yea, I was a “few” beers in at the time I posted it. Made perfect sense then!

Essentially I’m having an issue at idle, when revving the motor into the higher rpm range the dipstick will pop out and oil will come out of the tube. It does not happen under load, though.

Makes sense that there is little pressure to help seal the rings at idle, but definitely unfortunate to hear it should be doing that. It’s a pretty fresh motor. I could do a leak down test but I don’t see how it would result in anything other than a ring sealing issue. Any suggestions? Would y’all even worry about it? Seems like the only real solution would be to tear it back down.

[MadBill]

Any suggestions? Would y’all even worry about it? Seems like the only real solution would be to tear it back down.

A leak-down test first would at least pinpoint the affected cylinder(s).

[V12MECH]

I think he was referring to total advance ? Is there excessive smoke out the exhaust when free revving? Is the engine a crate deal , or if not , what are ring end gaps, and are they correct for piston type, when doing the leak down test, keep plugs in order and check for oil deposit on them. Or just make sure oil isn't over filled and put some more run time on it and do the leak down.

[superpursuit]

Like KnightEngines said above and it would be worse if the engine was cold (or pistons not up to temperature when you revved it) due to piston rock, especially if your pistons have a higher bore clearance.

Allan.