What cause's blow by? Type 4 engine

[Davey1188]

just trying to find out what cause's blow by?
Is it pressure getting by worn rings in to cank case.

Thanks LOTS TO LEARN

[cmonSTART]

When the piston rings become worn past their service limits compression gases can leak past into the crankcase, and crankcase gases can leak into the combustion chambers (resulting in that burning oil smoke you see on a worn engine.) A compression test can help determine things.

Sometimes other things can cause excessive blowby, like a bad PCV or plugged exhaust.

What is your Bus doing?
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'78 Bus 2.0FI
de K1IGS

[SGKent]

some blow by is normal. excessive blowby can be:
worn rings
worn valve guides
cracked head
cracked piston
scuffed cylinders
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[Wildthings]

All four stroke engines need some kind of crankcase ventilation to remove the build up of blowby in the crankcase. The PCV system tends not to give much trouble so most people don't realize its there and even if they do don't understand its importance.

[timvw7476]

It would be incorrect to blame 'worn' rings.
Rings can break on assembly from bad install, they can break via hydro lock or thinned lube thanks to a failed fuel pump diaphragm dumping gas in the sump.
And they can seize due to overheat.

But blow by is usually a tired piston, meaning collapsed skirt dimensions allowing rocking in the cylinder preventing a decent ring seal, or ring lands themselves opened up too much, so the ring again can't seat compression pressure properly, meaning that pressure gets vented to the crankcase.

No bueno. It's two measurements, ring side clearance, & piston to cylinder wall. Once those two, either one, are beyond VW spec, you've got issues, or you're going to have them real soon.
VW's .009" cylinder wall/piston clearance limit is a bit optimistic. Even .007" is a messy oil burning engine in a higher revving Type IV 1800. Easily 1 quart per 1,000 miles usage. I've done it, new pistons, rings & cylinders are the better route every time, unless you simply cannot get new & must re-ring.

[Davey1188]

[Davey1188]

[Davey1188]

[TomWesty]

Check to see if there is something restricting your crankcase breather system. Your engine may be fine otherwise, but something may blocking it from farting out through the normal breather system, so the gasses seek the path of least resistance. I don't know Type 4 engines, so I don't know the breather setup.
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[SGKent]

[Crashly]

Worn rings and worn liners are the results of blow by. I have seen a few things that normally cause it.
1. Dirt getting past the filter. Very fine sand makes it way past most filters.
2. Torn or missing air filters.
3. Wear and tear with good oil.
4. wear and tear with bad oil.
5. Poor quality parts
6. Parts that dont fit properly.
7. Sticking rings from carbon build up, or any other debris that has made it way into the cylinder.
8. Cracked rings
9. Bad valves
10. carbon on valve faces

Long story short it pays to have a " blow by" tester. It is fairly cheap and it will tell you where the problem lays. Another tool would be one of the cheap borescopes. This will allow you to see part of the piston and valves.

[timvw7476]

[SGKent]

depending on other conditions and wear, the piston skirts can sometimes be brought back to clearance using a rawhide mallet and a properly skilled whack. It is not optimal but it will last a reasonable period. It's a feel thing.
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[cmonSTART]

Lots of good info above. If it helps the OP read this:

https://mechanicbase.com/engine/what-is-a-blow-by/

It's pretty basic but mostly good IMO.

Again though, let us know what exactly you're seeing with your Bus and maybe we can narrow things down a bit.
_________________
'78 Bus 2.0FI
de K1IGS