OK, when it comes to leaking valve seals, I completely understand a puff of smoke upon start up. I can understand oil seeping through the seals and running down the valve guides into the cylinder when the engine is at rest and being burned off at startup. That all makes sense.

But another classic symptom of bad valve seals is a puff of smoke after idling for a while.

So here is the $64 question:

How does oil get down through the valve guides with the cylinders under compression?
I have tried and tried to wrap my mind around this. Am I missing something? 

I know somebody has the answer.
C'mon, out with it!

Well the compression is in the cylinder, The valve guide seal is in the vacuume
of the intake system. oil gets sucked down with the intake of gas and air.
BUT idle and loading up with oil is more likley bad rings, now don't you wish
you just left this question alone

 
Wild

now don't you wish you just left this question alone

If I had bad rings, I indeed would wish I hadn't asked. However, my compression test tells me it's seals.

So now what's the answer? 

I did my seals a couple weeks ago. Yup, need to pull the head again... the rings now show their old age. This happened to me before, and I said I'd never do another head job without replacing the rings. It was smoking really bad with the 5W30 oil until it would warm up a bit, so I put in 15W40 and it's not so bad now, so I'm going to wait until the fall and I'm going to rebuild the bottom end, at least the head is done 

All your compression test is telling you is that you have a good compression ring, what about the oil control ring and the scraper?

All your compression test is telling you is that you have a good compression ring, what about the oil control ring and the scraper?

Look, if the compression rings are good and compression is good and stable, then there should be no blow-by and therefore rings are a non-issue. I have rebuilt several motors and done a lot of top ends on dirtbikes and quads. I'm not a newbie to the whole diagnostic scene.

My original question wasn't if I need new valve seals - I have determined that my engine does, in fact need new valve seals - my question was why an engine with bad valve seals will smoke (upon acceleration) after idling for a period of time.
I've had more than one vehicle do this and valve seals did in fact fix the problem.

So while I appreciate the helpful spirit, I'm really looking for a physics lesson. 

Oil that leaked by the seals will soak into your carbon deposits and will not burn off untill you get it hot enough to do so.

Where did ya go I see your online?

Ok food for thought time, ok where are places that carbon could build up? I will start with the piston top, what does your egr valve do and when does it work?   your turn!

Ok food for thought time, ok where are places that carbon could build up? I will start with the piston top, what does your egr valve do and when does it work?   your turn!

Hmmm. Interesting. Hadn't thought of that. Makes sense. I have seen carbon buildup in small engines that was very oily - even under high rpm conditions. Guess it could soak in there and burn off slowly. I would think that it would gather around the valve ports before the piston top, but yeah, I'll buy that.

Here is a theory of mine - see if it makes any sense:

When the engine is at idle, the seals are relaxed and allow oil to bypass. The oil sits in the valve guides. Upon acceleration, vacuum builds as air/gases flow in/out of the valves. This vacuum pulls the oil out of the valve guides and into the combustion chamber and exhaust port where it is burned off. At the same time, the vacuum also pulls on the valve seals, effectively resealing them (why you don't get smoke under load) until the next time the engine is at rest or at low rpm (idle).

It's just a theory. I like to think and hypothesize about this junk.

Quote from: mrfuelish on April 30, 2008, 10:41:29 AM

Ok food for thought time, ok where are places that carbon could build up? I will start with the piston top, what does your egr valve do and when does it work?   your turn!

When the engine is at idle, the seals are relaxed and allow oil to bypass. The oil sits in the valve guides. Upon acceleration, vacuum builds as air/gases flow in/out of the valves. This vacuum pulls the oil out of the valve guides and into the combustion chamber and exhaust port where it is burned off. At the same time, the vacuum also pulls on the valve seals, effectively resealing them (why you don't get smoke under load) until the next time the engine is at rest or at low rpm (idle).

It's just a theory. I like to think and hypothesize about this junk.

Have you ever put a vacuum gage on an intake ?  the motor makes vacuum at idle, as the rpm's come up the vacuum drops witch is why you don't get smoke under load(if you only have bad valve seals).  I think you may want to check out the valve guides too

Ryan

I was playing devils advocate on the second post, but Mr Fuelish hit the answer
oil builds up and burns off when cyl temps rise.

I was playing devils advocate on the second post

Suuuurrrrre!

oil builds up and burns off when cyl temps rise.

OK, so why don't bad valve seals smoke all the time ie: highway driving? 
There has got to be a reason oil stops flowing down the valve stems/guides during normal operation.

Quote from: wildgoody on May 01, 2008, 07:40:25 PM

I was playing devils advocate on the second post

Suuuurrrrre!

Quote from: wildgoody on May 01, 2008, 07:40:25 PM

oil builds up and burns off when cyl temps rise.

OK, so why don't bad valve seals smoke all the time ie: highway driving? 
There has got to be a reason oil stops flowing down the valve stems/guides during normal operation.

no oil build up/ the acclimated oil gets burnt off when you start the rig after it's runing there nothing to burn 

no oil build up/ the acclimated oil gets burnt off when you start the rig after it's runing there nothing to burn 

But oil does build up at idle. What gives there? 

I guess it's not important -  I just wondered.
The valve seals are fixed. No more smoke.
It seems the question keeps getting re-hashed.