How much blowby is "too much"?
 

I was driving around in my new-to-me 300 when I spotted a lazy puff of yellowish smoke come out from the right side of the grille. I pulled over and lifted the hood with the engine running to discover that the crankcase vent tube had come disconnected from the air cleaner housing. I reattached it and before I went on my merry way I pulled off the oil filler cap for curiositie's sake. I've seen Russel's video on what a blowby-free engine looks like and lemme tell ya, this ain't it. I can see vapors coming out of the valve cover accompanied by a tubular-sounding pulse.
The engine starts easily and the exhaust doesn't smell like I'm having incomplete combustion. There is practically no smoke under any running conditions so I have no reason to believe that there is a drastic compression drop anywhere in the engine. I should mention that I don't believe that the ALDA is working at the moment. I haven't yet had an opportunity to clean out the banjo bolt.
There are a host of other nagging little problems, but this one bothers me the most. How much should I worry? Is it time to re-ring the engine? I've done it on a Golf with much less room to work so it shouldn't be THAT hard.

Did you rest the oil filler cap in it spot to see how much it dances?

Hah, no. I can though. How much dancing should I look for?

Ideally, no dancing, mine dances a bit, but does not try to fall out. Do a search on blowby or tea kettle dance, and you'll get quite a bit of info. There was a post a month or so ago that had a dancing oil fill cap.

When it becomes hard to start/ runs really rough/ fogs the neighborhood.

I believe a compression test is the only sure way to tell. There may be a couple of "used car lot" tests that will give a hint, such as the filler cap "tea-kettle" test. Most of these have no guage on what is passable or not, so they really only tell you "if" you might suspect a problem.

There is a guage to measure blowby but I don't remember seeing a posting here by anyone who has used one. It seems there is a lot of variation in amounts of blowby in engines that are otherwise fine, so i wouldn't worry.

according to the oil filler cap test my 240D has a lot of blowby but the car runs like a champ so im not at all worried:D

I think my next question would then be, how can I tell if the blowby is coming from worn rings or worn valve stem guides?

Same here, and my car can still start in negative temps (-9F before) without a block heater OR synthetic oil. I have both of those now, so it should start effortlessly this winter. :D

I've driven my car over 40k and its blowby has not increased at all since I got it, so I am not worried whatsoever.

Blowby is combustion product getting past the rings. Oil going down the guides is burnt in chamber and manifests itself by showing out the tailpipe. The broad range of blowby experience leads me to believe that until the air cleaner area becomes highly saturated in a few days after cleaning that area out one can live with it. To lessen the effect a lot just put straight 40 grade in the crankcase during warm/hot weather. Most oil additives quoted to lessen oil burning or increase performance are primarily viscosity increasers as well. Thicker oil aids sealing the rings a little better. Can reduce blowby to half in fact. Also if previous owner or car dealer has thin or very old oil in the crankcase it could be adding a little blowby as well. Unchanged oil does loose some of it's potential viscosity under heat I believe besides the damaging soot effect. Multi grade oils have always just offered to protect to their top rating. In fact they offer a lot less actual viscosity when compared to simular single grade oils in real world service. Internal work on these engines is so cost prohibitive if walls are tapered for example usually it is better to try to locate a good used engine or a nonroadworthy car to puchase cheap for the engine if good. Selling the other parts on the donar car could make this option almost painless. In your part of the country used part sales should be excellent. May as well cover a couple of other variables. If injection pump is too far advanced it will contribute to the effect but how much I really do not know. Also the actual valve clearances will have a vary slight effect as if too tight but still a little clearance the cyilnders mean average pressure will be a little higher where excessive valve clearances would tend to lesen cylinder pressure a little. But enough to produce any reasonable observation? That I do not know.

I used to run 15w40 dino Rotella oil, and now I run 5w40 synthetic Mobil 1 Diesel oil, and my blowby appears to be exactly the same (if not slightly less!) on the lighter oil....how is this possible? :confused:

Blowby is gasses escaping from the combustion chamber into the crankcase, so the oil used will not affect the amount.

I know....it just seems like I have less now.....its probably all in my mind...:dizzy2:

Pawosd, You are not really using a lighter overall oil but rather one with a lower rating on the bottom end. The attempt to mimic viscosity is a heat driven chemical function I believe. The oil is designed to thicken with temperature.There is perhaps a chance the synthetic is just more functional with the viscosity additives than the dino oil. In otherwords it is managing to sustain higher levels of viscosity at elevated temperatures or perhaps is just more responsive to manipulation than the dino oil. Oil today is a real engineered product. I am not sure totally of what I just mentioned but is my best guess. What you are experiencing does not have to be your imagination. My guess when 5w=50 is marketed you would probably be better off using it. Perhaps it's out there now. As long as the weight 5 is followed by a w it should be completly safe and actually better for a high milage engine. Chemistry will continue to move right along in a serious competative market. Wonder what comes next?

Barry,
I think it is actually not correct to sat the multi weight oils thicken with higher temps. I think what happens is that their viscosity remains more or less constant. Single weight oils get thinner as they get hot. So a 5W-30, for example should flow like a single weight 5W oil at colder temps, and like a single weight 30 weight oil at high temps. You could say they thicken at higher temps, but only as compared to how a single weight oil would flow at those higher temps. Clear as mud??