white smoke on cold start
05-13-2015, 12:21 PM
#46
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Recently did a 100km fast run, on the highway, parked car , idled 5 mins and switched engine off for a week. When I started again puff of smoke came out which did not last long. Car has done 80k km and had recent AOS change. Oil was changed about 2weeks ago running a tad below normal level.No warning lights. Is this normal?
05-14-2015, 01:03 AM
#47
Race Director
Macster, your theory about oil vapors through the AOS does not mesh with observations; it makes no sense that oil vapors from a cold engine would make it through the AOS and smoke the next startup while oil vapors from a hot engine wouldn't. Set up an experiment to test your theory; I did; that's how I know the smoke is only associated with parking the car cold. Oil leaking past cold rings is the only plausible source of that oil I've heard.
AOS simply failing from old age is plausible, but your theory of old oil contributing doesn't fit the anecdotal evidence which is a surprisingly large % of AOS failures are reported right after people have their oil changed or top off their oil. Remember- this isn't only subjected to oil vapor, but liquid oil as well. Lot of signs point to overfilling the oil as a major contributing factor. Regarding very frequent oil changes, it's well documented in industry that excessive maintenance can actually CAUSE failures just because it increases the risk of human error. Nothing in any of the oil analysis I've had Blackstone do shows my oil any less able to lubricate my engine at 8k or 10k than at 3k. At best, you're wasting money; at worst, you're increasing your risk of human error causing problems 3 or 4-fold.
Low AOS failures at high alt is interesting, too; lower ambient air pressures would lead to less of a pressure differential and a lower volume of air through the AOS. It should lead to higher % levels of oil vapor in the air (vapor pressure), but lower volumes of oil mist and other liquid oil, due to the reduced airflow pulling less oil up.
AOS simply failing from old age is plausible, but your theory of old oil contributing doesn't fit the anecdotal evidence which is a surprisingly large % of AOS failures are reported right after people have their oil changed or top off their oil. Remember- this isn't only subjected to oil vapor, but liquid oil as well. Lot of signs point to overfilling the oil as a major contributing factor. Regarding very frequent oil changes, it's well documented in industry that excessive maintenance can actually CAUSE failures just because it increases the risk of human error. Nothing in any of the oil analysis I've had Blackstone do shows my oil any less able to lubricate my engine at 8k or 10k than at 3k. At best, you're wasting money; at worst, you're increasing your risk of human error causing problems 3 or 4-fold.
Low AOS failures at high alt is interesting, too; lower ambient air pressures would lead to less of a pressure differential and a lower volume of air through the AOS. It should lead to higher % levels of oil vapor in the air (vapor pressure), but lower volumes of oil mist and other liquid oil, due to the reduced airflow pulling less oil up.
If there is any oil vapor it will sometimes make it past the AOS, regardless of the temperature. However, cold oil is less likely to generate oil vapor so when cold the AOS has less work to do.
One drives the car around and the vapor is created. Most gets removed by the AOS, but not all. The vapor that makes it past/through the AOS some of it impacts the intake manifold wall in not making that sharp turn. If one continues to drive the car this oil gets pulled into the engine and burned and because the engine is hot burns cleanly enough there is no smoke.
However, if one shuts off the engine with any oil on the intake walls, if there is enough it will run down into the engine, or on top of the closed intake valves, and the next time the engine is started, well, there's your smoke.
The techs tell me it is quite common whenever a manifold is opened up to find the intake walls oily. My belief is the AOS passes a goodly amount of oil vapor at various times.
Old oil, oil that has some miles on it and is contaminated with water will generate more vapor. This is why given the same driving style day in and day out my Boxster is more prone to smoking after nearly 5K miles on the oil vs. with fresh oil. I could almost time the 5K mile oil change by the the fact the engine smokes more often upon cold start.
I'm sorry but the oil will not seep past the rings. The rings seal real good against the cylinder walls. There is nothing pushing the oil past the rings, rings plural, as there are 3 sets: oil control, and two compression rings.
In fact if the engine is hot the oil tends to want to flow the other way, away from the heat of the hot piston crown and the heat it impart to the cylinder wall. One can see this in action with a cast iron skillet on the stove and some cooking oil in the skillet. As the skillet gets hot the oil tends to move away from the hotter spots of the skillet bottom.
No the oil that is the cause of the oil smoke on start up is from the AOS that simply isn't up to the task. I dare say all AOS's are rather crummy.
if one can't change the oil or give the car the other servicing for fear of making a mistake he should not be servicing the car. This is not a valid excuse to not properly servicing the car.
We can debate the "best" oil change interval. I'm not necessarily locked into 5K miles but it is a good round number and easy to track without having to keep records. Every 5, 10, 15, etc. K miles I change or have changed the oil/filter.
Personally I think changing the oil at less than 5K miles is a bit much unless one is using the car in a severe or harsh usage scenario. IOWs, with lots of idling, short trips, or some track time, I could see 3K miles changes being sensible.
'course, the owner can change the oil as often as he wants. I have no real problem with this, although if the owner doesn't at least change the oil as often as Porsche requires I would prefer not to know this.
For some owners possibly 10K miles is ok. A few oil changes ok my Turbo went 10K miles on the oil. I lost track of the miles as I had family responsibilities that simply took almost all my attention. The engine was fine.
As I have said before I do not want to run the oil right up to the ragged edge. If I have a choice, and I do, I prefer to change the oil while it is still doing an adequate job of protecting the engine. In the engine and engine oil combination, oil is the sacrificial item, not the engine.
As for what accelerates AOS failure I have no real idea. I do not overfill my engine with oil. I believe I treat the car with a reasonable amount of empathy for its mechanicals. And yet every 80K to 100K miles the AOS fails. The thing is just wear item.
So, as with other wear items it can wear out at any time. Some manage 80K miles or more. Some less. (On a related note back in Jan. when I had my Turbo in for what proved to be bad radiators the SM at mid-west Porsche dealer told there was a 2.5L Boxster running running around with over 300K miles on the engines and the original AOS and clutch. 300K miles on the engine and the clutch I can believe. My Boxster's engine and clutch are fast approaching 300K miles and I think will make it to 300K miles and beyond. But 300K miles on the original AOS? No way.)
I suspect infrequent use and oil changes contribute to the AOS's decline. Acid in the oil attacks the diaphragm and then the lack of use means the diaphragm suffers from lack of oiling. Group IV oils (POA) are not very seal friendly. There are additives in the oil to help the seals deal with the POA's lack of seal friendliness. However, the engine has to be run for the oil to contact the diaphragm. For engines that sit unused a lot this could explain shorter AOS life. I believe one reason why I get 80K to 100K miles out of my AOS's is because I drive my car lots.
