Smoke

Hi guys, a bit worried. When I have started the car cold in twice in the past two days I am getting a big waft of black smoke from the exhausts upon start up then it immediately clears. normal? Is it because it's being started and moved and not driven? Haven't noticed it before. It's in at porsche tomorrow so I was hoping I can have some understanding before raising it. thanks

Normal

Hi Steve, it's a lot! And I'm old school so smoke means issue. Is it oil sitting after build up? Thanks

Yup normal. Do yourself a favour and refrain from googling it. You'll be up till 4am reading countless threads and theories. My Carrera did it a bit, then it didn't, but sometimes does. Not much though. The 930 didn't, then did a bit, then didn't. Now sometimes after an extended break. If I park it on the wrong angle and oil doesn't drain out of the turbo fully, there's a smoke screen - only happened a couple of times - scared the sh!t out of me. My other one with the 964 turbo engine doesn't at all. So far. Normal. EDIT And I just realised this was in the watercooled section. So there ya go.

Mine won't do it for ages, and then  Wooshka garage is full of smoke, the 993 did it too.It's  just one of those things that boxer engines do occasionally  to cause sleepless nights.

My Boxster did it and it was the AOS needed replacement. It has done it again since a couple of times, scared the sh1t out of me. I had started it, let it go through the choke to idle and moved it, then switched it off. The next start up was smoky. Rare in my case.

Black smoke is generally not an issue (running rich). White/bluish smoke is oil, which can be more of a concern. Then it can be AOS (easy fix) or something more sinister, but that sounds fine.

They hate a cold start then turn off before getting fully warmed up. I always at least take the 64 for a drive around the block, if I just move it around the garage she will blow smoke on the next start.

Same here. They really hate not being driven, so if you're moving cars around, run it for a couple of minutes instead of a quick startup and shutdown - which can cause other false scares

Thanks guys, makes me feel much better. Past two days we have been working around the house with a skip so have moved her out of the garage into the road and back in twice, both occasions black smoke even though I have started and let it run for a minute or two before moving. will mention aop to Porsche tomorrow however have only had this come up in the past two days with not being driven so theory makes sense. Knowing their finely tuned makes sense. thanks again can sleep tonight.....?

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JB To give you a more complete answer the black smoke on start up when very cold is from the richer than normal mixture needed to aid starting.  Since the catalytic converters aren't up to temperature they allow the sooty hydrocarbon rich  exhaust to be seen at the tailpipe. The other blue smoke that the others have hinted at above comes from engine oil finding its way into the combustion chambers after sitting a while.   Since the cylinders are horizontal and in the Mezger engines the oil tank is higher than the sump, it's pretty much inevitable and part of the charm.  You won't notice it often, usually only occurs when your non enthusiast wife, family or friends are watching!

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Not sure this is so true with watercooled as it's a wet sump these days, but the possibility is still there since it pumps oil to the top of the engine on startup. So a quick startup and shutdown could cause what you are talking about on the m96 engine onwards since the oil pumps moves a significant amount of oil back up to the top of the engine during startup. GT series engines from my understanding are still all dry sump.

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Uncanny..?

I understood the rings can align at times. If they are all at '6 o clock' after shut down. Oil can make it's way along to the combustion chamber. Random.

Thanks Mike for that correction.  You are right about the different engine type and I regret I didn't give a clearer answer at all in the end! EDIT: Clarified my original post to reference Mezger engines especially

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ditto with my GT2.

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No worries Peter. Definitely filled in some knowledge for me on the aircooled blowing smoke

Hi guys, thanks again for the help and advice. got the car back today from PCB and along with some other minor warranty things they fixed, they replaced the AOS. Apparently there was something wrong with it, was blocked and taking in too much air.... All good now, driving like a dream. cheers

Glad to hear it's sorted. The AOS is a bit of a pain on the watercooled series. Was it an engine out job on a 4S since it has a different engine?

Hi mike, dont believe so, had the car for a day and a bit, replacing bushes, door lock and battery cables, so I would say no? they didn't seem too phased about it..

I had/ have the same. Normal and sporadic. The last time I pulled the car out the garage to clean it - no smoke. The started it to put back in the garage and ----- puff (always impresses the wife ) if it's ongoing it could be the oil separator needing repayment. as @shadyspeedway says, it's just oil making it was at irregular intervals.

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Interesting... when re-assembling a new motor, we all go to great pains to align the rings and 9, 12 and 3.. so that there is no chance for them to align or so that oil doesnt seep through if the ring gap is at 6pm position.. but if the rings all move around anyway, then why bother with alignment during assembly?

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Hi Mike, I am far from qualified. any explaination here as to why this might be happening?

Rings slowly rotate around the piston.  This is the reason that they are pinned on two stoke engines to prevent them rotating so that a ring end can't spring out the exhaust port and get jagged. As for aligning the ends of the ring ends so they don't coincide when rebuilding I don't know.  OCD I guess.

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Yeah. This is what I was told at trade school. evidently once the bores are glazed and the tension of the new ring has been lost through wear things can move apparently? An old P car guy explained to me.

Not quite right Luke: Ring rotation is required in order for piston rings to run-in and seal properly, they must be able to rotate freely in their grooves. The ring rotation is the result of the honing pattern (cross grinding) on the one hand, and the rocking movement of the piston at the top and bottom dead centre on the other hand. Flatter honing angles cause fewer ring rotations whereas steeper angles result in higher ring revolution rates. The ring rotation also depends on the engine speed. 5 to 15 revolutions per minute are realistic figures to get an idea of the dimension of the ring rotation. On two stroke engines, the rings are secured against rotation. As a result, both ring rotation and rebounding of the joint ends into the gas channels are prevented. Two-stroke engines are mainly used in motorcycles, gardening machines and the like. The irregular wear of the rings, a possible coking in the ring grooves and a restricted service life due to inhibited ring rotation is tolerated in this regard. in any case, this type of application is dimensioned for a shorter service life of the engine from the outset. The requirements made on a normal fourstroke vehicle engine that is in road use, are higher by far where the mileage is concerned. For Mike: The twisting of the ring joints by 120 ° to each other is only intended for facilitating the start of the new engine. After this period, any conceivable position of the piston ring inside the ring groove is possible, provided the rotation is not inhibited by design (two-stroke engines). Reference: Piston Rings for Combustion Engines - Motorservice Interesting comment regarding reduced life of two stroke piston rings being a result of the need to prevent them rotating.  I hadn't made the connection before.  As for the ring joints being set at 120 degrees to aid start up, I can't see that making any measurable difference nowadays with tighter ring end gaps (for improved emission reasons), the bores and rings generally being liberally oiled when first assembled and strong ignition and fuel injection being the norm.  However if we are talking Model T Fords I could see how that could be helpful to have them evenly spaced.

Peter so this means that on any of our motors, at any time, when we shut the motor off, its possible for one or more oil rings to have the gap at the 6oclock position, and hence we can get oil seep into the combustion chamber. Would this explain why sometimes we can get a small puff off blue smoke on startup occasionally and irregularly?

I have pulled the same motor apart about about 20 times and have never found the rings where I left them, however I have never found them lined up in a row either, so starting at 120 intervals means that they maintain some sort overlap during rotation.

Yep! I don't even think the position of the ring gap would be critical as remember the rings relies on the combustion pressure to make a good seal with the bore so when the engine is stopped there is only the ring tension to make that seal. Also the bores get doused with oil flung from the main and big end bearings and on the later cars, piston under crown squirters (used for piston cooling) so there is plenty of it around.  The oil doesn't necessarily drain away because of the horizontal cylinders, particularly if the car is parked on a bit of a side slope, giving plenty of time for a little oil to seep through to the combustion chambers. The only predictor of a smoky startup that I can see is it more likely with an audience! Steve, I don't think the rings would rotate at the same speed as there would be so many factors involved that would vary across each ring that there would be times when the gaps would have to line up occasionally.  Not that I think it matters since the gap is so small and the combustion pressure peak is so quick. Let me guess, pulled down a Ducati 20 times?

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You must be psychic