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Old 04-28-2011, 09:32 AM   #1
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My car smokes after 2 cold starts in a row

I've noticed my 2000 S Tip would occasionally smoke out the exhaust when I started it but most of the time it's smoke free. Yesterday I finally recognized a pattern - if I start the car and only move it a few feet, then restart it again much later (like the next day) it smokes quite abundantly. I can't tell if it's all oil smoke or a mixture of oil and unburnt fuel. Even at over 100k miles the car doesn't seem to consume any oil between 5,000 mile change intervals.

This week with the severe weather here in Arkansas I started the car to back it up a little farther in the garage so I could squeeze a motorcycle in front. I didn't get around to driving the car again until today and it smoked a bunch when I started it.



Any ideas what causes this and whether I should worry about it?
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Old 04-28-2011, 12:20 PM   #2
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The Flat Six engine used in Boxsters like to pool oil, especially after short "grocery hops".

The only cure for this minor issue is to drive your Boxster as if you stole it.

Seriously......When my wife drives ours, it smokes a little because she drives like
a good law abider while doing short runs. It doesn't smoke when I drive it.
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Old 04-28-2011, 12:23 PM   #3
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Same here on my 2000S...if I move and then come back and move it again it smoke a little. If haven't replaced the AOS...I would, mine went at 65000K.

I see you have Vet...I have had a 58, 67, and a 68. Just not into the new Vet look.
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Old 04-28-2011, 12:51 PM   #4
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My AOS is being replaced now at 59.9k along with the shift shaft gasket.
The AOS also made a "belt needs to be replaced" type of screeching sound, smoked off a traffic light and finally the CEL went on.

Do you have any other symptoms?
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Old 04-28-2011, 01:37 PM   #5
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Thanks for the replies. No other symptoms. I've been having trouble with a fuel trim CEL and the car was running rich at low load and lean at high load (didn't make any sense) and a new MAF sensor cured it per the Durametric readings and subsequently I've been totally free of CELs. It did the same kind of smoking before changing the MAF sensor.

I suspect the cold start enrichment might be the culprit but I have no idea how it works. My car acts like the first start is OK but then it uses that value as the starting point for the second start and enrichens it again, sort of doubling up on enrichment. That might not really be what's happening - you know how owners get an idea about something that's totally off base.

It's hard to see how oil migration could be different. I mean the oil has a full day to drain back down to the sump.
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Old 04-28-2011, 05:47 PM   #6
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I noticed a similar pattern on my 2001 2.7 Tip...

If I drive my car out from its usual parking spot to down the driveway to wash (total of about 20ft), it will throw out a huge cloud of smoke the next morning, almost guaranteed (if I don't go for a looong drive after the wash, I usually just drive around the block to dry the brakes and that isn't long enough).

Other than that, it only smokes once in a fairly long while.

The car used to throw out smoke once every 8 to 10 cold starts regardless, but I discovered that it was the brand of oil I was using - voltronic. It was a blast as the power was good, but it clatters on cold starts and the car used to throw out smoke more often.... changed to elf and almost reduced all the clatter, and greatly reduced the frequency of the start up smoke (but not the "morning after the car wash smoke").

I've always assumed that my car is normal after I had post about the smoking issue previously...
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Old 04-28-2011, 06:24 PM   #7
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Quote:
Originally Posted by Dale_K
I've noticed my 2000 S Tip would occasionally smoke out the exhaust when I started it but most of the time it's smoke free. Yesterday I finally recognized a pattern - if I start the car and only move it a few feet, then restart it again much later (like the next day) it smokes quite abundantly. I can't tell if it's all oil smoke or a mixture of oil and unburnt fuel. Even at over 100k miles the car doesn't seem to consume any oil between 5,000 mile change intervals.

This week with the severe weather here in Arkansas I started the car to back it up a little farther in the garage so I could squeeze a motorcycle in front. I didn't get around to driving the car again until today and it smoked a bunch when I started it.



Any ideas what causes this and whether I should worry about it?
Dale,
While the car is running open the oil fill cap. Does the idle go up or down? Do you hear a lot of suction from the oil file tube like a shop vac?

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Old 04-29-2011, 08:57 AM   #8
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Quote:
Originally Posted by Dale_K
Thanks for the replies. No other symptoms. I've been having trouble with a fuel trim CEL and the car was running rich at low load and lean at high load (didn't make any sense) and a new MAF sensor cured it per the Durametric readings and subsequently I've been totally free of CELs. It did the same kind of smoking before changing the MAF sensor.

I suspect the cold start enrichment might be the culprit but I have no idea how it works. My car acts like the first start is OK but then it uses that value as the starting point for the second start and enrichens it again, sort of doubling up on enrichment. That might not really be what's happening - you know how owners get an idea about something that's totally off base.

It's hard to see how oil migration could be different. I mean the oil has a full day to drain back down to the sump.


Horizontially opposed cylinders don't drain down very well. The worst normal operation you can do to any engine is start it & not allow it to fully warm up to normal operating temp. With over 100,000 miles on your engine I would assume your cylinder walls are worn & possibly out of round.
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Old 04-29-2011, 10:53 AM   #9
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Here is my best understanding of the start up fuel injection control sequence:

The DME controls cold start using engine temp and rpm as the critical signals that regulate injector pluse time and ignition timing. Normally, the injectors will pluse longer to enrich the mixture along with maintaining the timing at near TDC to get a clean and smooth start.

If the engine fails to immediately start (as sensed by low rpm), then the ignition timing is advanced and the mixture further enriched to get the engine to start but not to the point where the engine would flood with fuel.

Once the engine starts, the DME will keep the mixture enriched for 5-30 seconds and continue to adjust the timing based mostly on engine temp. This enrichment is gradually reduced as the post start timer (time since engine start) reaches the 30-sec mark.

After post-start, the DME transitions to the warm-up phase where the engine is controlled by the stored map control feedback loop.

Of course, this is all happening in a closed-loop feedback system designed to start the engine smoothly and then maintain a smooth post-start idle.

If you were to stop the engine 10-20 seconds after start, then upon re-start it would begin this process all over. However, with the engine shut off in a somewhat rich condition, this should improve the re-start (since the mixture is already rich) but then the engine could run slightly "over rich" for 10-20 more seconds until the post start timer reaches 30-sec mark because the DME hasn't transitioned to using the O2 feeback system to maintain the mixture until the warm-up phase.

However, the amount of fuel that is injected to enrich the mixture at startup is only a teeny tiny amount more than normal and even a slightly over-enriched mix would still only contain slightly more fuel than a perfect mixture. Plus, a rich fuel mixture rarely (ever?) causes an engine to smoke.

Engine smoke is almost always related to oil in some way.
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Last edited by thstone; 04-29-2011 at 10:55 AM.
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Old 04-29-2011, 11:03 AM   #10
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Blue = Oil Black = Unburned Fuel

Quote:
Originally Posted by Dale_K

Any ideas what causes this and whether I should worry about it?
If the smoke is blueish in color it's oil, if it's black it's unburned fuel.

I do not agree with the assessment that "Engine smoke is almost always related to oil in some way." Not so.

Don't worry about it unless it smokes ALL the time when warmed up and driving.. then you need to start looking for things to replace or repair.

Regards
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Old 04-30-2011, 03:56 AM   #11
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Thanks for the info on the cold start sequence. I'm sure I ran the car less than 30 seconds when I backed it up 3 feet in the garage to make room for the Harley. The car is smoke free on a cold start if I park it in a fully warmed up condition and I don't think the smoke I'm seeing is 100% oil.

It seems some other owners see a similar condition but not everybody. My oil cap doesn't have excessive vacuum, which makes me think the AOS is OK. I'm not going to obsess about it. Maybe somebody else will figure it out on their car.
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