This was the first time i experienced the smoke at start up for about 3-4 seconds. Never saw it before , should i be alarmed , or should there be something i am looking out for?

08 997 tt with 24k miles regular service , recent clutch replacement....

 

Totally normal. It happens from time to time. No worries.

 

Yup its normal

 

It usually happens in hotter weather i find. If you let the engine idle for 30 to 40 seconds before shut down it will usually stop it but it can still happen on the odd occasion. What causes it is fuel leaking off from certain offending injectors after shutdown. The fuel leak off, from being under pressure in the lines between pump and injector can leak out of the injector nozzle and sit in the respective cylinders, and the next time you start will ignite the excess fuel and cause the smoke. You will find the smoke is a greyish colour which also confirms this is a fuel smoke and not oil. The catalytic converters at the time of start are not at full operating temperature therefore are unable to burn off the smoke as it passes through the converters. Oil is a bluish smoke and also has that distinct smell. Fuel smoke under hard acceleration is black however, on start up fuel smoke will maintain a greyish white colour. It seems to be a normal occurrence in these engines, possibly due to the design layout of the engine and fuel system added to the fact the catalytic converters are not at a high enough temperature to arrest the smoke at the time of start up. Why it happens more in hot weather than cold is because in hot weather fuel has more of a tendency to expand placing further pressure on the injector to retain the fuel. As you may know once these turbo engines are shut down often they get hotter before they get colder. That again is because of the confinement of the engine and turbochargers which is why there are thermatic fans which kick in after shutdown to allow the engine compartment to exhaust the build up of hot air. This is also becomes a problem for the battery at one point....another story.

 

Great summary!

 

Excellent technical explanation of the white smoke. I have always thought it was oil, but it bothered me the absence of the distinct oil burning smell.
Obviously the injectors don't leak enough to accumulate large amount of fuel in the cylinder, because in that case it will lead to hydraulic compression at start up and then....New Engine.
But guess if the leak was significant it would have happened to several cars by now....

 

No need to worry. Leakage would be very minor indeed. You only need a small amount of fuel sitting on the piston or liner wall to cause that smoke. Once ignition occurs in that cylinder the fuel left on the piston or liner wall is usually only partially combusted and is then pushed out of the cylinder via the exhaust valves on the exhaust stroke cycle, out the exhaust system. If the cats were at temperature they would burn that unburned fuel off prior leaving the tailpipes and you wouldn't be the wiser. Once pressure in the fuel line has equalised between the high pressure fuel pump and, the injector, the injector is able to do its job of restraining the fuel from further leakage. It's not a case of the injector allowing fuel to run continuously into the cylinder...draining the fuel tank etc. That's not going to happen....so there is no risk of hydraulic locking or anything of that nature. It actually takes an abnormal pressure build up between the pump and injector to overcome the normal restraining pressure settings of the fuel injectors to then allow fuel to leak off into the cylinders. And once that excess fuel pressure is released, which would comprise of a small amount of fuel in the line at best, further flow/leakage becomes restrained once again by the injector's pressure setting. Similarly, it takes only a very small amount of oil to leak past worn valve stem seals or worn valve guides to cause blue smoke on the overrun....but in this case we are talking unburned/partially burned fuel departing the cylinder on start up....and not being arrested by the catalytic converters.

 

very interesting, I would think it is important to let the car run for about half a minuite or so after a drive. It is a good habbit to get yourself into/ i thought it more for the turbo oil pressures and tempature of the turbo but it also seems to be happening on non turbos aswell. I have been eager to get out of the car after a long drive and just shut it off as I pulled into the driveway, I noticed this on my gt3 the following startup the next morning. good info to know, im glad its comman and not just developing issues with my car. I need to relax when I get home and let her run a moment. Thanks for the info speed21 ~ plus 1

 

Intermittent smoking on start is common for all boxer engines - check the Suburu boards you'll see reports there too.

 

Normal. It happens to me if I start my car (cold), move it up a few feet to make room for my second vehicle (in my garage) and come back to it in a day or two then start it up again. It only happens occasionally. Letting you car cool off after a drive is a good habit to get into.

 

^ Exactly the same here, more in the warm weather.

 

I've gotten into the habit of letting car run for a moment before shut down and I use this time to check the oil level.

 

Never saw any smoke on my 50k Carrera S until a cold start last night (followed by yellow engine light illuminating). It seems odd to me that suddenly lots of grayish/blue chemical smelling smoke would appear. The car immediately stops smoking and continues to run like a top. No smoke with warm starts but this has happened three times already after sitting for 3-4 hours.

Any ideas why the yellow engine light might be on? My mechanic is talking about changing the oil separator (?).

Thanks,

J P

 

The yellow CEL is indicative of the presence of an error code or codes which can be accessed with a Durametric or PIWIS reader. Since the CEL came on when the smoke appears, chances are pretty good that the self diagnostic system will lead you to the faulty culprit. A little smoke is quite normal but in the presence of a CEL, you need to find out what its trying to tell you. Best,

 

This is a very good write up, but the OP didn't say what color the smoke is. White smoke on startup can also be an indication of a head gasket leak or other issue that's allowing coolant into the combustion chamber, or into the cat (though that will not be hot enough at cold startup to make it smoke). Of course in the OP's case on a low mileage car that's running fine this is highly unlikely, but it is another reason for white smoke.

Most of the time the smoke is from oil and not fuel though on boxer engines due to the flat configuration and the oil that tends to get into the combustion chamber after sitting for a while. As an owner of multiple 911 and Subaru STI over the years, it's consistent behavior from both flat engines, though the Subarus definitely tend to smoke less. I'm not sure if that's because of the lower number of cylinders, longer exhaust path to the tailpipes, minor differences in the engine design, or who knows what.

Yes, if you didn't have the CEL then I'd just say it was normal oil burning at startup, but the CEL definitely indicates something is wrong. You can get it read for free at any auto parts store, or go buy yourself an OBDII reader since you don't already have one. They pay for themselves over time in just peace of mind plus they let you reset codes to see if they reappear, either when you're trying to see if it was a one-time fluke that it came on, or after you replace a possibly or definitely faulty part.