I have a 2018 Impreza with about 50,000 miles. A few months ago we noticed that the car emitted a puff of smoke when it was first started in the morning (cold engine start), but only after it had been sitting for a couple of days. We took it to the dealer and the dealer said that the smoke was condensation and nothing to worry about. Since then the problem has gotten worse. The puffs of smoke are larger, and it no longer needs to sit for 2 days. It will emit smoke after sitting for one day, or even just overnight. It still only emits smoke when it is first started (cold engine start). It doesn’t emit smoke every time it is cold started but it happens frequently. The smoke is gray and dissipates very quickly. We have noticed that the dealer always overfills the oil when the oil is changed and the oil is currently overfilled. It is overfilled by about 1/4 to 1/2 quart. We’re not sure what the problem is. I don’t want to take it back to the dealer as they have already charged me to look at it without diagnosing a problem that can be fixed. I have owned the car for 5 years and what we are seeing now is not normal. I would appreciate any advice.
I found this. it might help to have it done. but others here might know more.
That could indicate condensation in the exhaust system, or a leaking head gasket.
Tester
If you are taking short trips, water can accumulate in the exhaust system and give that symptom.
The possibility of a head gasket leak is greater with some Subarus than with most other makes. I’d start by making sure the coolant is topped up in the radiator and the expansion tank and then marking its level when cold. If that level when cold goes down, and no external coolant leaks can be found, further investigation is in order.
Is the smoke grey or is it white? i.e. does it look like steam or clouds? White is likely condensation, grey, especially if darker grey, to me, first guess, that seems more like the engine is running a little too rich at start-up. If that’s the case usually you’d notice a gasoline odor at the same time. If you don’t note an gasoline odor, second guess, you’ve got some carbon deposits in the exhaust system. Especially likely if the car was idled a lot, or driven a lot of short trips, 1-2 miles at a time. Third guess, if the smoke has a blue tinge, that could be problematic valve stem seals. You’d notice some unexplained oil loss usually, if this was the problem.
If you think the problem is carbon deposits in the exhaust system, try driving the car on the freeway for 20-30 mile. Does that make the start-up symptom noticeably less?
Valve stem seals?
I was going to say, this sounds a lot like the old issue where valve stem seals would leak oil into the combustion chamber and produce some oil smoke at start up. The nose is pretty sensitive. Stand behind it when you start it up and produces the smoke. Oil smoke is distinct smell as is coolant. Water condensation will have no smell…
That is what I was thinking… But at 50K miles?? strange… 
Thank you all for your comments! This discussion has been very helpful.
I have been monitoring the coolant level and the car is not losing coolant. It also does not seem to be burning oil. I suppose if it was burning very small amounts I might not have detected it. The smoke is grey in color. We haven’t noticed an odor to the smoke, but we can pay more attention to that.
It seems like the most likely explanations are either condensation picking up carbon deposits in the exhaust system or leaking valve stem seals. I retired just before this problem started and I am driving the car less now, but it does get driven on the highway at highway speeds on occasion. It still emits smoke even after highway driving, sometimes in large amounts. I am concerned about the possibility that the valve stem seals might be leaking. Is there some way to test whether or not they are leaking?
Again, thanks to all of you for your help.
At 50k miles, they probably aren’t. That technology is well-tested by now. My prior VW Rabbit developed a valve stem seal leak problem at around 50K miles, but it was a known problem and was fixed gratis by the manufacturer’s recall. I knew it had an oil usage problem b/c it was losing a quart of oil every 500 miles. There was also a noticeable oil residue on the inner surface of the tail end of the exhaust pipe. If you aren’t noticing anything like that, suggest to not worry about the valve stem seals.
This thread reminds me I had a similar problem on my driveway-bound Corolla a couple of years ago. Corolla has not been driven since Covid problem began in spring of 2020 (long story), only idled in driveway to keep it lubed. I eventually had to add some gasoline to the gas tank, and not allowed to just drive the car to the gas station, so I had to pour gas from a lawn-mower gas container into the car’s tank. Might have been a bad idea in retrospect, but I did this while the engine was idling. As soon as I started pouring the gas into the tank, I noticed a lot black smoke coming out of the tailpipe. My theory is that gas fumes from the gas tank were being pushed through the evap canister into the engine, and that was making the mixture way too rich, and causing the black smoke. The point of this story is that your start-up tail-pipe smoke could also be a problem w/your car’s evap system.