Intermittent smoke

[iriejedi]

I have a 1993 Subaru Legacy that has a strong engine but it smokes intermittently (not always but usually) in a big puff when I am traveling down a mountain (I live in Colorado) on the interstate and I begin to accelerate after costing in gear a good ways down to help slow the decent. More recently will also let out a smaller puff of smoke while traveling down the interstate at a constant speed in a more level place like Denver if I start accelerating, although that doesn't happen as consistently.

 

I have replaced the timing belt twice (it's been happening the last 40,000 miles) and understand this procedure as well as have done it on multiple cars. It is definitely not a timing problem. I am curious if the issue is a result of some sort of blow by, rings, or if there is some other possibility such as intake parts/valves that could need replaced? Ideas? I would like to resolve this without having to take the long block engine apart if possible! Overall the EJ22 runs as strong as any I have had and there is no suggestion she is ready to quit. There is roughly 230,000 miles on this well maintained machine.

[mikaleda]

I'm interested in this as well, I've had this happen a few times myself. I am battling an oil comsuption issue as well. My engine has about 50k miles on it. I drive my car fairly hard though

[desertsubaru]

What color is the smoke. Does it use any oil or coolant?  Could just be valve guides. Also about 100 miles before your next oil put some seafoam or Marvels mystery oil in the oil to clean things up and it helps sticking rings.

[forester2002s]

Change the PCV Valve, and see if that makes a difference. 

[1 Lucky Texan]

yeah, I think the extended period of high vacuum just puts more than a normal amount of oil vapor in the PCV system so, the above are good suggestions.

 

could be some slightly stuck oil control rings on the pistons, weak PCV valve, etc. or just worn parts letting extra oil past. maybe experiment switching to a high mileage oil or heavier viscosity?

[Bushwick]

Are either cars mentioned in post 1 and 2 turbo'd? If you see smoke like you are describing on a turbo engine, quite often the turbo is failing, though most often the smoke is actually coolant being burnt off and will be white vs. bluish-grey (oil). If at all possible (goes to either of you with smoking engines) and safe to do so, immediately pull over and discern what you are actually burning by trying to get a whiff of it. Burning coolant is sweetly smell and it hangs in the air awhile. Sometimes when cold out and you first start an engine, you'll see a more white-colored vapor but it disperses almost as fast as it leaves the muffler, which is normal. Burning oil is rather pungent. So much so it can assault the senses and even be detected with windows up and heat/AC on IF it's not set to recirculate, just by driving behind someone that might be 5 cars ahead of you as it hangs heavily in the air.

 

As already mentioned, PCV valve can be an issue. Some engines are poorly designed with regards to their PCV system, others are VERY sensitive to a malfunctioning PCV valve that often gums up and/or the check-valve sticks. These are also problematic on turbo'd cars too as they often run to the turbo's inlet which pulls too much vacuum through the line which causes oil to get past the baffling in the valve cover, which then gets pulled IN to the turbo's inlet. Under lighter driving, oil will pool where it can until it either pools too much, or certain circumstances will allow it to get sucked up from the pooling and you'll see it burnt out the pipe.

 

Heavily worn valves can cause smoking too, though I think you'd see small puffs of it vs. large amounts. Rings when bad seem to be constant smoke and very clearly oil being burnt. 80-90's Chryslers were notorious for weak rings and puking oil, though their turbos were also culprit too.

 

Excessive carbon deposits on valves can also cause smoking. Usually it's a darker grey (w/o the blue oil burns with) and occurs when you step on it. Seems to be caused by people overly babying a car and/or running 87 (or low octane in general as this is a side-effect of it people rarely mention). Really dirty engines will have rather large deposits build up internally on the valve faces and piston tops. When you see a car tromp on the gas trying to merge with traffic on the highway (going from say 25 to 75mph) and there's a small amount of solid grey smoke that doesn't really linger or spread far, that's probably carbon deposits getting burnt. Even newer cars exhibit this if driven too lightly constantly.

 

Other thing that can cause smoke that's harder to deduce is a leaky master cylinder. Since your brake booster is powered by vacuum, and the master cylinder connects right at the brake booster, it is possible for fluid to get past the seal and end up in the booster. Going from cruising/accelerating to decelerating, changes the engine vacuum rather significantly, even on an NA engine. On turbo engines, you are producing vacuum at idle, and will go from vacuum to boost under harder acceleration. Very light throttle and you can actually keep it from creating boost. Whereas a straight up NA engine will get to right around 0 on hard acceleration then go back to vacuum on decel or idling. If you are in say 5th gear or OD and coasting at near idle RPMS, it should be going into near full vacuum. As you can realize, going from basically zero (no vacuum or boost) to a 12-15 vacuum can have interesting effects on liquids within an engine. Check the brake level. If low, top off with correct fluid and keep an eye on it. It'd shouldn't ever drop unless there's an actual leak, which you can smell though it's not as strong as say coolant. You won't see or smell any leaks if it's leaking into the booster. Foot pedal might feel softer and go to the floor or travel further than before, but pump 2-3 times and will firm up. Bleeding won't correct the pedal feel.

 

Also, to rule out PCV, pull your intake inlet hose off the throttle body or turbo inlet if turbo, and inspect either the inside of the TB or the impeller of the turbo for the presence of oil as that's a clear indicator. Just in case Subaru did something different due to the boxer design, find the PCV hose and follow it to where it connects to TB, intake, turbo, etc. and inspect the PCV line and whatever it attaches to instead for oil. REPLACE the PCV vs. trying to clean it with degreaser. It should rattle when shaken, but that doesn't means it's OK. It should seal nearly perfectly when blowing in the one side and blow straight through when blowing through the other side. Really old ones or ones that might have internal cracking preventing the sealing, will have you running in circles when paying $4 would have solved it.

[mikaleda]

I've tried most everything, mines a NA. I've replaced pcv with OEM new one, I've done an intake cleaning with seafoam, I've tried different types of oil to no avail. So far the only thing I can attribute it to is the way I drive must be causing excessive oil to be sucked into the baffels when I hit high rpms and then when I engine brake down hills it sucks the oil up into the intake and burns it. I've been going through about 1.5 to 2 QTS per 3000 miles.

[Fairtax4me]

Usually bad valve stem seals.

 

When coasting there is high vacuum build in the intake manifold, but little air flow because the throttle plate is closed. The vacuum pulls oil past the valve stem seals and it just trickles down the stem but then gets pulled back UP into the intake manifold. When you open the throttle plate to begin accelerating again, then there is a large flow of air into the manifold , which pulls the oil down into the cylinders and it burns off with the combustion of the A/F mixture. How much smoke you get depends on how long you coast.

Edited November 1, 2015 by Fairtax4me

[iriejedi]

The engine uses oil, it does not go through coolent. Probably takes a quart every 300 to 450 miles.

Sea foam isn't a bad idea. I've never tried it In the oil so not sure if that will make a difference but couldn't hurt.

I will also check the PVC valve and let you know, this sounds like a solid idea!