"Learn me" peculiar exhaust smoke

Saab 900 turbo, 16v engine; high mileage, but runs very strong. For the most part, the car blows no visible smoke at all, as verified by an observer following behind me. However, at one point on my commute (in the morning, engine's just starting to warm up), I drive down a long, steep hill. I stay in 3rd gear (at about 3500rpm), and maintain the speed limit all the way down without needing to dab the brakes. Once at the bottom, I get stuck at a set of lights for a couple of minutes. Upon taking off from this stop, without fail, every time, I'll notice a significant plume of grey smoke in the rear view mirror.

What's occurring that the engine will blow smoke (oily, I presume, as there's no water loss) at this point only?

Watching intently... my MX6 does this from time to time.

I THINK it's the sign of a turbo on its way out, or a failing PCV system.

Decelerating with a closed throttle will often draw oil past the rings and valve seals in a well worn engine into the combustion chamber.

No no NOOOOO!!!!!!!!

Don't tell me that!

I know my valve seals are good, it's a brand new head. I don't want to think that my freak of a motor is worn out in the nether regions.

Thanks! I suspected something like that, but couldn't articulate the idea properly.

(PCV system would be nice...but this engine has damn near 400K (kms) on it, with no evidence of having been opened up before, so Iceracer's answer makes all the more sense).

Grey is not blue, and blue is far more typical of oil.

You've been sluffing down a long hill on a just warming up engine. You could be pooling gasoline, you could have condensation building up in the exhaust. Blowing either out when you finally start to use the engine again after that light.

^^Interesting.

It's definitely more greyish than blue. And it's definitely not white (water)...

foxtrapper wrote: Grey is not blue, and blue is far more typical of oil. You've been sluffing down a long hill on a just warming up engine. You could be pooling gasoline, you could have condensation building up in the exhaust. Blowing either out when you finally start to use the engine again after that light.

I am inclined to agree. I have had cars that chuffed a lot of gray smoke on overrun during the warm-up cycle. If the exhaust isn't hot enough to vaporize it on contact and it is damp out both fuel and water can pool until the next time you make fire.

Would a car that runs perpetually rich also be more likely to exhibit such symptoms? Obviously it should be in lean burn decelerating in gear...

But i also get this if i come off the highway on an exit ramp and coast down it all in neutral then come to a stop.

Maybe the coasting cools the exhaust enough that it doesn't vaporize as you say? The car DOES idle at... (feel free to point and laugh) 12.7:1 AFRs.

I figure it's just a case of wetting the intake manifold with gas from the rich warm up fuel mixture.

Usually injectors stop squirting when the throttle is closed no matter what the warm up cycle is doing. If your throttle position sensor is not adjusted properly than your ECU is not aware of the plate being closed and will continue to squirt, wetting the intake runners with gasoline.

That's all I can think of, anyway.

The turbo in my old SAAB c900 sedan intermittently smoked similar to what you described. The turbo shaft had no descernable play, but the seals must have been marginal. Oddly it would do it right after an oil change everytime like clockwork. Than after a few days it stopped except for a little random puff once and a while.

The obvious test for the OP's situation is to drive down the very same hill with the engine fully warmed and see if the smoke appears. Then you'll have a better idea if you're looking for a fuel or oil problem. Are you seeing any oil consumption?

I've been chasing an oil smoke on decel problem for months; it's enormously frustrating when you fix the things that you suspect only to have the problem remain.

Grey is a too-rich fuel mix. Oil burns blue.

chaparral wrote: Grey is a too-rich fuel mix. Oil burns blue.

Oil leaking from a turbo seal on the exh side doesn't really burn, at least not the same way oil that's been through the combustion chamber does. Also the blueness of oil smoke can be difficult to see depending on the lighting.

turbos.. especially older turbo cars, usually emit some smoke on acceleration as they go rich when boosting. Not so sure on the brand new squeaky clean cars

some of the early mazdaspeed3's would do similar if you sat at a long red light, they'd either just start smoking like someone opened the tap, or would let out a big puff when you took off from the light. this was rumored to be caused by some bad turbo oil seals, but some people still had the issue after a turbo rebuild too

get somebody to follow you and cruise at a slow speed and then floor it.. I am betting you will see the same thing as the boost builds

Try putting the car in neutral and coasting down the hill. Then see if you get the puff when you pull away from the light. If it goes away, then it is the sustained vacuum and worn valve guides or seal. If you still get the same puff, then it is something else.

That makes me happy... Mine will puff if i do that sometimes.

Just a quick follow-up, in case this was keeping anybody up at night .

I finally had a chance to drive down the same hill on an up-to-temperature engine, and there was no smoke, pointing to a fuel issue when cold, as many of you suspected.

mad_machine wrote: turbos.. especially older turbo cars, usually emit some smoke on acceleration as they go rich when boosting. Not so sure on the brand new squeaky clean cars

It definitely does this, too, but not nearly to the same extent.

^^ That sure looks canoey.

Fuel smoke is black, oil smoke is grey. Unless you are using a lot of oil and the engine runs good. Don't worry about it.