Hi, after some rest, my 91 S10 Blazer 4.3L 4x4 emits a fume of thick white smoke on start up occasionally - just for 1 second, then it's over.
It doesn't happen every time - and the amounts of smoke also vary a lot.
My theory is that there is some coolant leak in one of the cylinders, and depending on how the machine comes to rest, there is a different amount of coolant seeping in that is burned off when the engine starts.

It's quite an embarassment, especially if there are parents and kids walking around the apartment complex that are in danger of having to breathe while the wind carries the unsavory fumes past them.

I guess I'll have to live with it for now, but I'm not sure how afraid I have to be the whole engine is going to fall apart on me soon.
How dangerous is this? Are there other theories of why this happens and is there something I could actually do about it?

 

Is it using coolant? Mine had both heads crack, just a hairline, between the valves.

If this is the case, depending on cooling system pressure and piston location it could fill or partially fill the cylinder and hydrolock. That isn't a good thing.

What you might be able to do is carefully let off the cooling system pressure when you park it, then after it cools see if it still spews at start up.

You can also do a hydrocarbon test on the coolant and exhaust.

Hope I'm wrong.

 

Interesting! I'll give that a try. I am not sure how much role pressure plays - I also happens when I drive it for a very short time without the engine ever getting real warm.
On the other hand, I couldn't use my car for 3 weeks in June, and when I finally started it up again there was absolutely 0 smoke. Go figure.

I'm not sure if they do that test during a smog check, but it passed smog with no issues whatsoever about three months ago, despite the problem (machine was warmed up though).

 

They don't, It's a test for exhaust fumes in the coolant and/or coolant in the exhaust. I think you can purchase a test kit, or take it to a shop and they can do it for you.

 

So, the evidence about releasing pressure from the coolant system when parking says that it doesn't make (much of a) difference. There is smoke nevertheless.
Overall, the problem has worsened somewhat over the last 6 months.
Didn't get around to doing this hydrocarbon test yet

Another interesting detail thing is that coolant and oil do not seem to not be mutually contaminated, so whatever is happening seems to be only affecting the coolant.
Somehow, the coolant seems to seep into one or two of the cylinders on park.
Wondering how big of a repair that would be.

 

is the overflow tank bubbling? compression in the cooling system is easy to find, just pressure check it when the engine is hot, even if it is a little over normal pressure it prob has blown head gasket or cracked head. say 20lbs pressure is over, up to 15 is normal.
are you adding water regularly? either way is a pretty big job replacing a head.

 

Hey Harley and Jigger!
Thanks for your replies.

I tested the coolant system and there has been no significant loss of coolant in the last 6 months actually.
I don't think I ever owned a car that needed so little radiator refill, pretty strange. Also, absolutely no bubbles whatsoever- even with radiator and reservoir caps off starting from longterm rest.
Oil looks good too.

Yesterday I finally got to use the compression gauge to test the cylinder compression, and they were all uniformly a little over 150psi. One was at 155. There were some minute differences in how long each cylinder took to reach the maximum but it was always either 3 and 4 strokes.

Somehow all of this seems too healthy for a blown headgasket to me.

Meanwhile, the problem itself got worse though. The car contiues having a 'whiteish' fume after the initial blowout.
But it all reverts to normal in less than 30 seconds.

I'm still hoping I will not have to unscrew the whole engine in order to fix this... from what other component could something get into my cylinder that causes this white fog on startup?

btw- the spark plugs showed no sign of discoloration or cleaning.
I will take them out in the morning one of these days though to see if there is water buildup in one of the cylinders.

 

Is it possible you have the classic 350/4.3 problem of oil leaking past the valve stem seals? The symptoms certainly fit what you're experiencing. I have that problem on my '93 Sonoma. I haven't bothered to fix it yet. You certainly don't seem to have any symptoms of cooling system problems. I would consider the leaking valve stem seals.

 

Sounds like valve seals.. have someone start your truck for you and smell the exhaust, you will know if its the valve seals. will have a distinct burning oil smell, mine does it for a second at start up, usually only after sitting over night.

 

So i did smell the fume a little and the smell did not evoke the idea of 'burned oil' at all.
It was more of a stingy smell that actually stuck around in my lung all afternoon.

It reminded me more of a chemical experiment - or, the more I thought about it - maybe this is the smell of unburned gas/air mixture?

Otherwise the valve stem seal scenario makes a lot of sense, thanks for pointing me too it, but I somehow couldn't get the stingy smell aligned with 'burned oil'...
hm....