Mystery Smoke Scare

[gophersnake]

I have a '68 GMC 2500 that I drive mostly for short distances: a mile here, five miles there on city streets, occasionally 10 or 15 miles on the freeway.

It usually starts easily (if you know how much choke to give it, how quickly to push the choke back in, etc.) Thursday evening it was a little harder to start than usual. I wondered if I'd flooded it or something, but I couldn't tell for sure. I'd driven it a ways up into the hills, maybe 3 or 4 miles, then it had sat for a couple of hours. The temperature must've been in the fifties and falling but that engine takes many hours to get stone cold. Anyway, it did start -- and then ran just fine all the way home.

Next day -- Friday, around noon -- I went to start it again. This time it acted like it was barely firing, maybe on one cylinder here, one there. After 15 or 20 seconds of cranking I saw smoke pouring out from under the hood. It subsided as soon as I stopped cranking; no fire, obviously. A fried belt? Nope, all the belts looked fine and still do. I opened the oil filler cap. Smoke poured out of the crankcase. I opened the air cleaner. Smoke was wafting out of the crankcase breather too. Clearly the crankcase was full of smoke. I checked the oil level -- normal.

I went to crank it again. It started, ran a little rough as if missing on one cylinder, then after a few minutes smoothed out and ran fine. I didn't see any blue, black or other smoke from the tailpipe OR the hood. I drove the truck a couple of mikes, parked it for a few hours, drove it another mile or so, etc., for a total of four starts that day. It started fine, idled fine and ran fine. Whatever had been the problem seemed to have cleared up completely.

So my question: what could possibly have caused all that smoke, then cleared up by itself? I've been thinking of things like a stuck piston ring that allowed a lot of blowby, then reseated itself. Maybe a PCV valve that somehow, by a fluke, let some oil drip from the rocker arm area into the intake ports. Maybe a badly sealing valve guide that did the same thing.

Any ideas? The engine is a 351 V6 with Bendix Stromberg WWC carb, manual choke, a PCV valve screwed into each head, a breather hose that runs from one valve cover to the air cleaner -- and not much else worth mentioning. I've been driving this truck regularly since 1980 and had lots of other interesting things happen, but this was a first.

[ChevLoRay]

TTT for you....maybe someone has a better idea than me. I'm thinkin' fuel issues, and maybe it did a little backfire (???) and the smoke you saw was from that. Who knows?

[WorkinLonghorn]

What did the smoke smell like? What color was it?
Did you check for a gopher snake across the battery? jk
Smoke WHILE cranking long sounds like starter/electrical. Smoke coming out from under hood would be external prob IF the PCV/vent ststem was working although at cranking/beginning start-up speed maybe internal smoke could escape?
Let me get some coffee in me and I'll think of something.

[Longhorn Man]

did it backfire throug the carb at all? Small carb fire maybe?

[special-K]

Sticky PCV?A plug fouled a bit,then burned off?Could it be an exhaust valve hanging up?I`d take a look at your plugs.

[gophersnake]

Thanks, folks. Today I came up with another guess: how about a little raw gas, or even fumes, getting past the rings into the crankcase, then catching fire -- say from a backfire through the carb, maybe even through the PCV valve?

On other cars I remember seeing wire mesh over the fitting where the crankcase breather hose connects to the air cleaner. That fitting on my truck is just an open tube. If anything like this ever happens again, I think I'll see about soldering some mesh on there for a flame suppressor.

The smoke sort of reminded me of burning oil, but not exactly. It was gray to white, not especially blue. If it was fumes in the crankcase burning, I guess that would have been some mixture of gasoline, oil and maybe a little crankcase crud.

That engine does sometimes backfire through the carb, usually after a cold start when I don't have the choke set exactly right.

Oh, and sometimes when I've just started it (in the morning, cold) and I'm maneuvering out of the parking stall, the engine will hesitate once or twice and there'll be quick puffs of whitish smoke out of the tailpipe. Other than that, though, I don't usually see any exhaust in the mirror.