1990 240: Help Solve an Intermittent Smoking Mystery!

Hi, I'm new to the forum but found you all because I don't want to give up hope on my mega mile (520K mi) 1990 Volvo 240 wagon, affectionately called "Social Suicide" by my teenage daughter. Hopefully somebody here has seen this behavior before and can help me get her back on the road. Its a two owner car who's whole history is known. And the entire driveline has never been touched. Up until this issue appeared the car drove like a dream.

The car just started to intermittently blow a fairly short but massive cloud of smoke out the tailpipe and then return to normal for a little bit before making another massive but short smoke cloud, then stopping and repeating the short smoke on then off for a bit smoking cycle. By this, I mean that it smokes about every couple of miles. Sometimes less, sometimes more, but never one cloud right after the other. Nor can I make it smoke all the time. I live near some decent sized mountains, and going up a 2mi steep incline which really puts pressure on the engine, it'll do it for maybe 10 secs then not again the rest of the way up. So there is definitely some sort of a refractory period to the behavior. The bottom line is that there seems to be no rhyme or reason to the short but massive smoke clouds the car is making, but it seems to be doing it regularly, just intermittently.

The smoke clouds are not engine speed nor throttle position nor engine/vacuum load dependent. They happen as equally at light throttle rolling down a country road as they happen at full throttle pulling hard up a steep highway hill at 70mph. And despite all the miles, up until this started, the engine literally ran like a new Honda. I do use the high mileage synthetic 5/50 oil. And it would easily go 10K miles between changes simply because the oil level never dropped, nor did it even really turn color. But now the engine also eats oil almost like it was gas!

Oh, and there is no extra pressure in the cooling system, nor is there coolant in the oil. From the plugs, it also seems like the cylinder closest to the firewall gets most of the oil, maybe 75%-85% of it, and then the cylinder right next to it gets a little. The other two cylinders seem to get little to no oil.

All indications are that neither the rings nor the valve seals nor the head or head gasket has suffered any sort of catastrophic failure. When its not getting sudden intermittent massive amounts of oil into the cylinders the engine runs like a top! It almost seems like for some reason oil is pooling in the intake and then after pouring into the cylinders.

Here's one curiosity to the puzzle. Last year, my mechanic "shimmed" the small air intake thermostat in the filter box that tries to set intake air to 190 and the car did the exact same thing, but only at highway speeds pulling up really big hills at WOT. Removing the shims and putting the little air box air temp thermostat back to the way it was stopped the issue 100% and the engine was perfect for another 20K mi.

Here's a second curiosity. It may just be a coincidence, but both times it started this smoking behavior it has also coincided with sudden drops in air temps that happen in Nov and Dec in the mid-Atlantic area.

And here's the 3rd curiosity. After it started happening, along with new plugs and a good compression test, the timing belt, pully and all front and back seals were replaced, and it seems like its now doing it more than ever.

Could it be that the fire box, or whatever Volvo calls the crankcase breather system, is clogged, and for whatever reason there is now a buildup of pressure inside the engine, or whatever, that is now somehow causing oil to pool in the intake to the point that its entering the cylinders? Or maybe somehow an oil pathway has become clogged, which is also causing oil to back up and then pool in the intake to the point that it enters the cylinders in the intermittent but very significant way??

I'd love to hear any all suggestions so hopefully I'm reaching an audience that's seen this happen before and can point me in a direction that gets Social Suicide back on the road. Our goal for the car is 1,000,000 kms, so we need about 4 more years out of her!

Thanks in advance!

Jimmy

 

OK, So a quick update. I haven't been driving it for about a week. At startup tonight it smoked a bunch, but it quickly when away after maybe 10 seconds. I drove it about 10mi and except at startup it didn't smoke at all. But, going up the last fairly big hill there was a hint of hesitation. When I got back I checked the dipstick, and it was down a mm or two, but most notably is that now the oil is turning black very quickly. before this all started to happen it could go 10K mi with the same amount of drop on the dipstick, and far less darkening of the oil.