Bad head gasket of even a crack in the cylinder head could be to blame.

Share story

Dear Car Talk: I have a 2003 BMW 325 with 158,000 miles that suffers from SOBS (sudden-onset billows of smoke). It happens only maybe once a month or so, but is very exciting. Imagine a James Bond car-chase scene, with a huge cloud of white smoke emerging from the tailpipe. After a short interval (less than a minute), the problem resolves. It can happen at startup, but more often it’s in the middle of a short commute or a long trip. When it happens, there is a noticeable loss of power. When the SOBS is not happening, there is no smoke at all from the tailpipe, and it will pass all emissions tests. I took it to my favorite hippie-Eurocar mechanic, and he drove it to lunch and back for a few days —  but, of course, he could not replicate the SOBS event. This is my daily car, but this condition is making me lose confidence in it. Can you help? — Art

A: Well, I’m not very impressed with the effort made by your hippie-Eurocar guy. Maybe his mind was on the Greek financial crisis and his future access to spanakopita.

The first thing he needs to do is actually test for a bad head gasket. That takes more effort than driving the car to lunch a few times and giving you back the keys.

What he should do is keep your car overnight. And before he goes home, he should put a pressure tester on your cooling system and run the car so it gets good and hot, and then shut it down. Then pump the pressure up to 20 pounds per square inch, and leave it overnight.

If you have a bad head gasket or, even worse, a crack in the cylinder head, pressurizing the cooling system often will force coolant through the breach and into the cylinders. Sometimes you can smell the coolant; other times, it’ll combust and produce something you’re familiar with, Art: sudden-onset billows of smoke.

If the pressure test is inconclusive, there are other tests he can do. He can carefully monitor your coolant for a very slow leak. Or he can do a dye test that looks for combustion gases in the coolant.

I’d say a bad head gasket is the most likely cause of your SOBS, Art. But if your mechanic really investigates it, and concludes that the head gasket and cylinder head are fine, then you go on to weirder explanations — like the brakes.

Now, I’m sure you’re wondering, “How the heck can my brakes send voluminous plumes of white smoke out my tailpipe?” I’m wondering that myself.

Actually, what we’ve seen happen in rare instances is that the brake master cylinder can leak, and brake fluid can drip into the power-brake booster. The power-brake booster is vacuum-operated. The vacuum comes from a hose that’s connected to the engine’s intake manifold. So if you get enough brake fluid in the power-brake booster, some of it can get sucked up into the manifold and sent into the cylinders.

Maybe you go over a bump, or make a turn or stop, and a couple of thimblefuls of the fluid get sucked into that vacuum hose and delivered to the cylinders. That would create plumes of white smoke, diminished performance and possibly even a desperate letter to some idiot car columnist.

Your mechanic can check for that by pulling the master cylinder away from the power-brake booster and seeing if it’s wet back there. Of course, he won’t be able to do this while he’s eating.

Got a question about cars?
Contact Ray through the website cartalk.com.