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Smoke Diagnosis

Smoke Diagnosis

Jan Wikstrom

The combination of (bluish) white smoke and black plugs indicates oil burning; too much fuel produces sooty (dry) plugs and black smoke, as well as "lumpy" running and overheating.

The problem is not a head gasket; in the V12, the oil would have to pass through the water to get into the cylinder. It's either worn/broken rings or worn valve guides. There's a simple way to diagnose:

  1. Bring the engine to normal working temperature.
  2. Let it idle for five minutes or so.
  3. Stomp the throttle and hold at about 4,000 rpm. If this produces a cloud of blue/white smoke, it's more likely rings. If the smoke doesn't taper off, it's *bad* rings.
  4. Keep the revs up around 4,000 until the smoke thins, then let the throttle snap shut. If this produces a puff of blue/white smoke, it's more likely the valve guides.
  5. If both tests produce a positive result, it's rings *and* guides.

Do a compression test, anyway. This is an easy job you can do yourself; I bought a compression tester from K-Mart for, I think, $15. It's a little awkward to reach in the V12, but worth the trouble. Take all the plugs out and test one cylinder at a time on the starter; it only takes three or four compression strokes to bring the gauge up to a steady value. The desired reading depends on the compression ratio of your particular engine. The main thing to look for is consistency; if one cylinder is much lower than the others, it has a damaged ring or piston, and if all are uneven beyond, say, 10%, you have a generally worn engine.

There's more info at www.zip.com.au/~wickers


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