Smoking doesn't necessarily mean that your powerplant is on its last legs, but it certainly does imply that it's seen better days. Black smoke is almost always the result of oil in the combustion chamber, so the best way to stop smoking is to keep the oil where it belongs. Bear in mind, though, that different colours of smoke mean different things; black smoke indicates oil burning, blue smoke means excess fuel or a malfunctioning ignition system and white smoke is actually steam resulting from a coolant leak in the engine.
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Pour a stop-smoke, stop-leak additive into your engine's crankcase. These additives contain seal conditioners that cause the valves seals to soften, swell up and seal against the valve stems. This is the most likely source of oil intrusion into the cylinder, so try this cheap fix first.
Drain your oil, flush the oiling system and replace the oil with a high-mileage synthetic containing seal conditioners. High mileage oils are specifically formulated to soften seals and reduce oil burning, and the synthetic element will help your engine to last that much longer. Get the thickest oil that you possibly can, preferably a 20W-50 or higher. The thicker oil will have a harder time making its way past leaking seals.
Replace the intake valve seals. Oil intrusion generally comes from the intake side; the vacuum created by a dropping cylinder will suck oil in through the intake valve seals, while high-pressure exhaust will push it back into the cylinder head. The complexity of this job varies by the engine, just be careful not to drop a valve into the combustion chamber while changing the seals. Before changing them, rotate your engine so that the piston at that cylinder is at the top of its stroke, and use a powerful magnet to retrieve the valve if it drops past the valve guide.
Tear the engine down and replace the piston rings. This is tantamount to a complete rebuild, requiring not only new rings but possibly an overbore and new pistons. The good news is that you'll ultimately wind up with an engine in like-new condition; the bad news is that you'll end up hundreds or thousands of dollars into the project by the time it's completed. If you're going this far, then you might as well have the bronze valve guides replaced and have your machinist perform a three-angle valve job to refresh and upgrade the assembly.
Tips and warnings
- If you track your problem down to failing piston rings, then you might also try a CSL -- aka copper-silver-lead -- additive to help restore the cylinder walls. The CSL will embed into your cylinder walls to smooth them and enhance the seal. However, this effect works more to keep high-pressure gases in the combustion chamber than to keep oil out. If your engine is getting enough oil to smoke, then smoothest bores in the world probably won't help. Still, it's worth a shot if you're looking at a complete rebuild otherwise.
- Make sure that your positive crankcase ventilation system works as intended, and replace the PCV valve if necessary. The PCV system uses engine vacuum to reduce pressure inside the crankcase, which can build up and force oil through the piston rings. A malfunctioning PCV system probably won't cause a well-maintained engine to smoke, but can certainly contribute to the problem if your bores are worn or your rings are weak to begin with.
- Turbochargers use a labyrinthine seal to keep oil in the turbo and out of the intake. If your turbo engine begins to smoke, check the turbo oil seal.
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