Diesel Exhaust Smoke Analysis
By J. D. Neeson, President
Diesel engines, like everything else, have become increasing complicated over the recent years. These improvements have made marine diesels much more efficient and more environmentally compliant as well as lighter and quieter. (The EPA Tier I, II and III regulations have been the big impetuous for these newer designs.) But this added sophistication has meant the newer engines are dependent on their computers to operate and require electronic diagnostic tools for most trouble shooting. These new electronic tools are extremely accurate and give detailed reports on how the engine is operating. They are also expensive and most people are unlikely, and in some cases not allowed, to purchase them.
However, the basic way that diesels operate hasn’t changed and by keeping in mind F.A.C. (Fuel, Air and Compression); it is still possible to analyze the exhaust smoke to help diagnose problems with your engine.
Black or Gray Smoke
Most diesel engines, even the new ones, create some smoke (black or white or both) when they first start up. But if the smoke continues beyond the first few minutes, after the engine reaches operating temperature, there can be problems.
Solid black smoke is caused by unburned carbon from the diesel fuel while blackish gray smoke is a combination of this carbon and incompletely burned fuel.
So remembering F.A.C. lets start with FUEL related causes.
- Incorrect Fuel Grade or Contaminated Fuel. Most people don’t have a choice when it comes to a fuel grade so grade problems are rare, but there has been a great increase in poor quality fuel. When fuel prices go up if often seems the quality goes down. So check the fuel from your tank and change the fuel filters often.
- Excessive Fuel or Irregular Fuel Distribution. This is sometimes hard to determine, but it can be caused by:
- Improper rack adjustment. This can be measured, but it is pretty rare.
- Improper Timing. This requires a fair amount of knowledge and special tools.
- Faulty Injector Firing. If the injectors haven’t been checked for a bit, it may be easier to have a mechanic remove these and Pop check them for you. The newer engines have a common rail and operate under extreme high pressure so if a faulty injector is suspected you will have to call a certified marine mechanic. On older engines with separate injector lines (mechanically driven fuel injection pump) you can check a suspect injector by disconnecting the fuel line while the engine is running and see the effect on the engine. WARNING – Please don’t forget the pressure in the injector lines are in excess of 2000 pounds per square inch and can push fuel into your body. People have lost fingers and arms not to mention blowing themselves up (especially easy on common rail diesels). Don’t do this unless you are knowledgeable and have good insurance.
- Lugging or Overworking Engine. This happens surprisingly often. The engine is overloaded and sounds like it just can’t get up to speed. It can be an incorrect propeller (over-sized or over-pitched) or something wrong with the transmission (stuck plates) or outdrive (grinding gears). It can even be something as simple as a fouled propeller or substantial growth on the hull.
Next is AIR related causes.
- Insufficient Combustion Air. Diesels require an amazing amount of air so you should check:
- Air openings in the engine room or external air vents. We have seen people attempt to sound proof their engine and cause problems. Sometimes during hull repainting the openings can be partially blocked.
- Intake air filter. Remove and replace the intake filter. This can be hard to see if it is clogged so it is easier to just replace it when in doubt.
- High Exhaust Back Pressure. This is usually caused by some obstruction in the outgoing exhaust path. Sometimes, on the older diesels with outdrives, the rubber flap in the exhaust pipe breaks loose and gets stuck across the opening causing both this problem and overheating.
And finally COMPRESSION related causes.
- Faulty Turbo-Charger. If the turbo charger is not working properly, the engine doesn’t develop the sufficient air/fuel ratio and since the turbo allows the engine to create more power (why a turbo engine has more horsepower than a non-turbo one), the engine acts as if it is lugging. Fortunately, most times turbo chargers either work or they don’t work. You can check the turbo, when the engine is off, by removing the air filter and spinning the impeller. The impeller should turn very easily and not wobble at all. Incidentally this impeller is turning in excess of 20,000 RPM so there is any grinding, resistance or play then the turbo needs to be replaced.
- Incorrect Valve Clearance. Incorrect valve clearance can also affect the compression stroke of the engine causing the engine to appear to be over working or under-powered. Rarely does this happen quickly unless the head has been replaced. This may be beyond the casual mechanic.
Blue smoke is almost always caused by excess lubrication oil somehow entering the cylinders. The smoke, besides being blue, will have a burnt oil smell that is relatively easy to identify. There are a number of causes of this:
- Worn or Broken Piston Rings. When the condition gets bad enough the engine will begin to use up oil and can cause base pressure in the crankcase. When the engine is running you can sometimes feel the pressure at the top of the dipstick hole. The engine will have to be rebuilt.
- Scored Piston Liners. This can cause the same symptoms as the bad piston rings and means the engine will have to be rebuilt. It can be caused by contaminated fuel which is why it is so important to change fuel filters often.
- Faulty Turbo Charger Seals. This is relatively rare, but sometimes the seals in the turbo charger fail. Turbo chargers are oil cooled and if the seals fail the oil can be exhausted. The only fix is to replace the turbo.
- Excessive Crank Case Pressure. It is possible that the crank case filter can become clogged enough to allow the pressure in the crank case to rise. Most times however, excessive base pressure (see #1 and #2) causes the filter to become clogged rather than the filter causing the pressure. In any case you should replace this filter relatively often.
White smoke is caused by water vapor or unburned diesel fuel in the exhaust. It reflects that there has been misfiring in the cylinder. Often you can tell the difference by smelling the white smoke to see if the smoke has a diesel smell or not. The typical causes of white smoke are:
- Head Gasket Leak. This is usually accompanied by overheating issues and doesn’t happen all that often in most diesels unless there has been a catastrophic overheating problem.
- Cracked or warped Cylinder Heads. This again is usually due to overheating and is usually not subtle. The engine runs poorly, puts out white smoke and overheats.
- Faulty Injectors. This means that an injector (or maybe more than one) is not firing properly. It often gets worse overtime and sometimes causes the engine to start hard. Rarely, but at times, it can cause some overheating. The newer engines have a common rail and operate under extreme high pressure so if a faulty injector is suspected you will have to call a certified marine mechanic. On older engines with separate injector lines (mechanically driven fuel injection pump), you can check a suspect injector by disconnecting the fuel line while the engine is running and see the effect on the engine.WARNING – Please don’t forget the pressure in the injector lines are in excess of 2000 pounds per square inch and can push fuel into your body. People have lost fingers and arms not to mention blowing themselves up (especially easy on common rail diesels). Don’t do this unless you are knowledgeable and have good insurance. It is probably a better idea to have the injectors removed and tested. The injectors can be checked for their opening pressure (Pop tested), for leaking nozzles (contaminated fuel can do this), and for the correct spray pattern. This requires special tools.
- Low Compression. There are a number of reasons for low compression and it always makes sense for the compression to be checked (special tool again). But whatever the cause, the reduced compression causes the fuel/air mixture to not fully burn and to allow the partially burnt fuel to exit with the exhaust. Most times this does not cause overheating, but it can affect starting and the power of the engine.
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Marine Parts Express is a division of Water Resources, Inc., a privately held Maine Corporation.
For all your marine engine parts needs, call us toll free at 877.621.2628, or outside the U.S. at 207.882.6165.
August 7, 2011 / JD Neeson / 0
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