Black smoke is caused by soot in the exhaust.
In today’s emissions-conscious society, a smoking exhaust can mean a citation, and some states even measure “opacity,” or how dark the exhaust is, in order to make the ticket stick. Citation or not, exhaust smoke of any color is a negative for the truck owner. Engines that don’t smoke give better fuel economy, last longer, run cooler and require less frequent maintenance.
The first thing you need to know about exhaust smoke, says Tom Freiwald, senior vice president of marketing at Detroit Diesel, is there are three types- white, blue, and black. White is from unburned fuel, blue is from lube oil getting into the cylinder, and black is the result of incompletely burned fuel (too much fuel and not enough air).
Combating black smoke
Diesel engines naturally make a little soot during the combustion process. Small amounts form during injection and burn off before the exhaust stroke, but by then a little has sneaked around the rings and into the oil. This soot causes oil deterioration, making the oil too thick or “viscous,” and it is also abrasive. And it represents wasted fuel. So, the last thing a trucker wants is soot in the exhaust, no matter how romantic those old country music lyrics made it sound.
If you see black smoke, the first thing to worry about is air. One way to minimize soot is to make sure there is always more than enough air to burn the fuel – a job that’s now performed by the engine electronic control module.
Begin with the air intake system. Check the air cleaner’s air restriction gauge, if you have one, under maximum load conditions to see if the restriction is too high. Doug Wilson, Caterpillar’s on-highway engine product analyst, says these gauges don’t always retain the maximum restriction indication. If there is any question, examine the air cleaner for clogging and check the air intake for any kind of foreign object, dirt, or damage (like a dent) that might restrict it. It’s a good idea to replace the air cleaner if it’s accumulated the miles or hours for recommended replacement, as deterioration can happen even without clogging.
Next, check the boost pressure gauge. Zack Ellison, Cummins’ director of technical support, says that low boost pressure due to leaks can result in black smoke. If boost pressure is below what you normally see, inspect all the connections in the pressure side of the air intake system. Pay close attention to the charge air cooler because it has a number of leak points and thin-walled tubes. Inspect the labyrinth seals where the tubes fit into the headers. If there are leaks, you may be able to hear (as a squeal or rush of air) or feel them when someone accelerates the engine and builds boost pressure.
A dirty charge air cooler can be even more of a problem than a leaky one. Reduced cooling of intake air will reduce the amount of air getting into the engine, even if boost pressure is normal, and the higher air temperature is particularly likely to produce black smoke. Make sure the charge air cooler fins are clean and the tiny passages between them are unobstructed. Also, check all the junctions between hoses and fittings to make sure they are properly connected and that clamps are tight and in good condition.
Turbo trouble will show up not only as slightly reduced boost pressure, a black smoke producer, but a change in the “pitch of the sound of the turbo,” according to Wilson. If the turbo is suspect, says Volvo’s Peter Blonde, you will want to shut the engine off and pull the plumbing off the engine side of the turbo compressor. “Make sure there is not excessive oil leakage into the charge air cooler from worn turbo bearing seals,” he says. Clogging of the turbo bearing’s drain line is another sign of trouble, and will make such leakage much worse. Disconnect the line and inspect it to make sure it is fully open.A defective wastegate or variable geometry control may also cause black smoke via reduced boost pressure. Such trouble is best left to an engine dealer, though on variable geometry systems (used only on engines with EGR) you might want to replace any filter used in the air pressure line and check electrical connections for cleanliness and tightness.
Have you set overheads at the intervals specified in the owner’s manual? The initial overhead setting after engine break-in is especially critical. If not, especially if the engine has run long miles, the valve settings could be loose, restricting airflow.
All types of injectors, from the latest electronic ones to the pre-electronic unit injectors used on Detroit 92s, will provide improved performance when overheads are set properly. Overhead settings are especially critical with Cummins PT type injectors because a loose setting can leave carbon in the cup at the bottom of the injector, according to Ellison. They’re also critical with hydro-mechanical (pre-electronic) Detroit injectors because they are timed in the process, says the 92 Series manual.
Detroit Diesel actually invented the electronic unit injector. This is one from a late-model Series 60. Working at extreme pressures means nozzles will eventually wear, giving black or gray smoke. Replacement with a remanufactured unit restores top performance.
Gene Walker, Detroit Diesel’s national service manager, and Volvo’s Blonde, both report that if an engine with modern electronic injectors has 600,000-700,000 miles on it, injector tips may have eroded. The nozzles will eventually wear, reducing atomization and slightly increasing fuel delivery. Wear may happen earlier if fuel filtration has not been up to snuff. The latest electronic injectors are normally replaced with factory remanufactured ones.
The latest electronic injection systems will also de-rate the engine, reducing fuel delivery, if boost pressure drops, and the symptom is often a lack of power rather than smoke. However, a failed boost sensor, one that indicates boost pressure that’s higher than what’s actually there, can cause black smoke. This problem will normally result in an engine check light and appropriate trouble codes stored in the electronic control module. A fuel temperature sensor sending a reading that’s warmer than the actual fuel temperature can also cause smoke. You want to inspect wiring and connections to such sensors if you get this kind of symptom or trouble code.
Earlier injectors, such as those used with traditional hydromechanical fuel systems, may need to be rebuilt periodically. Pop pressure, spray pattern, fuel delivery, and nozzle needle lift can be checked on a test stand and the unit overhauled by a diesel specialist. The same checks, minus fuel delivery, will help injection nozzles used with in-line pumps.
Cummins PT injection pumps need to be calibrated on a flow bench periodically. But both these and traditional in-line injection pumps are subject to tampering. PT pumps can have manifold pressure control settings modified for faster response or the Button changed to up the power rating when the turbo and other engine parts are not up to it. In-line pumps can have the manifold pressure control settings or the rack settings changed. Manifold pressure control tampering creates smoke only when you hit the throttle. Incorrect rack settings or the wrong Button will produce smoke whenever the engine is under load. Getting an injection pump re-calibrated is not especially expensive, and correcting these settings will help your engine and oil last longer, and it will improve fuel economy.
In a few cases, internal problems with in-line pumps or unit injector pumps may cause smoke, though the normal result of wear on this equipment is reduced fuel delivery and power.
With the Detroit Diesel Series 92, tampering would consist of altering the settings on the throttle delay or fuel modulation system, or installing injectors that provide more fuel than the original ones. Get a diesel injection specialist to check injector sizes if the engine smokes constantly, or recalibrate the throttle delay/fuel modulator if smoke shows up only at sudden throttle opening.
Backpressure can result if you install the wrong muffler or up-rate the engine when the existing muffler lacks the necessary capacity. An EGR valve that has stuck open might also cause the problem, but you’d get the applicable trouble code. Burnt valves might also be a cause, and they’d likely also cause irregular running and a chugging noise. Have a cylinder pressure test done at an engine shop. Low cetane fuel might be responsible – consider that if the problem shows up suddenly after refueling, especially if changing suppliers.
White smoke comes from fuel or other liquids that go right through the engine without evaporating. One cause could be an engine that runs too cold and needs a new thermostat. Another could be very low cetane fuel or a clogged fuel filter. The latter also gives slow throttle response.
Low compression on one cylinder might give puffs of white smoke, but you’d feel rough running or misfire. If something (for example water in the fuel) should cause an injector tip to break off, the resulting complete lack of proper atomization could give white smoke. Here again, you’d feel misfire and rough running.
Coolant leaking into the cylinder because of faulty injector sleeves or O-rings, a cracked head gasket, or an EGR cooler with internal piping that has corroded through could also cause heavy white smoke. Check the radiator or overflow tank for low coolant level.
Blue smoke means significant amounts of oil are leaking into the combustion chamber. Lube oil burns poorly, it isn’t atomized and properly distributed in the combustion chamber.
This can occur if the turbo drain line is clogged, or if the turbo bearing and seals are worn. Check as described above for oil in the intake system downstream of the turbo. If it’s just turbo trouble, you’ve gotten off easy. However, turbo trouble often indicates oil change intervals have been over-extended, and that makes it all too likely you also have worn piston rings, cylinder liners, main bearings, and valve guides. In this case, you will see a significant loss of oil from the crankcase. A compression, cylinder pressure, or blow-by test at an engine shop will confirm or disprove this possibility.
Heading off trouble
Smoke, like other engine troubles, usually results from a lack of maintenance. On earlier engines, tampering to improve performance is a frequent cause. Don’t tamper with the engine. If somebody already has, get the injection pump or other fuel system parts set to spec’.
You should also change oil and filters as the factory recommends, or at intervals determined by an oil analysis program. Use quality fuel filters and air cleaners, and replace at recommended intervals. Have overheads adjusted as recommended, and get hydro-mechanical injection pumps set to spec’ as frequently as recommended, too. Watch for leaking air intake parts, and go over wiring to make sure insulation is sound and connections are tight. As the engine accumulates high miles, replace electronic injectors when they wear out. Doing all this will head off smoke trouble most of the time.
The right muffler means not only peace and quiet, but also peace of mind
Think of yourself standing waist deep in the ocean at the seashore and being hit by a wave. This is something like what a muffler does to interrupt sound waves in the exhaust. As the exhaust flows around various baffles and through holes inside the muffler, the sound waves are broken up, greatly lowering the volume you hear.
Mufflers are a valuable component of the exhaust system. When working properly, mufflers provide not only a quieter ride and enhance engine performance, but are also shown to reduce fatigue associated with sustained noise.
Ken Kicinski, chief engineer OE exhaust products at Fleetguard Nelson says, “The total volume of the muffler or mufflers dictates the end result in terms of sound level.” This tells us why so many trucks use dual mufflers: You can easily get twice the muffler volume available with single exhaust. And, that increase in volume means quieter operation.
“Internal design is the next factor,” he reports. Clever design will lower sound level with a minimal effect on backpressure.
These are the M100465 (horizontal) and M065140 heavy truck mufflers from Donaldson Co. The M065140 is an off-road muffler with aluminized steel construction, and black matte high-temperature finish.
Just what is backpressure? It’s the restriction of the muffler and exhaust system piping that creates pressure in the exhaust system that begins where the exhaust exits the turbocharger.
Expansion is the key to engine operation. Your engine takes in air, and after compression and combustion, expansion begins during the power stroke, as the air flows through the turbocharger’s exhaust nozzle. As expansion occurs, the energy added during combustion is converted to power to drive the truck down the road and to spin the turbo. If there is too much backpressure in the exhaust system, the expansion process is stopped prematurely, costing you money and reducing engine performance.
Wear and Tear
There is disagreement about just how much condensation may rust exhaust system parts, but all parties agree diesels create relatively little of it. Yet it’s pretty clear a truck chugging away at a low idle with the coolant well below operating temperature will be hard on its exhaust system. So, at least idle the engine fast enough to keep coolant at 180 degrees.
Ron Buchler, a senior engineer truck and bus product with the Walker Heavy Duty Exhaust Division of Tenneco Automotive, says that even though trucks rarely sit for long, it does happen. Shutting a truck down with condensation in the muffler and then allowing it to sit for a long time would be a bad idea.
Kicinski points out that Caterpillar diesels with ACERT, and some medium trucks, already have catalytic mufflers. These definitely need to be run hot – preferably under load.
Another helpful hint, says Kicinski, is to try to keep wash water out of the exhaust system. “Drive it after going to a truck wash, to dry out the system,” he says.
Unfortunately, rain water can also be detrimental because “