Coughing and Wheezing?

| April 07, 2005

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.

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