what causes disiels to smoke?

extra fuel not being burned in teh cylinder

What he said.. when diesels smoke, they are either REALLY high miles, getting worn out, or the owner has modded it to make it a speed demon, causing the black smoke.
Scott

oil,and what they said...

White smoke from a diesel is the engine running to lean.

so all the unburned fuel, is that bad for the motor? Is mild modding bad for a diesel?

Well, not quite. It's alot more complicated than that.

Basically, a diesel should burn cleany when operating correctly. The exhaust is actually signifigantly cleaner than a gasser, athough there are some complex chemicles produced that give it the characteristic smell.

Smoke can be divided into 2 classes - white / blue and black.

White smoke can be oil, or can be fuel being burnt at low temps. The reasons for buring oil are pretty obvious, and generally indicate a failure somehwere. White smoke from fuel can be caused by a number of smaller issues, some entirly normal, some not. If you let a diesel idle for an extanded period of time, the clyders cool off to the point of incomplete combustion, cuasing fuel washdown of the walls, and when the engine is returned to load, you can get some white smoke from the fuel burning off. Smoke can also be caused by bad timing, fialing injectors, failing injection pump, air in the fuel, mechanical problems, sometime bad fuel, etc. Bascally, white smoke is indicative of a problem, athough not always a major one.

Black smoke is the result of overfueling. If there is an eccess of fuel for the oxygen presant, the diesel is disacociated, and becomes paritcuate carbon (soot) and probably various other gasses. (Not sure what happens to the hydrogen, but it must go somewhere. HC's, mabey? I doubt straight H2, mabey momoatomic hydrogen that then react once it meats air?) This soot create the characteristic black color, and the soot lining almost always seen in the exhaust pipes and one offending hondas that follow too closely. :) Under normal operating conditions, a diesel should not be making black smoke, exept for a very light haze at full throttle. However, since there is no air metering, if the air charging of the cylinders is decresed, for instance by a clogged air filter or high alltiude, the full throttle fuel delivery is the same, resulting in overfueling at full throttle. Also, any diesel will make black smoke if it's lugged, for reasons I do not understand fully. With turbed diesel, the full throttle fuel setting is setup for max boost, so when you punch it, there will be some overfueling untill the turbo spins up. Also, mods that increase the fueling will create smoke.

As to if it's bad, not nessesarily. Smoke is a good indicator of what's going on in the engine, though, so it should be payed attention too. White smoke is generally bad, athough when starting and after idling it's not that worrysome. Black smoke in and off it's sefl is not that bad, but it indicates you're over fueling, which means high combustion temps, which can be very bad. It also indicates fuel not being burnt, so the engine is not getting optimal fuel economy if it's making smoke. Making smoke is not nessessarily bad, but it's an indicator. As long as you know why, don't worry about it. If you get black smoke cruising at part throttle, or exessive amounts, thenworry a bit. Figure out why. Could be a plugged air filter, could be your lugging it, could be from pulling a hill at max gvw at 10000 ft, whatever.

As to bombing, if done carefully, it shouldn't hurt much. The biggest thing is to keep an eye on things, and know the limits. Definetly get a pyrometer, and keep and eye on it. Pre turbo, anything over 1150 is questionable, and I would stay under 1050 or so. Aluminum melts at 1200, and guess what those pistons are made of? Of course, the egt's aren't exactly the same as the piston temp, but none the less, I happen to lthink that keeping everything inside the engine inside the engine is slightly more important than pulling 25000# up the grapevine at 75 mph. Of course, anything you do to increase the power output will be incresing the stress on everything, so keep that in mind. Yes, most of the common diesel trucks will handle it, but it can't be good for things. I won't get into the arguement of want can handle what, or who's truck is tougher than whos, but just be rational in what you do. Don't expect to get 500 hp out of a stock 92 ctd, or at least not without major mods, or a life expectancy of 2 minutes, but it's not unreasonable to get a bit more out of it, and if you're careful, probably woln't hurt anything.
-John