[jwholden]

I was driving the highway today and a 3-5 year old chevy went by (pre-duramax). When this guy shifted gears there was a plume of nasty black smoke that you could barely see through. Otherwise there was no sign of exhaust at all. I know diesels smoke once in a while but this seemed extreme.

What could it be?

[dan deutekom]

Injectors set a little rich or worn.

[scl]

Program bumped up and boost cranked up. Lots of little tricks to pump mega horses out of them diesels. Too cool!

[Bill Schwab]

Several things could be going on here. Since black smoke indicates unburned fuel, it could be a plugged air filter, which, allows pleanty of flow when at lower RPM's, or it could be he juiced it and put in larger injectors. The last thing is dependant on what GM engine it was. If it was a 6.2, when the pumps got worn, they passed alot of fuel at high RPM's under load. If it was a 6.5, the front pump gears would wear causing port timing to be slightly off. All you need to be with a diesel is 1/2° off on timing and you could either predetonate, or run too rich. If an computer was incorporated into the mix, it could be a simple elctronics glitch that would cost you $1,000.00.

Lastly, if the valve guides are so sloppy that the valve stays open too long, or allows too much juice in without giving the compression the engine needed to fire the fuel, that could also make the truck burn black. The first thing one should do is pull compression, which in any of those engines should be a minimum of 380lbs.

Diesels are very easy to work on, just black dirty and messy, and require absolute precise timing. The largest cause for this type of symptom with today's fuels is in the way they extract the sulpher to make sulpherless fuels. ( it is best for the engine to run off road fuels and pray you don't get caught by the IRS police!) They use a process that freeze drys the fuel, and it leaves microscopic particles in the fuel, causing premature wear on the pump and injectors. Since pumps are made under strict tolerances, and I mean like hundreds of thousands to the inch, it does not take an aweful lot to throw one out of whack.