Cummins 8.3 idles rough...injector problem?

[Yet Another Driver]

I'm new to heavy trucking and just bought a used 1993 Ford L8000 dump w/ Cummins 8.3 engine a few weeks ago. The idle was set up way high - about 1,200 to 1,300 rpm. Got the idle to adjust down to about 800, but the truck got real rough with a lot of vibration at that rpm. Is this a sign that one or more injectors arn't working too well? I also have reports from a family member following me home that the exhaust is pretty smelly. What's involed with troubleshooting or repairing? Does the Bosch pump need overhaul? Fuel filters have been changed but it doesn't help. Can a bottle of injector cleaner in the tank clear this up? The truck's odometer is at 271K. Thanks for responses.

[yoopr]

might help.
That 1,200-1,300 RPM at Idle is outrageous. I'm guessing they kicked it up for some reason. Be nice to know why

[bob h]

seeming as how you didn't even notice the excessive base idle speed... you need to have a "trained ear" listen to it... many conditions can cause rough idle; air or fuel inlet restriction, low fuel supply pressure, air in the fuel, water in the fuel, poor fuel quality, low compression, worn cam, injection timing, base engine timing, worn injection pump, injectors... this is only the beginning of a very long list...

does the engine have a miss? is there any smoke; @ idle, @ startup, under load? is it hunting or surging? is there base pressure? is there fuel in the oil? is there fuel or oil leaking from the exhaust manifold, turbo, etc? in your "smell report'... what kind of smell; raw fuel, oil, rotten eggs... incense?

there is a few basic tests/checks that should be done before the papers are signed............... but, hindsight is 20/20 ;0)

[Yet Another Driver]

Thanks Bob for your reply. Yup, I didn't know for sure what to look for, see, smell, or feel with the big diesel before buying, so now it's the bad time of getting my education on what the the problem is and what the costs will be to get it running better. I decided to purchase the truck because it was the right weight and configuration for my needs and the cab was not trashed out. The paint and body were good and the dump box is brand new (it was a converted day tractor, single axel). Tires are all very good and the brakes are excellent. Those things are sometimes hard to find all in one package in a used truck.

The turbo seems to be spinning well. I can manually rotate it if I take the duct off, and it sounds like it's spinning well when in high gear on the road.

The roughness (vibration) is bad at 800 but goes away when I idle at about 1,300 to 1,500. The truck seems to run on the road pretty well at 55-60 mph. I get up to 65, though, and seem to hit a wall where the power runs out and vibration comes back. Don't know if the rear end gear ratio figures into this? The engine doesn't seem to miss and there is no smoke from the exhaust, but the roughness may be a sign that it is not hitting on all cylinders on the entire rpm curve? No oil leaks visable, except a bit around the oil pan, but I was able to get a 1/16th turn out of those bolts, so think I got that tight now. The truck is a beast to get though the lower gears smoothly. Too much lugging and surging, which is probably because the idle was set too high. Forth, fifth, and sixth gears go smoothly. Don't know if there is fuel in the oil or water in the fuel.

I guess the truck will have to go to my local Cummins dealer for a check. Hopefully there will be a low to medium cost solution.

If you have a better idea of what could be bad from my description, please reply. If no other ideas, then thank you for getting me thinking on all the possible trouble areas.