I'am getting 12 mpg on interstate @ 75 with 2005 3500 6-speed empty.
the dealor has changened the limiting valve & new o-rings on injectors,but still blows black.
the truck also has long cranking when starting warm. it is getting worse the more miles.
any ideas?

 

hmmmmm

first of all I would go to another dealership, it sounds like they are just trying to throw parts at it in hope something will fix it. bad karma there.

anyway, I would suggest one or more injectors are leaking. the way to tell this is to try a cylinder shutoff that can be done through the dealership ODB computer. this way they can tell if this is realy whats going on. and I would say that since you are blowing alot of black smoke.

if it comes down that the injectors are bad and the dealership is starting to haggle with you, I would suggest a set of KAX injectors, or some aftermarket injectors direct from Cummins, or these from Piers. installing aftermarket injectors will increase the power of the engine, and also increase fuel economy.

the sign of black smoke is not good on a stock engine under no load conditions. I can get mine to smoke a bit, but I'm also flooring it.

 

they also tell me I have 30% idle time that is part of the problem.
but I have 17000 on the truck didn't do any of this stuff when brand new was getting 17 to 18 mpg

 

idle time realy has nothing to do with it. but if they quoted you 30% that is a bit high, but I also believe that is something that the dealership can use to wiggle out of warranty work. so take it with a grain of salt.

I dont know what my idle time is but I do know over the life of the engine I have been averageing about 43mph when you take the engine hour meter and divide that into the total mileage.

something quick you can try if youre ok with droping about $5 is go to wally world and get a 32oz bottle of power service (grey bottle) and dump the entire contents into the fuel tank at your next fill up. this is 2x my normal dose I give my truck. then procede to run the truck as hard as you can, preferably pulling a load if you can. this might blow out anything that is jammed up in the injector, it'll burn any carbon build-up you may have. you might see ALOT of black smoke but running the engine hard might help what ever is keeping the injector open.

one other thing I need to ask as well is this, how is your turbo lag? or can you even hear your turbo at the higher end of the RPM range under load. if it sounds muffled, laggy, or kicks in late then pull the plumbing between the turbo compressor houseing and intercooler and see if you have oil in it. I dont want to put a gremelin on your truck and say the turbo might be blown, but that might be something else to look at. my turbo normaly starts talking to me around 1200rpm and she lights to over 30# past 1800rpm.

I'm hopeing the $5 bottle will work though.

 

another part I forgot to mention is I can get up to 18 mpg with stop and go driving but it goes to 12mpg when on long drives if loaded as bad as 8mpg .
some people have told me maybe my watse gate is not working .
thanks for the help!!

 

waste gate stuck open, I could see that.