Fuel mileage drop !!

[michaeloac]

Hey guys, I recently bought a 99 250, auto 4x4, it has 66k on it. It has a lift on it, banks exhaust and a K&N air filter. Iwas getting around 20 mpg until the last two fillups and it's now getting 10-11 mpg. The only thing I have done was taking the k&n out and cleaning it(shaking it out-no solvent). Could that have caused the problem? I am changing the oil, fuel filter today, and am thinking about going back to an oem air filter. I am diesel challenged so please talk in simple terms. It also seems to be blowing black smoke at start up, thanks for your help

[indyF-350psd]

if youre going to run the stock airbox then change back to an oem filter. the k&n will lift off of the mating surface and let unfiltered air go by. these trucks are well known for not being very friendly with k&n in the stock box. as for the mileage are you basing that off of the overhead computer or are you calculating that yourself? that it is a 99 you might want to check the up pipes for leaks. the up pipes also called uppies are in the back of the motor behind the turbo. its what sends the exhaust gasses to the turbo to spin it. if you have a leak there that would cause a mileage loss. also chech all of your intake boots on your intercooler and intake spider. if you have a boost leak that will also lead to a loss of mileage. the black smoke on startup kinda sounds like maybe an injector problem but others more experienced will shime in to verify that or not. also the overhead computer isnt all that accurate most of the time and since you have a lift on your truck the mileage will only be worse. to get 20 mpg's you have to drive roughly around 60 mph so if you drive faster than that youre definitely not going to get 20 mpg. realistically the normal mpg should be around 15-17 but probably on the lower side with a lift. the other thing to consider is winter diesel. winter diesel has alot more additive in it and will drop your mpg's by a noticeable amount. living in florida it may not affect you that much but up here in indiana i was shocked at how much i lost due to winter diesel......

also do you know if you have an early 99 or a late model 99 truck? what month was it produced?

[michaeloac]

thanks for the reply, I'm pretty sure it's an early 99. I check the mileage with my hand held calculator. Like I said I am not the sharpest knife in the drawer when it comes to diesels, but I would like to learn. After I change the oil-rotella 15/40, and the fuel filter and oem air filter I will recheck. Is there any good fuel additive for cleaning the injectors?

[rebelchevy02]

Did you get your fuel from the same station, as the last two fill ups?

[michaeloac]

Yes, the station is only about 2 miles from the house.Not to mention they have the best price around.

[rebelchevy02]

The additive most people, myself included like is Powerservice DieselKleen. You can get it at walmart, and most part stores, some gas stations. I am not sure if/ or when you guys go to winter fuel down in florida, but you may give it another tank or two, before you do anything hasty. A 9 mpg drop, in one or two tanks, would often lead one to believe it was one of the following situations

1) Significantly more towing
2) Significantly more hot-rodding
3) Bad fuel.
4) Switch to winter fuel

You can add Diesel Kleen, and expect about a .5mpg gain on the tanks you add it, but I would give it another tank, and see what you come up with. Just my .02

[michaeloac]

Thanks for the info, I will try that additive and see if it helps. I still don't understand how the mileage can drop in half.

[papadelogan]

Before you go putting an OEM filter back in, consider one of the best, cheapest, and easiest mods you can do: the KWIK or 6637 filter mod. Basically, it's a MASSIVE round filter that was originally used on Cummins engines in dusty environments. With a 4" section of 4" exhaust pipe (commonly gotten for free or very cheap from a local muffler shop) and 1 worm-gear clamp, you can significantly improve your engine's breather AND filtration at the same time.

You can get this filter from a nearby NAPA (part# 46637), order from fleetfilter.com, or contact Riffraff Diesel (a sponsor & member here). Best option in my opinion is Riffraff; buy a pair to make the shipping "worth it" and have a spare.

[michaeloac]

Thanks for that tip Brendan. I will get r dun !! Any comments on whether lucas oil will help the engine?

[totin_tools]

Dont use Lucas the oil system doesnt like it if I remember correctly. Where abouts do you live in Orlando? I have a house up in Apopka but am hardly ever home.

[papadelogan]

Do not use ANY oil additives, Lucas or otherwise. Our injectors fire off high pressure oil, and additives can just mess up the works.

[michaeloac]

Thanks for the heads up on lucas oil and other additives. I still own a house by seaworld but currently live outside of Louisville Ky in Crestwood.
Thanks again for the help and I will repost mpg after the next tank, Mike

[afonda]

putting the lucas in the fuel to add lubricity.

Is that what you meant?

[michaeloac]

I was talking about lucas for the oil, I know they make an additive for the fuel system also, is it worth while?

[afonda]

for the fuel addative. I think it lube's well and really quiets down the injectors too.

I have used the lucal fuel addative too with no issues.