Bad Mileage

Dusty Roads · 09-18-2008, 02:49 PM

I have an 04.5 LLY and have been getting pretty bad mileage lateley. Around 80,000 miles I have seen a decrease in mpg performance. I used to avg 17-18 @75 mph. Now lately I have been driving slower 65-70 and only getting about 14 on freeway and 9-10 on the farm. I had the BANKS 6-gun on it before and took it off around 95,000. Currently I have 110,000 mi (5,000hrs on engine). I did not see any difference in mpg with it before or after. I changed filters and ran diesel cleaner in the fuel as well but no change. I also have the Banks intercooler, air intake and exhaust on it as well. Has anyone else have this problem? Any recomendations on maybe better programmers/tuners?

I have heard that around 80,000-100,000 mi the CATS start to plug real bad. I wonder if this is the case since I have a lot of idle time. Also been thinking about cutting my CATS off but am leary of losing performance and throwing codes. Any ideas?

86burgandyss · 09-18-2008, 02:54 PM

You would gain performance when you remove the cats, not sure about the codes. I throw an egr code on occasion, but I have a turbo back system.

**bigdaddydiesel** · 09-18-2008, 03:04 PM

Idle time will kill your overall MPG results, no matter what you do, and yes, removing the Cat should help.

Dusty Roads · 09-18-2008, 03:25 PM

I know idle time will kill mpg, but recently All I have are hwy miles w/ no idle time and I am still only getting about 14 mpg. It just seems like my mileage really dropped off the map. My range is only 350-360 miles before I need to be back at the fuel pumps.

Dusty Roads · 09-18-2008, 03:37 PM

Would you know if I would need a fingerstick or an EGR Blocker?

midieselguy · 09-18-2008, 05:53 PM

Dusty Roads,
I have the same engine with the mods listed in my sig. I get 20-21 MPG at 75mph. My biggest gains came with the turbo back exhaust and KB's IOH kit. Around town my mileage drops to 15-16. I'd take the cat off and let it breathe.

**bowhunternkbay** · 09-18-2008, 07:36 PM

I would bet that with that kind of mileage, soot buildup is pretty extensive...both in the cat and in the intake from the EGR. Research for getting rid of the soot buildup in the intake is nothing short of removal.

Dusty Roads · 09-18-2008, 10:34 PM

Quote:

Originally Posted by midieselguy

Dusty Roads,
I have the same engine with the mods listed in my sig. I get 20-21 MPG at 75mph. My biggest gains came with the turbo back exhaust and KB's IOH kit. Around town my mileage drops to 15-16. I'd take the cat off and let it breathe.

This might be a stupid question but what is the KB'S IOH kit?

Dusty Roads · 09-18-2008, 10:42 PM

Quote:

Originally Posted by bowhunternkbay

I would bet that with that kind of mileage, soot buildup is pretty extensive...both in the cat and in the intake from the EGR. Research for getting rid of the soot buildup in the intake is nothing short of removal.

Do you have pics/ instructions on how to put in the block plate? Do you also know where to get a finger stick and the proper way to install it?

**Hook'em_Horns!** · 09-19-2008, 04:45 AM

Quote:

Originally Posted by Dusty Roads

This might be a stupid question but what is the KB'S IOH kit?

A flaw exists with the design of the intake turbo mouthpiece on the LLY motor. Porting the mouthpiece is a simple solution, or as mentioned above, an IOH (induction overhaul) kit will update the MP to the current GM LMM design in use.

Review these threads for more info:
ported mouthpiece pic
LLY mouth piece????

While this mainly addresses performance and overheating issues, is does offer a slight increase in mileage. Your problem is somewhat different as your mileage has decreased with more miles put on. I have the same issue, I once saw 17.3 mpg running unloaded, while the best I currently have seen is just under 15. Check my sig.