Engine/Transmission (1998.5 - 2002) Sudden Loss Of MPG

[MRiley]

over the last year this truck is driving me nuts. Last summer the VP44 died. I replaced it with a Blue Chip. Everything seemed normal. Then the pick up tube in the tank broke and sucked air every time the tank went below 1/4. I put in a new draw straw, again things were normal. About 2 months ago i noticed that the milage was falling, power seemed good but MPG was down from 19 to 14. I went on a trip with my 5er last month and went through fuel like i have never seen. Then the ECM died, I was hoping that it was maybe the cause of the MPG drop. I just got the truck home today and it averaged 6mpg! I have never gotten this bad towing, usually I get 12 -14. Power seemed normal. The air filter does look dirty so I'll clean it tomorrow.

My truck has super low miles 65,000 (I'm the only owner, bought it new). I shouldn't be having these problems. I run a Smarty on #9 and have for about 8 years never any problem. Boost is set to top out about 30psi, my FASS lift pump holds steady about 12psi, and I just put in a new fuel filter (less than 200 miles on it). I run the stock injectors and due to the low milage I've haven't adjusted the valves.

I'm at a loss here. I love the truck, but 6mpg isn't going to do.

 

[JR]

I would start with the basics and look at anything that can be causing drag. Jack up each corner and do a thorough inspection of brakes and bearings. Same with your trailer. Tire pressures all up to spec? Sorry if I'm sounding too basic but sometimes it's the simple things we over look.
Is your 4 wheel drive possibly engaged? Should be able to spin the front drive shaft.

Do you have a CEL on? Where did you get your ECM from? Has your mileage been in the dumps since you installed the ECM? Is it specifically for an HO truck? How about the VP44? (HO?)

 

[RonaldLarson]

I would start with the basics and look at anything that can be causing drag. Jack up each corner and do a thorough inspection of brakes and bearings. Same with your trailer. Tire pressures all up to spec? Sorry if I'm sounding too basic but sometimes it's the simple things we over look.
Is your 4 wheel drive possibly engaged? Should be able to spin the front drive shaft.

Do you have a CEL on? Where did you get your ECM from? Has your mileage been in the dumps since you installed the ECM? Is it specifically for an HO truck? How about the VP44? (HO?)

Could be that the air leak from the draw straw injected enough air to damage the Blue Chip IP. Call them for clues.

 

[SAndreasen]

never hurts to check the fuel filter again and check the air filter ive only had one truck out of 5 that the filter minder actually worked no matter how plugged the filter is . also wouldnt hurt to do an oil analysis

 

[MRiley]

So there are no CEL's I assume the problem isn't the ECM as I noticed a decline before that went out. I did check the wheels on the trailer for heat build up about half way through my trip and they were cool, nothing to indicate drag. The air filter is dirty, and no the filter minder has never moved. I run a K&N and i'm also wondering if I could have pulled oil to the AIT sensor? If the sensor is reading off but still sending readings would there be codes? I recently put new tires on but they are the same size I've always run so I'm guessing that isn't it.

I was thinking air because it seemed extra smokey on the drive home. But, I'm a little confused because I was seeing good boost pressure. If the filter was to blame wouldn't boost fall off? The smarty was always smokey but ti seemed worse that I remember. I was also wondering about injectors? would they throw codes?

 

[JR]

Normal boost, no hesitation, no loss of power yet mpg suffers... That's a strange one.

Injectors won't cause a CEL as they are pure mechanical. Signs of a bad injector are loss in power, stumble at idle, etc. You could also be "making oil" where the injector dribbles fuel into the crank case and causes the oil to rise.

 

[MRiley]

I tried to check the resistance values for the AIT sensor. I can't seem to find the correct values anywhere. I know I've seen them here before. if anyone can find the link I sure would appreciate the info. It was super dirty so I cleaned it and reinstalled.

The air filter was really bad. Much worse than I expected.

 

[JR]

What does the intake piping down stream of your Turbo look like? Is it dirty or oily?

The intake air temperature sensor is likely fine, they rarely ever go bad and would throw a CEL if it did.

 

[Vaughn MacKenzie]

Does your truck have an exhaust brake? If so the AIT will be grimy by now and pulling it out a d cleaning it with carb cleaner should help. But I would also read over Blue Chips' troubleshooting section for ideas too.

I would also caution you on the Kenny & Norm air filter, it's one Cummins specifically warns against using...if you want a reusable gauze style element go with an AFE or similar brand. But nothing stops dirt better than paper.

 

[MRiley]

so I think it was a dirty AIT. I cleaned the AIT which was covered in black, (surprising since i do not have a exhaust break) and then took off for a road test. I went about 120 miles round trip, i figured it would be enough to see a difference at the pump. I got 20mpg hand calc. I'm happy with that. I also noticed that I was running lower boost pressures. Anyway thanks for all the input.