1999 K2500 continuously gets 5mpg

[Philip Parker]

Who tuned it? Did you do any logging for the tunes?

Troy Brown at Howell tuned it.

 

[stutaeng]

This is on my short list, however my long-term fuel trims are showing fat for some reason.

Interesting. What does fat mean? Rich? Edit: you answered that already. So negative values, right?

On the slightly newer ECUs, I want to say the ECU can control up to +/- 25% fuel trims. Basically, the ECU can control fuel by extending or reducing the injector pulsing to compensate for a lean (lean condition can be real such as vaccum leak or perceived such as mis-reporting O2 sensor,) and driveability is not affected much. But the CEL will come on after 10 or 15%, I can't recall right now. Beyond that, the ECU won't do much and drive-ability will suffer. I honestly don't know on the black box though, just know that it's not as refined as the later ECUs on these things. Are you running the black box or the 0411? 7% doesn't actually seem out of ordinary; that's pretty much normal. You are supposed to add the long term and short term and they should add up to zero, ideally, but up to 5% is okay, or even slightly over.

Regarding the EGR, we've had some conversations here and I recall it was more of an emissions thing? So I don't think that will make a huge difference, as long as it tuned out.

 

[Philip Parker]

Interesting. What does fat mean? Rich? Edit: you answered that already. So negative values, right?

On the slightly newer ECUs, I want to say the ECU can control up to +/- 25% fuel trims. Basically, the ECU can control fuel by extending or reducing the injector pulsing to compensate for a lean (can be real or perceived,) and driveability is not affected much. But the CEL will come on after 10 or 15%, I can't recall right now. Beyond that, the ECU won't do much and drive-ability will suffer. I honestly don't know on the black box though, just know that it's not as refined as the later ECUs on these things. Are you running the black box of the 0411? 7% doesn't actually seem out of ordinary; that's pretty much normal. You are supposed to add the long term and short term and they should add up to zero, ideally, but up to 5% is okay, or even slightly over.

Regarding the EGR, we've had some conversations here and I recall it was more of an emissions thing? So I don't think that will make a huge difference, as long as it tuned out.

Yes rich. What you've stated is also my understanding about the ECU's ability, and why the tuner said it isn't really anything to worry about. He's convinced that the ECU (black box) is getting bunk information form something but I have literally swapped every sensor on the engine to no avail. At this point the only things I can think of is either a ground somewhere that is funky or driveline drag.

Yes, you are right - EGR would not drop milage by this much.

 

[454cid]

Is it staying in closed loop?

What are the front 02 sensors doing?

 

[stutaeng]

I run HP tuners and have done about half a dozen scans. The long term fuel trim show fat around 7% and the guy who tuned it said that’s nothing that will be causing such poor mileage.

I have not figured out how to see torque converter clutch activity within HP tuners but I will look again. This is of specific interest to me because when towing the truck feels way down on power, almost like it’s just not putting torque to the ground. However it is totally drivable and if you didn’t know something was up because of the mpg is you wouldn’t notice anything as far as drivability is concerned.

I see. Can you post a screen shot of the O2 and fuel trims graphs from the HP Tuners screen? Let truck idle, then rev it to 1,000, 2,000 and 3,000 RPM.

Yeah, I agree with your tune guy. I've got a truck running right now at 11% fuel trims due to mis-match injectors (L59 engine that has those high-low flex fuel injectors with LM7 tune) and MPG is not an issue. That's on a P01 ECU.

The real question is for me now based on what you describe is: can you trust your fuel trims? If your O2 sensor graph profiles look okay, I would say, yes. I'll have to think more about that one...

 

[Philip Parker]

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[Reluctanse]

this may be a dumb question, but how are you calculating mpg? is it coming from the computer or is it miles traveled / gallons into the tank? are you certain the speedo is correct? sorry if this was already covered-

 

[Philip Parker]

this may be a dumb question, but how are you calculating mpg? is it coming from the computer or is it miles traveled / gallons into the tank? are you certain the speedo is correct? sorry if this was already covered-

No worries, certainly need to verify! I am calculating from odometer and gallons added. The city has a speed radar display on my street and it matches to within 1mph to the needle on the dash.

 

[Philip Parker]

I should add as well: I installed a new OEM fuel rail when I did the top end and the injectors were multi hole instead of the single hole version.

 

[OutlawDrifter]

Tire size, lift?