1999 K2500 continuously gets 5mpg

[Philip Parker]

Hey all - I have a k2500 ccsb with a modded 7.4. Stock bottom end with Brodix short runner heads and all the valve train goodies. 510 cam both sides with a nice lsa for efi.

The truck never gets better than 5mpgs (empty or towing). I have done it all: maf, o2s, new ECU, three new tunes, blocked off egr, distro, timing is dead nuts, air filter, etc. map shows strong vacuum. I’m running out of ideas.

I’m beginning to think it’s the torque converter or something down the line. There are two very small exhaust leaks, but nothing that would yield half my mileage. Years ago is had an o2 sensor low voltage code, hasn’t popped up since.

I need ideas.

 

[Nad_Yvalhosert]

Why on Earth did you block off the EGR? Do you know what EGR does?
It allows inert gasses to partially fill the cylinders on the highway, taking up space, and lowering cylinder temps on the highway, thus gaining MPG.

What's the air pressure in your tires? Too low hurts MPG
How do you drive? Jackrabbit starts from the light...
You have a heavy duty big block, do you expect Prius MPG?

My 2500 Suburban got 10 loaded or empty. My '00 Denali gets just under 12. I can't ask for more than that myself

 

[0xDEADBEEF]

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

 

[stutaeng]

Did you have a baseline MPG before all of the mods where made that you can compare to? Did the MPG drop off after modifications where made? Or did you buy the truck with everything already made?

I believe the 2500/3500 gas trucks get somewhere on the 10-12 MPG; they are heavy beasts. Unless it's a turbocharged diesel, then probably around 16 MPG?

But 5 MPG is way too far out of wack. What is your elevation? You also need to check driveline. Dragging brakes, e-brakes, and binding bearing in an axle and things like that. Jack up the rear and make sure everything spins freely, then do the same for the front. I wouldn't think it would be torque converter.

Plenty of guys here with the 7.4 Vortec, maybe they can offer more advice.

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[Schurkey]

Connect a scan tool, look at the data stream. Verify EVERY sensor, and all the computer outputs--like spark advance.

I'd be especially interested in:
Short- and long-term fuel trims
Misfire counts
Coolant temperature
Torque converter clutch operation
MAP and MAF

 

[Erik the Awful]

There are two very small exhaust leaks, but nothing that would yield half my mileage. Years ago is had an o2 sensor low voltage code, hasn’t popped up since.

Fix your exhaust leaks. Ambient air getting in means the O2 sensor reads a "leaner" mixture, which also causes that lowered voltage. In response your ECM tries to richen the mixture.

 

[Philip Parker]

Why on Earth did you block off the EGR? Do you know what EGR does?
It allows inert gasses to partially fill the cylinders on the highway, taking up space, and lowering cylinder temps on the highway, thus gaining MPG.

What's the air pressure in your tires? Too low hurts MPG
How do you drive? Jackrabbit starts from the light...
You have a heavy duty big block, do you expect Prius MPG?

My 2500 Suburban got 10 loaded or empty. My '00 Denali gets just under 12. I can't ask for more than that myself

I blocked out the EGR because it was stuck open causing bad map readings so the ECU is pulling timing like crazy. Tires are pumped up, I don’t expect amazing mileage but I do expect better than five.

 

[Philip Parker]

Fix your exhaust leaks. Ambient air getting in means the O2 sensor reads a "leaner" mixture, which also causes that lowered voltage. In response your ECM tries to richen the mixture.

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

 

[Philip Parker]

Did you have a baseline MPG before all of the mods where made that you can compare to? Did the MPG drop off after modifications where made? Or did you buy the truck with everything already made?

I believe the 2500/3500 gas trucks get somewhere on the 10-12 MPG; they are heavy beasts. Unless it's a turbocharged diesel, then probably around 16 MPG?

But 5 MPG is way too far out of wack. What is your elevation? You also need to check driveline. Dragging brakes, e-brakes, and binding bearing in an axle and things like that. Jack up the rear and make sure everything spins freely, then do the same for the front. I wouldn't think it would be torque converter.

Plenty of guys here with the 7.4 Vortec, maybe they can offer more advice.

You must be registered for see images attach

Before doing the engine stuff the truck is probably getting around eight average.

Before I did the motor I redid all the suspension components including bearings on all four corners. Like literally tore the entire front end out and rebuilt it. The rear brakes are disconversions and I have checked for them dragging but they spin freely.

Long-term fuel trims are showing fat and 7%-ISH. The map was an issue because when it went into close loop for some reason the EGR got stuck open and it would start pulling timing like crazy because the ECU thought there was a huge load on the motor. I have HP tuners and have done about 12 scans over the past year or so which I can send along if that is something of interest.

 

[Philip Parker]

Connect a scan tool, look at the data stream. Verify EVERY sensor, and all the computer outputs--like spark advance.

I'd be especially interested in:
Short- and long-term fuel trims
Misfire counts
Coolant temperature
Torque converter clutch operation
MAP and MAF

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.