Fuel mileage sucks! Please help!

[Scottfunk]

For those who aren't familiar my truck is a 96 GMC Yukon with an '02 24 Valve, it is backed by a 6 speed Allison transmission. The tires are 285/75/17 and I have regeared so that at cruising speed (70 MPH) I am right at 1960 RPM. I am currently getting about 10 MPG and I am trying to figure out why. I run a Quadzilla on standard tune, level 4 and I have a bit of a lead foot but most of my driving is freeway miles. I also run a FASS 150 and a 5" straight exhaust with 4" down pipe.

 

Now then...at cruising speed I am approximately 20% throttle and my EGT's are in the low to mid 800's. My load ranges between 30 and 36% based on the slope of the road. The boost at cruise is 11psi and I never really paid attention to the IAT's until recently but they stick in the mid 130's; because of space constraints I use an air to water intercooler and I noticed a drip from the filler cap recently so there's a chance the fluid is low. I recently adjusted the valve lash and I have plans to clean the IAT sensor and MAP sensor in the next week but from what I understand about the sensors if they were dirty they would read low not high. I am admittedly not an expert in the field, in fact this is the first diesel truck I've ever owned or even driven.

 

So the question is: Where can I start looking for inefficiencies or out of service conditions that are burning extra fuel? I would really like to take the old girl to the coast in the spring but at 10mpg I'd need a second on the house. Any help is appreciated.

[Me78569]

well 11 psi at cruise is telling me your sucking down the fuel.   

 

What is timing doing at cruise?   

 

what turbo?  

[Mopar1973Man]

Cleaning sensors will do nothing.

 

IAT has no purpose with Quadzilla tuner. IAT has no effect with the Quadzilla active.

 

Need to slow down and get the EGTs under 600F. Also proper timing will help.

[Scottfunk]

The turbo is the stock holset 35. The timing is in the 16's. Occasionally the load pct will drop below 30 and I'll see 20°. The truck has zero mods internally. At idle the egt's are over 400, 2 psi boost. I thought something was wrong so I checked valve lash following @Mopar1973Man 's procedure, they were out but not ridiculously. I put a dab of oil on the exhaust manifold at each cylinder when I left work and they were all burned off  by the time I got home (not very scientific I know, but I believe it proves all cylinders are burning). @Me78569 I know your theory about it taking x amount of fuel to move a vehicle at any given speed so if that's the case what level I'm running on the quad at cruising speed shouldn't matter, it should just take less pressure on the pedal to maintain a given speed. What should I check next?

[NIsaacs]

3 hours ago, Scottfunk said:

The turbo is the stock holset 35.  At idle the egt's are over 400, 2 psi boost. 

 

Are you running any belt or gear drive accessory's? What are you using for an air filter and piping?  You should be running under 300* and no boost at idle. Is it possible your auto tranny is in gear, in park? Maybe linkage or electronics not quite right on the tranny?

[Mopar1973Man]

7 hours ago, Scottfunk said:

At idle the egt's are over 400, 2 psi boost.

 

Sound like there is a lot of drag. Typical idle EGT's is like 250°F for me. 

[Dieselfuture]

Using level 4 means you're using wire tap settings. I use level 3 only when cruising down the road. But I know what you're saying about giving the amount of fuel to move down the road. And like mentioned above, 11psi cruising on flat ground it's too high, should be more like 2-5 psi. Definitely have some drag or something. Brakes dragging, aerodynamics, 

[dripley]

With 0 boost on top of that with mine. Cruising at 70 in mine, 2000 rpm and 3 to 4 pounds of boost. Definitely some drag there somewhere. I can get 11 mpg pulling the 5th wheel at 65 and it weighs15k.

[Mopar1973Man]

Typically 55 MPH is 1-2 PSI of boost and roughly 16% engine load. Now at 65 MPH 2-3 PSI of boost and roughly 19 to 20% engine load. EGT's are still under 600*F for both speeds.  

 

I still think those 285 tires and final gearing don't help. Most likely too low of a final gear ratio to the ground. 

Edited December 18, 2018 by Mopar1973Man

[NIsaacs]

25 minutes ago, Mopar1973Man said:

  

I still think those 285 tires and final gearing don't help. Most likely too low of a final gear ratio to the ground. 

 

If the OP's complaint was 1 or 2 mpg low, then maybe. But that's not his issue at 11 mpg empty. He is driving a Yukon, maybe 6k lbs, he has a serious issue, in my opinion.

 

Transplanted electronic automatic transmissions can give you fits. He is using too much power at idle and cruising. Like the tranny is partially in two gears or something...

[Scottfunk]

When I mentioned my EGT's in idle I was referencing in gear, in park they hover right around 300. I know this probably isn't the right forum for discussing the ins and outs of an Allison transmission but when I built it every clutch pack clearance was dead center of the service range. The trans appears to operate normally and at a constant throttle position maintains consistent RPM's throughout the gears. Hooked up to EFILive it shows all of the correct clutch packs engaging at the correct time and TC lockup pressure is 0 in free mode and 145 in lockup. Granted the Allison's internal parts take more effort to spin than your NV but I wouldn't think it would be that drastic. Also I'm running much higher than stock on the operating pressures, maybe 50% over. Perhaps generating that kind of pressure creates substantial parasitic losses?

 

I am due for some tires and I will definitely be considering weight when I research, and possibly some aluminum rims could reduce rotating mass drastically? But still, stock trucks these days come with 31-32 inch tires so I don't feel like I'm way outside of normal operating range. @Mopar1973Man I used your recommendation for cruising RPM of around 2K when I regeared my truck with the 4.10's is there a chance that something about my setup would rather be a little lower? @NIsaacs the attached picture is my air filter and it is clamped directly on to the turbo. I'm sure it gets a little warmer than it would tucked in a fender well and that's something I have on my list to remedy, but watching my IAT's they never get way out of range...is there something about the length of my intake system that could adversely affect fuel consumption? And also, I haven't been across a scale yet but by my calculations the Chummy weighs much closer to 8K lbs with the added weight of the Cummins/Allison combo and 3/4 ton axles. 

 

If we consider that the Cummins calculates load pct based on boost psi at any give throttle request is there a chance that adding or removing fuel at any given point will change that relationship enough to move the load pct up or down? I'm just shooting in the dark here because I really am in the dark when it comes to tuning a diesel.

[kzimmer]

Maybe drop the pan and check for material? If a binding condition exists, there should be some junk in there by now.

[Mopar1973Man]

Figure out your final gearing to the ground. More you get the final to the ground between 3.55 to 3.73 is optimal.

https://tiresize.com/gear-ratio-calculator/

 

Rotational mass does play a role in this too.

http://hpwizard.com/rotational-inertia.html

 

Optimal IAT is 100 to 140*F...

[NIsaacs]

Do you have a tranny temp gauge? My '01 with nv5600 (360lbs) and D-80 new with me in it weighs 7k. Your filter is fine, as is, as far as fuel mileage goes, not sure about filtering tho Your 11 mpg is not caused by small issues, you have a biggie somewhere.

 

 

[Me78569]

agree this is not a tuning thing, I think it is a drag thing.  the engine is working really hard to generate enough exhaust flow to get 11 psi at cruise state.  

[LorenS]

Have you tried putting the truck in the air and spinning the tires to check for abnormal driveline resistance?

 

Your 4.10 gearing with a 0.61 over drive is equivalent to my 3.55 gearing with 0.69 over drive.  I run 265/70 tires, you are on 285/75s.  Revs per mile for my tires are 638, yours 596, or 6.5% fewer revs/mile. I'm not if that explains your 10 MPG vs. my 19 if you have a big lift affecting aerodynamics.  I may not have a 3rd row of seats, but I do have a camper shell on my long bed truck.  I do not have a body or chassis lift of any kind.  I also rarely go over 70 MPH unless tucked in behind some traffic.  For comparison, 245/70/17s are listed at 661 revs/mile.

Regearing is expensive, but if you have a pile of parts to grab from, a set of gears closer to 4.38 may be a real improvement, if they exist.  4.56 would put you right there with the 245/70/17s. 

 

If injector pop pressures drop too low, could this cause this, due to negative torque resulting from earlier injection?

 

Another thing that comes to mind, is your transfer case engaged?  That could suck some fuel.
Also, if your steering toe is seriously out of whack that would cause lots of drag. If your tires look normal or aren't burning hot you can  probably rule that out.

Edited December 18, 2018 by LorenS
tire and gearing data added

[Scottfunk]

@Mopar1973Man I like the calculator at http://www.grimmjeeper.com/gears.html because it allows you to calculate your final drive ratio through your transmission, transfer case, axle, and tire size. With your 245/75/16's / NV4500 / 3.54 and my 285/70/17 / Allison / 4.10 my sixth gear is only twelve hundredths lower than your fifth gear in final drive. Rotational mass is definitely a consideration here but it sounds like I'm off to a tranny shop so see what they think could cause drag in the trans. @LorenS out of alignment is also a definitely possibility since I've never had it professionally aligned since I built the suspension in my driveway, but that being said other than being low on tread overall I took it to a tire shop to see what they would charge me for an alignment and they said "it didn't look that bad." Who knows what that means?

[Scottfunk]

So what if we take the trans out of the equation... What's a normal load/boost reading in park, idling? I read 6-7% load with 2 psi boost. Is that normal?

[kzimmer]

Cant remember load numbers as my truck hasn't ran since April, but I've never seen more than 1psi in park, and that was probably just the sensor being not zero'd out. What do you read for boost with key on and engine not running?

Edited December 19, 2018 by kzimmer
Typo

[Mopar1973Man]

This is a reason why I don't look at boost pressure on Quadzilla and have ISSPro gauges that are accurate. The MAP sensor is known to be inaccurate for boost reading. Use a mechanical gauge to verify.