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.

@kzimmer  @Mopar1973Man  My boost with the engine off is 2 psi so I guess there's no telling how bad it's off at cruising speed. Is there any way to clean/calibrate it or do I just replace it? 

Edited December 19, 20186 yr by Scottfunk

Late year sensors sometimes have odd offset.  If it is stable and increases semi smooth it is prob fine.

You will have to get data logs from other guys with stock injectors.

I'd say that's good news, you aren't actually building boost at idle due to some strange parasitic loss.

11 hours ago, Scottfunk said:

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?

Sounds normal, mine is also off 1-2   psi. Have you checked for boost leaks

Mine is close to right on down low but skews as boost pressure go up. This why I tend to use my a-pillar gauges more so than Quadzilla display. Quadzilla isn't the only one with this issue. Edge Juice does too. 

1 hour ago, Mopar1973Man said:

Mine is close to right on down low but skews as boost pressure go up. This why I tend to use my a-pillar gauges more so than Quadzilla display. Quadzilla isn't the only one with this issue. Edge Juice does too. 

My question then is, is this a mathematical issue within the Quadzilla programming or is it an issue with the sensor? In other words is the information skewed before the ecm uses it to process fueling values or is it skewed when the quadzilla is processing the information for display? If the information is skewed before the ecm processes it then the fuel tables are worthless because they're basically using an arbitrary value to compute fuel values.

Just now, Scottfunk said:

it skewed when the quadzilla is processing the information for display?

 

Bingo. Again this is a problem both Edge juice and Quadzilla both have in using the boost information from the MAP sensor. Being there is 3 different MAP sensors used and at least two different versions of ECM software used for boost information it does make things a bit of a mess. The only thing the Quadzilla does is verify the sensor is working. As for accuracy I wouldn't suggest use the Quadzilla for boost readings. 

So the information the ecm is receiving from the sensor is correct and the value displayed is incorrect? Just making sure we're saying the same thing. Maybe I'll try a different MAP since they're relatively cheap and see if it helps.

Edited December 19, 20186 yr by Scottfunk

I noticed a cruise psi @ 70mph go from 2-3psi to 4-5psi when I installed a new map sensor (quadzilla).

Feel mechanical is the best bet for monitoring. 

9 hours ago, Scottfunk said:

So the information the ecm is receiving from the sensor is correct and the value displayed is incorrect? Just making sure we're saying the same thing. Maybe I'll try a different MAP since they're relatively cheap and see if it helps.

No electric sensors are not always %100 accurate, they don't need to be.  They are used to sense change in boost as a reference point.

Is there any risk of harming engine components by performing a dead cylinder test mechanically; holding a rag against the back of the injector and cracking them open one at a time to observe changes in operation?

On 12/18/2018 at 8:16 PM, Scottfunk said:

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?

My 99 idles in part at 200F with 100's popped at 320bar. back when they were at 280 bar I was seeing upwards of 250-300F.

However, under load I was seeing higher boost and EGT's because I had to have my foot farther into the throttle in order to keep it moving.
 

2 hours ago, Scottfunk said:

Is there any risk of harming engine components by performing a dead cylinder test mechanically; holding a rag against the back of the injector and cracking them open one at a time to observe changes in operation?

 

I would pull and get the injectors popped personally. Find out what size they are and what your popping at. I have done what you speak of, but your not going to hear the change. the only thing that really works is the 3 Cylinder high idle mod, and you can swap injectors around, but that's a waste of time.

If you have say 100's or 150 hp injectors and your programmer isn't tuned right for them you will see issues. Pop to low, same thing.

I had issues with my brakes dragging, transmission binding and other dumb stuff including a wheel out of alignment. But 11mpg is still better then a ford Excursion getting 6mpg. I just think you should be 17-20mpg easily in that GMC.

 
Is this realistic? It's not that I can't afford to pay for pop testing but I can't afford to have my truck down and I can't afford to drop the injectors and have a guy tell me I can pick them up thursday and then listen to some excuse about how they won't be able to get to them until next month.

Check eBay, there's some cheap pop testers around. You'll also need the Cummins adapter. I bought one, I think from India. The gauge was crap, so I replaced it with one I sourced locally. It's cheap-ish and it works. I've re-popped my injectors a few times on the Cummins, and even in my VW TDI.

Edited December 20, 20186 yr by kzimmer
Typo

I thought if a guy can find a junk head for free, chop one cylinder off and use it as a jig for pop testing 

They sell the collars for pop testing.

44 minutes ago, Mopar1973Man said:

They sell the collars for pop testing.

Chunk of head sounds more fun and you can sell the other five pieces 

 

Heck of lot easier to set up that heavy piece of cast iron. 

It sure is, I guess I never looked at these to see just how simple they are

@Mopar1973Man do you know of anywhere local that sells them? I'm going to have some extra time on my hands over the next few days and if I could use it to put this behind me that would be super.