6.2 L92 Volumetric Efficiency

[goldata81]

Sorry this post got longer than I was planning....

So I've been dealing with a few error codes the past 2 months... P0121, P0171 and P0174 on my '07 Yukon Denali with 113k. Vehicle is completely stock besides a K&N air filter that I've had on for about 60k. There has been no engine work ever done. New plugs and wires at 100k. I've been logging OBDII data looking for a trend to point me in the right direction for a fix the past 2 weeks. The long term fuel trims max out at just over 24% under moderate to heavy acceleration but range about 0 to 5% at idle. MAP and MAF values appear normal(MAF is 6-8 g/s at idle) but I am not sure what they really should be. Commanded throttle and relative throttle position track each other nicely. Fuel pressure is 60 psi at idle, I haven't had a chance to check under heavy acceleration. I have taken my logged data during a few brief WOT runs and have calculated about 62% volumetric efficiency. I can't seem to find any info on what VE I should be expecting. To me, ~62% seems low?
This past weekend, I cleaned the MAF sensor and reran a WOT run today and now see about ~72% VE and my LFTF are about half of what they were before I cleaned the MAF and I haven't had any codes pop up over the last 3 days. The P0171 and P0174 lean codes were very repeatable with moderate-heavy acceleration prior to cleaning the MAF. The P0121 seemed more sporadic but just letting the engine idle in park for about 15 minutes would trigger it.
At idle the MAF reads between 6-8 g/s but read the same at idle before the cleaning. At the WOT run after the cleaning, the MAF is reading higher as seen in the VE calculations. My LTFT are now around 12-13% under heavy acceleration which still seems a tad high. So my questions are:

1. What is a reasonable VE for a stock L92 6.2L?

2. The MAF reading seems improved after cleaning, but is it still under-reporting based on the VE of ~72% I calculate?

One other thing I found interesting... I figure the MAF reading is a combination of RPM and throttle opening. I took the volumetric airflow(cuft/min) calculations I got during the VE calculations and divided it by the engine rpm, then scaled that by 1000 to get a value that was easily readable and cross plotted that result with the commanded throttle actuator control. By looking at the plot, it seems the airflow per RPM increases linearly with commanded throttle then hits a wall at about 50% commanded throttle, after which you get no more airflow per RPM at larger throttle opening. Maybe I still have an issue or maybe that is how these are designed. After cleaning the MAF, the plotted values did increase.

 

[Jason_S]

On most normally aspirated engines a maximum of 85 percent VE is achieved at peak torque. I do not know specifically about the L92 though.

 

[YukonandtheHOE]

Go to the HP turner or Efi live forum's.

What are you trying to achieve? Modify or just compare data? Im sure theirs a 6.2l base tune in the HPT Repository. If you cant find what your looking for on the forum's, let me know ill get a base tune and do some screen shots for yah

 

[goldata81]

Thanks for the replies back. I'm not attempting to tune but was trying to figure out what the expected airflow should be to see if the MAF was the root cause of the 3 trouble codes I was seeing. I was trying to correlate airflow to throttle opening to engine speed but the 3d plots in Excel weren't working out so I just divided the airflow by engine speed to get the airflow per rpm (then scaled by 1000) and plotted that against the commanded throttle. So far after the MAF cleaning my CEL has remained off.

jason_s, good point on measuring VE at the peak torque. I am not sure where that is on the L92 but will do a bit more digging to find out. The number I had noted above I am sure was not close to peak torque.

As I think about it more, I may need to take a second look at the data I plotted since for every 2 rpm (4-stroke engine) the airflow should be close to 85% of 6.2L of air at the peak torque at WOT. According to the plots, any throttle command over ~50% doesn't net you any more airflow per engine revolution since the plot looks to flatten out.

I ended up ordering a new MAF but have yet to install it. I figure I'll install the new sensor and log the data again to see how it compares to the old cleaned MAF data.

 

[Mister381]

Did you ever log any more data?

 

[j91z28d1]

Did you ever log any more data?

what kinda data are you looking for?

 

[Doubeleive]

for future reference another thing to check in these types of circumstances is back-pressure
I had bizarre related codes (that only occurred under a load) and performance issue's, turned out I had a clogged 3rd cat
2.5psi is max, I had 5 at idle and 10 with throttle. gutted 3rd cat, problem vanished
often it is usually the MAF though, it can cause of host of problems/codes seemingly unrelated.

 

[Foggy]

171 & 174 are lean codes.. prob have a vacuum leak or an intake manifold gasket
leak - very common with age on these.. You'll need to fix these first before
you'll get any help on HPT forum... your info is all skewed until you get this fixed

 

[Marky Dissod]

1. What is a reasonable VE for a stock L92 6.2L?

Regardless of engine & cam & heads & intake manifold & exhaust SCAVENGING,
VE varies across MAP readings & RpMs.
Multiply the VE by a displacement (cylinder or engine), and you have the amount of air 'expected'.

VE @ closed throttle @ 600RpM is less than VE @ closed throttle @ 1000RpM, but not by much.
GM VVT improves VE below peak torque RpM, for example.

I figure the MAF reading is a combination of RpM and throttle opening.

MAF directly measures how much air is passing thru it, moment to moment.
There are other tables that basically say,
'based on whatever the MAF just said - which better be pretty damn close to the expected VE table area we're in, or I'll throw a gottdamnt code! -
inject this much fuel, then light this much BTDC, then check with the O2 sensors to see how close the calculations were, and by how much to adjust them next time.

VE is a very educated guess of how much air SHOULD be in the cylinder / engine as a function of RpM vs KPa.
Short Term Fuel Trims are how far off the bullseye the guesses are moment to moment.
Long Term Fuel Trims are STFT performance trends.

During very slow / small throttle / RpM changes, the more the MAF is prioritized over the VE table.
The quicker the throttle / RpMs change, the more the VE table is prioritized over the MAF, until things slow down a bit.

I'll get a base tune and do some screen shots for yah

Please show him a VE table or two, as well as an MAF table, for starters?

 

[petethepug]

Someone on the forum purchased an OEM Bosch MAF for their L94 and discovered it wasn’t in spec for his truck. The solution was to find the OEM part number from a GM website via the VIN.

He put a lot of time into figuring this out.