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Very horrible Gas mileage help please

12K views 26 replies 4 participants last post by  TimD. 
Fuel mileage

OK So just for the record MAF readings in Grams per Second with car warmed up, sitting in park should idle about 3.8 to 4.0 Gs-ish, too much more more and computer will think its getting more air than it actually need and will conversely add fuel much as you are seeing. So lets explain GM tune strategies. (basically computer is reading O2 in through MAF and left over O2 after combustion)(its a constant dance)

OBD1 GM used basically a large block scale if drawn on graph paper 256 across, 256 down. The set numbers were 128 on Block Learn and 128 on Fuel Integrator. Now in each square had separations of 10 inside. if the o2 sensor saw lean condition it upped pulse width, rich it drops down. If it maxes out the block, then it switches to next highest to richen, or lowest to lean out the fuel trims. The integrator is long term fuel use learned.

OBD2 they compacted this to long term/short term trims. In a a stock mode long term and short term should NOT add up to more than 10+/-.
So internally the computer reads B1S1/B2S1 and requests more or less fuel. Each percent on scan info more or less has 10 points inside but you never see this is its just for calculations. If the short term maxes out at 1 it goes to 2 then 4 then so on and so forth.
Now after set calculation time/run time/number of revolutions the long term REMEMBERS and maintains this number so its not relearning.

So basically if we saw this as digital, car running you would see
Short Term 1.0% -9.9% Long Term stays at 0%
It goes to the next level of ST10% LT goes to 1% ST resets to 0%
So ST goes 0% LT 1%
So every time ST goes over 10% the LT jumps.
IF your running right and the actual program is in control ST in 0%-5% is great.
So in general if your ST was 3%, LT was 10% you have an issue.
ST3% LT3% 3+3=6 your good.
Long term evaluation should always be after at least 20 minutes of driving , preferably high way.

NOW lets talk about WHAT controls these ST/LT percentages.
O2 sensors are the heart and soul of fuel control.
After your primary O2s are active you should see mV below .200mV and over .800 mV .
I prefer AC Delco, NTK or Denso O2 sensors. Bosch are the Autolite of the O2 world except for specific factory used applications.

you want O2s going low and high. low is lean, high is rich on most O2s.
Most secondary O2s are simply for converter condition/operation. SOME newer cars have tried using after cat but it just isnt required in actuality.

If your ST+LT = closer to 10% in performance applications your either air leaking into after maf, or exhaust leak OR misfires.

Checking O2s should range freely and use below .200 and above .800mV.
Now some cars use O2 crosscounts but generally those are just condition indicators not critical to diagnostics.

And this all should be with NO BOOST AT IDLE.
The rule of 1 gS per liter of engine still holds.
By the way I am a 30yr tech, diagnostic tech, performance builder/tuner for many of those years.


QUOTE:
okay so at idle STFT its Either at 0.0 to 1.6 bouncing back and forth and my LTFT is at a constant 8.6
So now at 2000rpm
STFT iseen 0.8 , -0.8 and 2.6
LTFT 7.0
And my O2 sensor ready at idle is bouncing around in the .100s so itd bounce any where from 0.100-0.760


ok SO HERE WE GO that high on the O2 of .760 means you seem to have a lean bias O2 and this will lead computer to adding fuel hence your higher than needed LT readings. O2 should read 0.100-0.860(or higher) and I bet LT will drop down.

Basic strategy is O2 drives the ST, ST drives the LT
and all of that drives the injector on time which uses those number plus vacuum/boost, air flow, coolant temp, air inlet temp, throttle opening to calculate the amount of fuel required.



See the engine is an air pump, the computer starts reading exhaust oxygen content(running parameters hard coded into computer program/design) and tries to balance remaining air in exhaust gases to air going into engine. Any screw up and you get a rich or lean condition.
Easy things to remember
1 gram per second per Liter hot/idle/park/no boost
Primary O2 below 0.200 and above 0.800 and active
ST/LT needs to be as close to 0 to 3 when added together to see proper tune.
Remember when ST hits 10% the LT jumps another 1% so if your LT is 8.6% your ST has reset 8 times and is working on another jump, if you add them its over 10% total and your running rich, which if your O2 readings are accurate, your O2 has a lean bias and computer thinks its running lean.
 
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