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My question is, why is the load value on the scan tool not changing during different engine speeds, coasting, accelerating, decelerating etc. Its stays at exactly 0.8%
Ive never given a whole lot of credence to the calculated load value. Some cars will idle 2% other will idle at 35%... Some wil run down the highway at 80mph at 15% others will be at 85%.... Brand new off the lot.
Here's some freeze frame data from when the p0171 was set. I've only one bank on this small engine!
I've cleared the code a few times and swapped out the o2 sensors. Code keeps coming back ....
Since your total trims (combo of both) is much higher off idle, I'd still look at fuel delivery or mass air sensor. That wot test might help determine if it's starving for fuel.
According to SAE International SAE J1979 / ISO 15031-5, the formula the computer uses for calculating load is:
LOAD_PCT = [current airflow] / [(peak airflow at WOT@STP as a function of rpm) * (BARO/29.92) * SQRT(298/(AAT+273))]
- STP = Standard Temperature and Pressure = 25 °C, 29.92 in Hg BARO,
- SQRT = square root
- WOT = wide open throttle
- AAT = Ambient Air Temperature (in °C)
Characteristics of LOAD_PCT are:
- Reaches 1.0 at WOT at any altitude, temperature or rpm for both naturally
aspirated and boosted engines.
- Indicates percent of peak available torque.
- Linearly correlated with engine vacuum
- Often used to schedule power enrichment.
If your Load value is fixed low for all conditions and RPM ranges stationary and while driving, my first thought is there's something wrong with the MAF or its wiring.
I'd check and see if your MAF readings change at all, or if it's fixed at a constant value.
What is the linear Correlation with vacuum? Like, what percentage would different vacuum numbers be? Or where would I find out? Because that would be the first time I ever found a use for this PID...
The way I understand how the equation works is this:
Load = (Qmaf) / (Qmax(RPM) * TEMP ADJ.)
Qmaf is the air flow rate from your MAF
Qmax(RPM) is the maximum air flow rate at WOT for a specific RPM range at 25C stored in the computer (set by the engineers)
TEMP ADJ. is an adjustment factor to modify Qmax(RPM) based on what the air temperature is, since the temperature will affect the density of air (Cold dense air moving at 10 m/sec will have a higher mass flow rate than warm less-dense air moving at 10 m/sec).
Another way to think of load, is the force that is resisting the power produced by the engine.
If you're going up a hill in third gear, to keep the engine at 2.5k rpm, the engine will have to pump a lot more air than if you're cruising in third on flat terrain.
I find that load can be more useful than rpm when you're trying to determine if you've got a fuel delivery issue, since the power demand on an engine at say 2.5k rpm will be much higher when you are driving up a hill compared to 2.5k rpm when sitting in park.
see that was my understanding.... I don't see how that equation would output a value that has a direct linear correlation with vacuum.... Am I missing something?
