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Old 08-18-2013, 08:07 PM   #193 (permalink)
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Join Date: Dec 2008
Location: lone star state
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lesson 2; Accuracy, first off ,it's no instrument. it's only a toy. (but nothing better to learn to drive frugal !)
when your ECU and scan tool boots,
your scan tool does a valid pid scan, to see all valid scans.

1: if not 01 5E pid valid, the tool must use MAF reverse calcs. most older cars dont have this.
these are only accurate on a hot engine and in closed loop, and are not accurate, otherwise.
Can be very accurate, on flat ground cruise near 50mph, and never at WOT or near 75% of WOT. (if no MAF it uses the MAP)

2: if the car has 5E available and the tool is smart enough to use that? it gets fuel rates from the
core of the ECU injection rate algorithm.

This may be, vastly more accurate at all times. but not on the low end.
read how injectors work , at low rates,( not so good, is the simple answer)
the low end is a very tiny flow, at cut fuel mode.
Does, this help understand the limits for ECU OBD2 scans?
The ECU also uses the injector to keep the TWC, three way CAT hot.
If car is CALIF car , you can be sure it does. yes, this wastes fuel. but makes the air clean.
Case in point:
Here are some more facts about OBD2, 1996 to present
I have 2 examples here, a 96, Suzuki 1.6L and a 96 , ford v6 3L
they are totally opposites (book ends)
the Suzuki; if you kill the CMP (cam sensor , the spark dies on purpose.) Dead Distributor.
this is because the ecu MUST have cam location data.

the Ford keeps running. CMP cut. (it has no Distributor "DIS" system it is)
the Ford, runs off the crank sensor, it can do that magic, because, it CAN.
it has wasted spark ignition.

But the FORD goes into to OBD fault mode. P0340? and that fact
shuts down the mandatory spark misfire monitors.
and goes to limp-home, which wastes fuel. (fuel guessing mode)

I used many types of ECUs to test my OBD tools, I make.

Point 2, if you go into Fault mode, the SYSTEM can be slow to recover in to full
closed loop. after say, dumping the clutch , coasting. (bump starting)

That is because , this ECU you have , drops all the fuel trims on the floor.
It did that due to limphome mode.
Depends on year, make of car and even if car is from Calif. how it does that and how fast.
my point?. your readings will be way off. (off, dead,or pure lies)

BTW, my TI30IIS hand calculator. (brand new) shows 45mph/0 =
"Divide By 0 ERROR, on the LCD. ( nothing new under sun? huh)

on most microprocessors, this is a NMI, "non mask-able interrupt."
That means, in uCode jargon, the last math instruction was (DOA).
how you or your scan tool handles NMI is your job,.

Me ? I trap them, and use a very tiny number, to avoid all errors of this type.
mine is really, in OBD2 parlance just a 16bit integer. PID rules.
so in binary that would be B0000000000000001. (i add the LSB 1 at end)
here is 1 more, fly in the ointment.
the ECU may produces wrong numbers in the live data stream , when in LIMPHOME.
each ECU works different, Iv'e run many and they all have odd , results in Limphome.

I have a Plamer Software scan tool software, and comes with the MPG algorithm for free.
I does in fact never divide by zero, it shows about 150 mpg. coasting, in cut fuel.
and you can add in your own macro instructions an display any thing you want !

my question to you, why is 150mpg not good enough.?
That number btw is very inaccurate. on all cars. it's a huge number yes but is not accurate.
It's not because no ecu was ever designed to do that. not 1. at all times and all modes.

but does work well , hot engine, flat road , easy cruise.
and the O2 sensor fulling engaged.
on my car and my guage, i have a closed loop LED.
i keep the LED going for the longest time and MPG is MAX !

just 1 little LED, can do that.

IN the end , only the fuel tank refill , is the RAW truth. Rubber meets road.

BTW 4, be careful slowing down with no brakes (lamps), the guy behind you can eat your can.

hers is my proof, nothing new under the sun..... 84 mpg (very light car)
Has carb that my thumb cant fit into... too...
note JOCKEY!

Last edited by jtgh; 08-19-2013 at 05:00 AM..
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