Just got a OBDII scanner, what are normal settings for ex. fuel trim, intake pressure - Ranger-Forums - The Ultimate Ford Ranger Resource

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Old 02-15-2015
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Join Date: Feb 2015
Location: Davenport, IA
Posts: 3
Just got a OBDII scanner, what are normal settings for ex. fuel trim, intake pressure

Just got this little bluetooth scanner and app for android, tons of info but I'd like to know more about what it means in a diagnostic sense. For instance, are there "normal" values for fuel trim, intake pressure, MAF rates, etc.? If there is already a thread or even sticky for this then I'm sorry for wasting your time, just did a quick search of those terms on this site and didn't see anything that applied to me. If there is NOT a sticky for this subject, maybe we can make one. I'd have loved it as a new to ranger person to have as a guide "this spec should be within this amount of degrees" etc. Don't need all the answers handed to me, just would like a guide from those with more knowledge of this. Thanks in advance,
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Old 02-15-2015
Thread Starter
Join Date: Feb 2015
Location: Davenport, IA
Posts: 3
For instance, just logged a drive, and my LT fuel trim levels varied from a low of -11.72% to a high of +14.84%, is that normal? I had previously read that ST fuel trim levels would vary from -5 to +5%. That's just one example... Perhaps I should focus on the ST data, of which I did not log. BTW, my vehicle is a Mazda b2500 with a late 90s Ranger engine. 240000 miles on the body, 87000 miles on the engine. 2.5L 4cyl 5 speed. My last car was a 1980 Mercedes diesel, so getting used to electronics and the different fuel motor has been interesing, frustrating, and fun, to say the least... Another example is the vacuum data fluctuated a bit and I know I have a vac leak behind the glovebox (recirculating door actuator is missing. I have plugged the tube, but hasn't changed anything I've noticed...) Is there a "normal" vac range? Again, not expressly looking for specific answers to my ?s I've brought up, but am wondering if anyone knows of a fairly compressive list of "norms" for many of the typical data components. If not, and it's a good enough suggestion, I'd start a list right here.

Last edited by mhue; 02-15-2015 at 08:11 PM.
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Old 02-16-2015
Thread Starter
Join Date: Feb 2015
Location: Davenport, IA
Posts: 3
Just found this:
MOTOR Magazine Article | MOTOR Information Systems

Several examples of typical data, as well as a bit about what it means...
Answered a lot of ?s I had, and has sent me off on another round of googlin'. I don't know any of my new truck's history, and this new scan tool seems a good help to self diagnosis as I'd like to save as much moola as I can fixing it up. Cheers,
Matthew from IA
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Old 02-17-2015
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Sorry missed this post the first time around.

Yes, Short Term fuel trims change very fast.
Long Term fuel trims change slower, LTFT is used by the computer to run the engine without O2 sensor feedback, O2 sensors can not function when they are below 650degF, which is why they are heated but when the engine is cold it will take a few minutes for exhaust to get O2s hot enough.

Fuel trims are not a %, just FYI
+ means fuel injectors were opened longer, "pulse width" is the term used, longer pulse width means longer open time for fuel injector.

So - of course means less open time for injector, shorter pulse width

The MAF sensor reads the volume of air coming into the engine, this sets the "0" for the fuel trims, it is obviously a Floating 0 because RPMs change, increasing and decreasing air volume flowing thru the MAF sensor.
Computer starts by calculating 14:1 air:fuel ratio(based solely on MAF sensor data) and adds the amount of fuel needed for the 14:1 ratio, thats the 0
It then gets the O2 data, when available, and fine tunes the air:fuel mix
O2 sensors only see Oxygen, not fuel, too much Oxygen in exhaust means Lean air:fuel, too little Oxygen means Rich air:fuel
The computer constantly changes the pulse width to keep air:fuel as close to 0(14:1) as possible.
Yes, in a perfect world -5 to +5 while driving gives the best MPG.
But at idle and at Wide Open Throttle the engine needs a richer air:fuel mix so +10 to +20 is not unusual.

All vehicles change over time.
MAF sensors get dirty and fuel pumps have less pressure
If MAF is dirty it can report less air than is actually coming in(vacuum leak does the same thing, because the leaking air isn't coming thru the MAF sensor)
So computer is basing the the 14:1 on bad data, there is actually more air than reported by MAF sensor.
Computer see this in the O2 data as too much Oxygen in the exhaust so it increases pulse width(+) to add more fuel, the computer has a set parameter limit when it will warn the driver of a possible problem, it will set a code and turn on the CEL(check engine light)
Usually it will do this if fuel trims are +20 to +25 for any length of time, engine will run fine, computer is just notifying driver of a possible issue.
People often start replacing sensors when they get a code, DO NOT do this, a code usually means sensors and computer ARE WORKING, but a parameter limit has been reached so you should CHECK ENGINE, lol.

Computer has no way to monitor fuel pressure, on 2001 it expects 65psi of fuel pressure, over time fuel pump may drop down to 50psi, this means fuel injectors need to be opened longer(+) so that could cause +20 to +25 and a CEL

So the "normal" fuel trims will change over time as parts age.

Sensor #1 is the upstream O2 sensor, the one closest to the engine, this is use for air:fuel trim
Sensor #2 is the downstream sensor, located after the Cat converter, this is used to check if Cat converter is cleaning up the exhaust, it should always show a leaner exhaust

Last edited by RonD; 02-17-2015 at 12:10 PM.
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