Mass air flow readings

Camaroguy · Aug 4, 2018

Ok guys so my truck is getting about 20mpg on the highway and I don’t even want to know mpg for in town. So I’ve been watching the data on my scanner will driving and the mass air flow is reading 3-3.5 gps at 700-800 rpm and 50-58 gps at 3000 rpm witch is highway speed is that normal or is the mass air flow out of calibration? Also the sort fuel trim bounces around rich and lean the way it should but at cruising speed on the highway it will jump to 29.1 short fuel trim. I’m at a loss of where to go
A little history bought this truck had rod knock rebuild the engine has 175 across the board replaced both o2s sense one was dead the other slow replaced spark plugs with oe then smoke tested and found idle air and Egr leaking replaced them still no improvement in fuel economy
Thanks

ericbphoto · Aug 4, 2018

What size tires? Is the truck lifted? As my other significant modifications? Tire pressure good? What makes you think your mpg is off or MAF is wrong?

This message composed solely of recycled electrons. Go green!

Camaroguy · Aug 4, 2018

18-20 mpg on the highway and 235/65r16 on the back and 225/60r16 on the front rears are almost stock height as for the mass air flow the gallons per second at 3000 rpm seem to be high and the short fuel trim keeps going crazy and locking at 29.1

adsm08 · Aug 4, 2018

Where is your long fuel trim? 29.1 is pretty much maxed for the short, but if you are cruising on the highway it should be high because you are using fuel.

Camaroguy · Aug 4, 2018

Long is up to 8 but closer to 2 in town

adsm08 · Aug 4, 2018

That's not too bad.

Camaroguy · Aug 4, 2018

If it’s not at least 27 mpg than it’s getting another motor or sold not trying to be an a###ole but my Silverado gets 24 and that’s 4x4 350 extended cab

RonD · Aug 4, 2018

Wouldn't expect 27MPG from a 1999 Ranger, would be nice but not "normal" for regular driving

1999 2.5l is older engine technology, it is a Lima or Pinto engine, a SOHC first used in 1974 Pinto as 2.3l, last used in 1998 to mid 2001 Rangers as 2.5l, stroked 2.3l
22-24MPG would be about "normal"

That being said 20MPG would be on the low side of expected MPG in a 4cyl regular cab 2WD Ranger.
Check some spark plug tips, see if engine is actually running Richer than it should, darker brown tips.
Upstream O2 sensor needs to be changed every 100k miles, they run out of chemicals, just like batteries do, and they start to cause Rich running and lower MPG as they do.

Check your Math, look on drivers door label for the Stock Tire size, then look at the size on the truck now.
If not a match then Odometer will be off, so MPG calculation is off, if you are basing it on odometer.
Larger diameter tires lower odometer miles traveled, so you could have gone 250miles but odometer would show 230

I see nothing wrong with MAF data
If upstream O2 sensor is older that could cause the higher + fuel trim, and that would show up on spark plug tips as darker brown

Downstream O2 sensors tend to last longer, 200-300k

Camaroguy · Aug 4, 2018

New o2s up and down stream 22-24 seems low for a 4 cylinder and plugs where white tipped changed over to the proper Platinum plugs

RonD · Aug 4, 2018

If spark plugs have good color then you are not running rich, simple as that

Fuel trims describe fuel injector open time based on Computers calculated open time.

Computer has a base air:fuel mix set for engine size/displacement, a 2.5 LITER engine will pull in 2.5 liters of air every 2 RPM at Wide Open Throttle(WOT).
So computer knows the mix from factory tables in memory for any RPM and throttle position

Gasoline's mix ratio is 14.7:1, and it is a WEIGHT ratio
14.7 POUNDS of air to 1 POUND of gasoline

Gasoline's weight doesn't vary much by temperature, it does a little, but not much

Air's weight changes quite a bit by temperature AND elevation above sea level
So MAF and air temp sensors are used to aid computer in its calculations of the Weight of the incoming air so it can add correct weight of gasoline to it.

Computer sets that calculation as 0 STFT(short term fuel trim)
It then checks O2 sensors voltage, if .1 then too much oxygen in exhaust(lean burn), if .9 then not enough oxygen in exhaust(rich burn)

If Lean computer opens injectors a bit longer adding more gasoline, +2 STFT
If Rich computer shortens injector open time so less gasoline, -2 STFT
All this happens in a few seconds so engine would never be running Rich or Lean, Rich or Lean "codes" mean computer calculations are off so it is letting driver know that something isn't working correctly, i.e. vacuum leaks, low fuel pressure, leaking injectors, ect.......
Computer sets these codes if -15 or +15 STFT are seen over longer periods

Accelerating will always get to or above +20, computer ignores this since Fuel Trim is economy based, and driver wants POWER not economy if they have pressed down on the gas pedal, at WOT computer also ignores the O2 sensors entirely.

So.............if engine is not burning the gasoline, spark plug tips are good, then there is some other reason for the poor MPG
Driving habits

Dragging brakes
Loaded truck
Low tire pressure
Bad math
????

Craig0320 · Aug 4, 2018

My 98 2.5 5 speed 2wd single cab stock tires got 24 mogs

Mass air flow readings

Camaroguy

New Member

ericbphoto

Overlander in development

Camaroguy

New Member

adsm08

Senior Master Grease Monkey

Camaroguy

New Member

adsm08

Senior Master Grease Monkey

Camaroguy

New Member

RonD

Official TRS AI

Camaroguy

New Member

RonD

Official TRS AI

Craig0320

Well-Known Member

Sponsored Ad

Sponsored Ad

Staff online

Members online

Member & Vendor Upgrades

Latest posts

Truck of The Month

Recently Featured

Follow TRS On Instagram

Latest Album Photos

TRS Events

25th Anniversary Sponsors

Check Out The TRS Store

Sponsored Ad

Sponsored Ad

Sponsored Ad

Amazon Deals