1997 Ford Ranger XLT 4.0 liter automatic 2wd. Trouble codes that caused the check engine light to just come on are p0171 and P0174. My check engine scanner indicates that both banks are running lean. What would cause this? What should I be looking for? Any advice or recommendations? O2 sensors?

 

Just FYI, neither bank is actually running Lean, that's not what the codes mean

It means the computer's CALCULATED air/fuel mix was Lean so computer is having to add more than 15% more fuel than it had calculated base on sensor input

The computers main job is to calculate how long to open each injector to get a balanced 14.7:1 air:fuel ratio
Air/fuel ratio is by WEIGHT, 14.7 POUNDS of air, to 1 POUND of gasoline, 14.7grams of air to 1 gram of gasoline

Computer already knows(is programmed for) what engine its running, a 4 LITER engine, so it knows how much air will be pulled in at any RPM and throttle position, its just math
What it doesn't know is the WEIGHT of that air, that's what MAF sensor is for, it weighs the air, really, lol, and air weight changes ALOT

Computer is also programmed for the Fuel Pressure at each injector, in 1997 that should be 30psi, and it knows the weight of gasoline which doesn't change much
So by opening an injector for 100ms(milliseconds) in knows grams of fuel added

So its does its calculation and then watches O2 sensors, if O2 shows Lean(too much air) then computer adds a bit more fuel on the next RPM, then a bit more and more until O2 show "Good Burn", this takes about 2 seconds
If it has to add more than 15% more fuel it will notify the driver by setting a Lean code and turning on CEL
Its still adding the extra fuel, so engine is not running Lean

So why is computers calculation off?
Well ALL the air coming into the engine needs to pass thru the MAF sensor, if there was an air leak between MAF and O2 sensor, vacuum leak, then MAF data would be incorrect, so calculation would be incorrect
If MAF sensor was dirty, same thing, incorrect data to computer

If fuel pressure was say 15psi, then computer is basing calculation on wrong info, 1/2 the expected pressure so less fuel flows out

Clean MAF sensor, its easy to do

Warm up the engine and let it idle
Unplug the IAC Valve's 2 wire connector, idle should drop and engine may even stall, if not you have a vacuum leak

Test fuel pressure, engine running, and then increase RPMs to 2,500 and hold it there for 15seconds or so making sure pressure isn't slowing dropping, if it is change fuel filter first, but could be a pump issue

 

How can one go about testing the MAF? Do I need a scanner with live data or is there any bench tests that can be performed?

I brought the engine up to operating temperatures and disconnected the IAC valve 2 wire plug. The engine's idle idled down to 500 rpms and stayed running steadily at 500 rpms..

Have not checked fuel pressure yet.

 

You can test MAF with volt meter but its not really definitive
4 wire MAF connector
2 wires on the outside should read battery voltage, engine running thats 13.5-14.5volts
2 inside wires should read 0-5volts, about 1v engine idling, then volts should go up when you REV engine to higher RPMs

A $15 Bluetooth OBD2 reader would be a better way to see if MAF and everything else is working OK

Always Clean MAF sensor first

Idle dropped so no vacuum leaks
So it comes down to dirty MAF, cracked or leaking air tube(between MAF and intake), or low fuel pressure

O2 sensors do wear out, only sensor that can wear out, they run out of chemicals to detect Oxygen, but BOTH going Lean at the same time is very long odds
Upstream O2s last 125k miles or 10 years, after that they will lower MPG a bit so often worthwhile changing them just to get best MPG back, they pay for themselves over the next 125k or 10 years in fuel savings

Clogged exhaust can cause Lean codes as well, but you would notice poor running at higher RPMs/Speeds as exhaust pressure built up, which you don't mention

 

Is the test you mentioned with a voltmeter performed while engine is running? while the connector is unplugged or plugged in? Am I just probing the wires while engine is running? Want to make sure I don't fry anything like the ecu before performing this test. I do have access to a Zurich code reader with live data capability. What should I be looking for while using this tool?

 

Yes, engine running with MAF connected

You can usually get a meters probe into the wires slot, but if probes are larger then no, so what you do is to shut off engine, key off, then get 4 sewing needles/pins and pierce each wire with a needle and space the needles out so they can not touch, so no shorts
Then start engine and test the wires that way

The needles will not damage the insulation on the wire
You can also put needles into the connector so you now have a contact to test

 

Thanks Ron. I'm going to do some testing and will let you know. I am suspecting my MAF sensor to be faulty. I just cleaned it 2-3 months ago. Is there a tutorial on how to replace my MAF sensor? It is a 1997 ford ranger xlt 4.0 ohv with the conical cone type air filter, so round housing??

 

replacing the MAF is pretty straightforward. It's right after the airbox. Only thing is it may be a bit pricey. I have an aftermarket one on mine that cost about $90. Its recommended to use a motorcaft part here. That may be more expensive. Certain parts like sensors I've heard to always use motorcraft parts because if they read slightly off it could give you engine running problems. Use the cheaper off brand parts where accuracy doesn't matter as much.

 

Oh yeah much more expensive using OEM. $300 part on rockauto. $50 refund after core.

 

Just tested the MAF with a multimeter per your procedure. Everything checked out fine except the two inner wires. It read 1volt at idle (the middle wires). Then as I revved/tached it up in rpm the voltage actually dropped instead of raising. It fell well below 1volt as i revved the engine. I'm thinking it for sure the MAF. Can anybody help me with a instructions or step by step guide to replacing it. I think I am going with motorcraft through rockauto. Please and thanks for all help.

 

Reads like its working but Ford may be using a Pull Down circuit so test the 2 inner wires to Battery ground with key on engine off
One should read 5volts, other one less than 5v
The less than 5volt is the signal wire you want to test with engine running, with same battery negative ground

 

Would you mind explaining testing the signal wire in layman terms, lol. I feel a little confused. I know what to do to find signal wire, but what I am doing from there? What is the test procedure? Thanks for you all your replies and efforts/time.

 

Set meter to 12v DC(20vDC on most)
Ground the meters black probe on battery negative, and use red probe on signal wire with engine running, should see about .8-1volt, REV engine

 

What I am looking for while I rev engine?

 

An increase in voltage, there is no specific number that matches RPM number

 

Okay. I did as you said. I tested the two middle wires with a multimeter set on 20volts. From battery ground on black lead and middle wires on red lead. Both middle wires read .03votls when I test them. Is that a sign of a bad MAF? I have a Zurich ZR15 with live data capability. Should I be using that along with these testing procedures?

 

Yes, you can use the OBD2 reader
Set MAF data for grams/second, g/s

Should see 4.2-4.8 g/s at idle then it should go up from there as you increase RPMs
You want it to be steady if RPMs are steady

General rule of thumb is 1gram for each liter of engine for each 500rpm
So 4 liter engine would pull in 4grams/second at 500rpm, idle is usually higher than 500 so 800-1,000

 

The Zurich ZR15 measure MAF in lb/min (pounds per minute). At idle 900 rpms the reading was 0.59lb/min and would increase with rpms. I went ahead and reset the check engine light for now. I am kind of at a lost here. Any suggestions?

 

Google: lb/min to g/s

.59lb/min is 4.4g/s , so that part is fine

Lean on both banks
Air leak between MAF and intake valves
Low fuel pressure
Exhaust blockage
Lots of misfire in both banks
Failing MAF
Old O2 sensors
Computer failing

That's all I can think of