Hello,

This truck has become the bane of my existence.

98 ranger xlt 4.0 ohv 4x4

I got my truck up and running with a new (to me) motor.
it was throwing lean codes and I just started going through and replacing things that could be the issue.( I went wild researching this place was definitely a Wealth knowledge )

i have replaced all of the vacuum lines with softer vacuum hose since all the hard lines became brittle and cracked.

replaced fuel injectors with ultr-power fj579
replaced the gaskets from lower intake to fuel rail fuel rail to upper intake
replaced egr valve gasket and o ring.
cleaned maf and egr valve
replace air filter
replace PCV valve

now the truck is running rich and throwing p0172/p0175

i just don’t know where to go from here
any suggestions would be great.

I am not a smart man but I try.

ive attached pics of my live data



Fuel trim but with raised idle

 

Not sure if you truck has a "Fuel Pressure Regulator"

Or a "Fuel Pressure Damper" they look very similar



Check the small vacuum line going to it (red circle) there should be NO gas in it, the diaphragm may be ruptured and flooding the engine with to much fuel.

 

Welcome to the forum

1998 won't have the Fuel Pressure Regulator on the engine, 1997 would
1998 can have a Pulse Damper on the fuel rail, it has a vacuum hose attached so can suck in raw fuel if pulse damper leaks

Lean and Rich codes have nothing to do with how the engine is running, you need to remember that one thing
Lean and Rich codes are only about the computer

If an engine is actually running Lean it will ping/knock when accerating
If an engine is actually running Rich it will blow smoke out the tail pipe

The computer calculates the Open Time for the fuel injectors, this is STFT(short term fuel trim)
Computer then watches the O2 sensor voltage to see if that calculated Open Time is correct
O2s can only see Oxygen, not fuel
0.1 volt is high oxygen, "lean"
0.9volt is low oxygen, "rich"

Computer tries to keep O2 voltage average at 0.4volt

On a V6 or V8 engine there is one O2 sensor on each side(bank) of the engine, called upstream O2 sensors
If just one Bank of a "V" engine shows lean or rich code then the issue is specific to that ONE Bank of the engine
If both banks show lean or rich codes then issue has to be something they have in common

How the computer calculates open time for the injectors
Computer is programmed at the factory for a few specifics
4 LITER engine size, so it knows EXACTLY how much air is coming in at any RPM and throttle position, its just math
Fuel injector size
and
Fuel Pressure, so it knows exactly how much gasoline will flow out of that injector at that fuel pressure

Fuel pressure should be tested any time there is a lean or rich code to take that off the table, as this is a MAIN number used is STFT calculations
1998 should be 60psi fuel pressure, +/- 8psi, it has a fuel pressure regulator in the gas tank

Sensors fill in the blanks for the computer to do its STFT calculations
Air TEMP and air WEIGHT, the mix ratio for gasoline is 14.7 to 1, 14.7 POUNDS of air for each POUND of gasoline, or 14.7grams of air to 1 gram of gasoline
This is why fuel injectors are rated in POUNDS per hour of fuel flow
Gasoline's weight doesn't change much when its cold or warm, its vapor does but that's another story, lol

Air weight changes ALOT when its cold and warm, that's why/how "hot air" balloons can fly, warmer air inside is lighter than the cooler air around the balloon so it floats, like wood floats in water
There is an Air Temp sensor for this
And a MAF("mass" air flow) sensor, MAF measures the density(mass) of the air coming in, air at sea level is denser(heavier) than air at 5,000ft elevation(Denver) so computer needs air Temp AND air Density to do correct WEIGHT calculations

O2 sensor are FEEDBACK sensors, real time feedback to tell computer if its STFT calculations were correct
-10STFT to +10STFT are fairly normal as an engine ages
Codes are set when -20 to +20 STFT or LTFT levels are reached
LTFT(long term fuel trim) is what computer saves in memory when its turned off, so on the next start up it doesn't have to relearn the status of the system all over again
When computer does it's STFT calculation, which is STFT 0, it then adds LTFT to it
So if LTFT was -10 then STFT 0 is -10 to start with, if STFT changes to -5 from O2 sensor feedback then its actually -15, -5 + -10 = -15

If you fix a problem and LTFT was -20, then as you drive you should see +10STFT for awhile and that will slowly bring down the -20 LTFT closer to 0 LTFT
LTFT is a LONG TERM average of STFT


New parts are an UNKNOWN, first assumption should be they DO NOT WORK, not that they work, unless they were Motorcraft parts, because these are tested
So swapping in new parts is a roll of the dice, and the very very last thing you should do, and only swap in one part at a time, if there is no change then swap back in the old part, for now

Running issues, if there are running issues like misfires these need to be addressed first, misfires generate Lean Codes, and are NOT caused by lean codes
Most common cause for misfires after old spark plugs, are burnt exhaust valves

If no misfires then lean or rich codes can be this
Low or high fuel pressure, test it
Clogged exhaust, vacuum test
Dirty injector tips, injector cleaner in gas tank
Dirty MAF sensor, clean it
Leaking EGR Valve, test it with vacuum hose