Hello,
I have tried everything I can think of to fix these lean codes but nothing has seemed to work yet. So far I have replaced Fuel pump and filter, Fuel pressure regulator, intake gaskets, cleaned the MAF sensor (readings check out too), replaced the o2 sensors, and checked every vacuum hose for leaks, including the intake and such. Idle air control and TPS seem to both be where they should be. EGR works fine i tested it with another vaccum line. Brake booster and line works fine. PCV is working well too and the elbow isn't broken. Fuel injectors sound fine when I took a screwdriver to them and listened. LT Fuel trims are maxing out at 25 and the short terms are almost always above 10. I've run out of ideas. Is there something I have missed? It occasionally misfires too, but i think that's because it's running so lean. Any Suggestions on what I should do next?

 

Welcome to the forum

Lean code does not mean engine is running lean, it means computers calculation was Lean so computer had to correct it based on O2 sensor feedback, add more fuel that calculated
So engine is NOT running Lean, or it would be pinging like crazy and your pistons would melt

After engine is warmed up and idling...............unplug the 2 wire connector on IAC Valve
IAC Valve will close and RPMs will drop to 500 or so, engine may even stall, either is GOOD, no vacuum leaks

If RPMs do not drop then there is a leak somewhere you just haven't found it yet

Yes, fuel pressure should be 55psi in 2002, there is no fuel pressure sensor so computer just assumes 55psi, if its running at 25psi then you would get Lean codes

The way it works is the computer will start to calculate "on the fly" air/fuel mix AFTER engine is warmed up, well after O2s are warmed up they need to be above 650deg to work properly
The computer has a few assumptions, fuel pressure for one, and engine size for another, a 4 Liter engine
So computer knows how much air is coming in to the 4 Liter engine at any given RPM, its just math
And computer knows how much gasoline will flow out of a 14lb injector with 55psi pressure if opened for say 100ms(milliseconds)

If you changed injector size then computer would be lost, or engine size or fuel pressure, because these are all assumptions

The MAF sensor WEIGHS the air, air weight changes quite a bit, with elevation it gets lighter(thinner air the higher you go), colder air is heavier, which is why "hot air rises"
So the reason why MAF is used is to see how heavy the incoming air is

And this is important because gasoline engines use 14.7:1 air:fuel ratio, and its a WEIGHT ratio
14.7 pounds of air to 1 pound of gasoline
14.7 grams of air to 1 gram of gasoline
Thats why injectors are rated in pounds(LBS), rating is the pounds of gasoline that will flow out in an hour

Volume doesn't matter only weight matters
So computer uses the weight of the air to calculate how much weight in gasoline needs to be added
And that calculation is 0 STFT(short term)

O2 sensors see Oxygen, not fuel, in the exhaust, too much Oxygen was a Lean burn, too little Oxygen is a Rich burn

So after 0 STFT is calculated then computer watches O2 sensor, if too much oxygen then it tries +2 STFT, if too little oxygen then it tries -2 STFT

Computer will go to +25 or -25 to get a good Oxygen level in exhaust, at +/-20 it will set codes

LTFT is an average of STFT over months and months
If LTFT is +25, and STFT is +10 thats actually +35 STFT in real time, just FYI


From the things you have done already and the current fuel trims I would say your computer is most likely the problem
If you can check fuel pressure, that would be good
And if you know injectors haven't been swapped to another size then you are left with computer problem, because it would have to be a darn BIG vacuum leak to hit +35 STFT

 

Ron, Thanks for writing that whole explanation out, I do appreciate it. After looking at my PCM and it's respective numbers I found out that the one that is currently installed is for a 2001 Ranger, NOT a 2002 ranger. I will check fuel pressure tomorrow and update you on that, but right now i think you may have been correct in the suspicion of the computer being the issue. If the fuel pressure ends up being fine would my next steps be purchasing a new/reprogrammed PCM? Thanks.

 

That would be my guess

You have covered the bases pretty well, and +35 STFT(+25 and +10) is way high, for bad MAF, O2s or even vacuum leak(you should hear it if its that big)
+35 means computer is having to add 35% more fuel than it calculated............

Low Fuel pressure or smaller injectors might do it

Any history on this Ranger?
Is it new to you and had issues from the start or are the Lean codes something new

Just wondering if someone put in a 3.0l Computer?
Which is 35% smaller engine than the 4.0l

Maybe blocked exhaust but engine would barely run after a minute or two, wouldn't be driveable

 

How did you do this? I had the same problem on my '99 4.0. Fuel trims peg at 35. Tried finding leaks in all vacuum line thru visual
inspection. Even tried the Carb cleaner method to no avail. It was only when I did a smoke injection test that I finally found
a leak between the upper intake and fuel rail. I replaced the upper to fuel rail, and fuel rail to lower. Been about six months with no
CEL and fuel trims at between 3 and 7.

 

+1 ^^^^

Smoke test is very good to find hidden vacuum leaks

 

Yea thats a possibility. I bought it about 6 months ago at 126k miles (it just hit 130k recently) with the lean codes on it, the guy didn't tell me about them (he cleared the check engine right before we met) and i made the mistake of not bringing a scanner to check everything (dumb I know i've yelled at myself plenty for it lol). When I changed the fuel pump it seemed fine for a few weeks at max I dont remember exactly, but the codes came back. Anyway i checked fuel pressure today and it turns out it is really low (barely reaches 30 PSI AFTER I crank it, not when i turn on the key which is a problem),which is weird because i already changed the fuel pump, filter, etc. Im sure the wrong computer isnt helping, but I think fuel presssure is the source of the problem right now. Unless the computer is messing with the fuel pressure? Not sure. I dont really want to throw another fuel pump at it, cause if its just gonna fail again (if it is failing), i'd rather find out why it keeps going out. I havent used a smoke test yet, but I will here soon i just have to make one of the homemade smoke testers. I used carb cleaner and butane all around the engine, but couldnt seem to find it.

 

Cycle key on and off a few times and see what fuel pressure you get

The fuel pump only runs for 2 seconds each time key is turned on
Pressure should build up to 60psi or so

Also behind the passenger side kick panel at the top is the inertia switch, it passes voltage to the fuel pump
You can hook up a Volt meter to either wire on this switch and to a ground
Then cycle key on and off to see if fuel pump is getting full battery voltage, should jump up to Battery voltage(12.3 to 12.8volts), for 2 seconds, then drop to 5-8volts, that's normal
If fuel pump isn't getting full 12volts then pressure will be low, and its actually 14volts when alternator is working
Its possible to have a corrode connection so fuel pump is only getting partial voltage

In the 2002 Ranger the Fuel pump assembly has the pump and pressure regulator both, these are what hold the pressure after engine is off, and pressure should hold above 30psi for MONTHS, and MONTHS

Pump Assembly seen here: https://images-na.ssl-images-amazon....AC_SL1000_.jpg

Just an example, there are 2 fuel lines at the top
The pump at the bottom is connected to one of those lines and that line goes to filter
The 2nd line at the top of the assembly is the RETURN line, FROM the filter
And it runs to the black and silver disc about half way up the assembly, that is the fuel pressure regulator, its set for 50-60psi, so 55psi average

So fuel pressure control is ALL in the fuel pump assembly, with the exception of a leak in a fuel line, which you would smell easily

 

So I tested the intertia switch wires and they jumped up to 12 for a second then dropped to a consistent 6-7 volts so i think that checks out, but I suppose the corrosion could be after the switch going to the pump? Also when the engine bogs down the o2 sensors read really low (.045v-.065v) and the loops status reads as OL-Fault. It's pretty random, but happens more lately so its hard to pinpoint what it could be.

 

If you want you can leave the volt meter connected to inertia switch, and start and drive the vehicle, should show vehicle voltage, 13.5-14.5v, should drop below 13.5v with engine running

O2s have a range of .1v-.9v, under 1 volt, .1 is Lean(high oxygen), .9 is Rich(low oxygen)
.35-.45 is the sweet spot, best oxygen level for upstream O2s

under .1 is very lean, high oxygen in exhaust

Downstream O2 should be .7-.8v, low oxygen, because Cats should use most of it up burning emissions up

 

Long story short, I fixed the problem by just replacing the fuel pump again. The fuel trims stay at a solid +/- 10% long and short. Weird solution. Not sure why and I hope it doesn't go out again.