ranger issues

I have a 99 ranger 4.0. Its missing like crazy and cant figure out why. It getting good fire and good fuel pressure. All the injectors are working. Short term fuel trim is showing bank 2 (driverside) is way rick and bank 2 O2 sensor is showing lean. Any suggestions?

It is 1 of 4 things

1. Coil pack
2. Grounding issues
3. Plugs or wires
4. O2 sensor (outside possibly)

Short Term Fuel trim with -1 or lower is rich, +1 or higher is lean, these numbers represent Fuel injector Pulse Width, how long an injector is open.
-10 to +10 is in the normal range, computer will usually turn on CEL(check engine light) when short term fuel trim gets to -20 or +20 area

If you have a - or + you need to have a 0, that 0 comes from MAF(mass air flow) sensor and the computer's 14:1 air:fuel ratio calculation based on the MAF sensors air volume data.
So if fuel trim is -20 then computers initial 14:1 calculation is adding too much fuel to the available air, there is less air than reported, so computer doesn't open injector for as long(leaking injector possible)
If +20 then there is more air than reported(vacuum leak possible) so computer is opening injectors longer, adding more fuel

O2 sensors read Oxygen levels not fuel levels, high oxygen = lean exhaust, low oxygen = rich exhaust.

Unless engine is pinging/knocking(lean) or blowing smoke(rich) it isn't actually running Lean or Rich, computer is adding more fuel or reducing the fuel so engine runs normally, a CEL for either Rich or Lean is just letting the driver know something is wrong, the calculations are not adding up.

A misfire dumps oxygen into the exhaust, it didn't get burned up, which means O2 sees lean(high oxygen levels) and computer adds more fuel.
So a misfire can cause Lean fuel trim.

So I would address the misfire first, 4.0l engines are known for being hard on spark plugs, over the years I have had a spark plug just fail while driving, motorcraft tend to last longer but they can still just fail one day.
Runs fine then it doesn't, just like that.
One spark plug died and looked fine, tested it on an old coil I have and it was deader than a door nail, oddest thing, not that old either.

So if you have a steady miss, i.e. same cylinder, then I would check each spark plug.

Also people often check the coil pack and sometimes goof up the firing order on these.
3 4
2 6
1 5
Front

That 5 6 4 gets me every time

The fuel trim numbers may indicate low fuel pressure but long shot, would need to see the actual numbers at idle and then driving at 40mph

How many miles on the O2 sensors?
The front O2s are rated for 100k miles, if they are well past that then replace them, they may not be the problem but you will get better MPG for the next 100k miles so worth it just in general maintenance.

The short term fuel trim for bank to reads 26.3 all day. Rev it up or at idle it doesnt changed. Iv checked every where for a vaccume leak and cant find one. Also i pulled the fuel rail and cleaned that and the injectors as well. All the injectors seem to be working right. I feel like iv narrowed the miss down to the number 6 cylinder but still cant find the problem. Ran a compression check on that cylinder and it is fine. The fuel pressure still sets at 64psi so thats good. When you spray brake clean through the breather tube on the intake boot it smooths out. But still cant figure it out

Short Term Fuel trim not changing means the O2 or it's circuit is bad.
If you don't have an "O2 sensor not switching" code then I would suspect Computer issue, as it is not detecting the fault, just reacting to the +26 so other bank is rich.
Unplug O2 sensor on that bank, see what code you get, should get Heated O2 fault and no O2 fault.

And when engine actually runs lean it will ping/knock all the time when accelerating or going uphill, any load on engine will cause that with lean fuel mix.
Rich running causes very hot Cat converter

Iv tried unplugging the O2 on that side and it doesnt anything. It doesnt throw a code for that O2 just keeps saying bank 2 lean. With it idling i can pull plug wires off and make it miss worse but the computer never shows a miss fire. Do you think that is a computer issue?

Certainly reads like one.

Misfire code takes a bit to set, it doesn't just come on when engine misses, so not a good test if computer is working.
Computer stores RPM and throttle position when a misfire is detected, and the misfire must repeat a few times with the same parameters before misfire code is set.

If you have a live scanner watch the O2 voltage reading on that bank, sensor 1, it should change alot and very fast.

Watch the other bank as well, sensor 1.

Just FYI, sensor 2 is the downstream O2 sensor located after the cat converter, it doesn't change as much, and except for long term fuel trim and test cat is working it won't effect engine operation.

My thoughts were more about the O2 sensor circuit in the computer being the problem, but that would require replacing computer.
The fact the computer is setting some codes means part of it is working OK, unplug the MAF sensor, and start engine that should cause CEL very quickly.

One bank is lean +26
Other bank is Rich, but whats the trim number?

The fuel trim on the other bank bounces back and forth from around -4 to +6 at idle

That is not Rich it is normal, so either upstream O2 on the other bank is bad or computer circuit reading it is.
If possible swap O2 sensors around.

-5 to +5 driving is normal, +3 to +10 at idle is normal as well, idle should have extra fuel so higher "+" number
And the numbers should always be changing and changing very fast

Ok so you think O2 on the bank with the + fuel trim is bad? Or possibly the computer?

Micky123 said:

Ok so you think O2 on the bank with the + fuel trim is bad? Or possibly the computer?

Click to expand...

Yes, if it is not changing.
But swap it with the other O2 to confirm if it is O2 sensor or computer.

Computer looks likely because there should be a code telling you the O2 sensor on that bank is not changing.

After 100,000 miles upstream O2 sensors often start to fail, and will lower MPG in any case, so general maintenance is to replace them, downstream O2 sees cleaner exhaust so will last longer