I have a 99 Ford Ranger. Just did a tune up, replacing all 6 plugs and wires (after 3 and 5 had been misfiring). Now, it threw up a code that 3 was misfiring again. No idea why. Also threw up two other codes.

All codes from reading are: P0303 Cylinder 3 Misfire Detected // P1131 Lack of H02S11 Switches - Sensor Indicates Lean // P0171 Fuel System Too Lean (Cylinder Bank 1)

Poured some seafoam in to help clear up fuel, didn't seem to do much in regards to the misfire. Misfire seems to happen around the 60 MPH mark. Any ideas/advice would be great! Thanks!

 

Welcome to the forum

Bank 1 on a V6(or V8) engine is the side of the engine with Cylinder #1, which on Fords is the passenger side

Lean code means computer is having to add more gasoline than it calculated to get O2 voltage on that Bank to stay near .4volts
An O2 sensor voltage is between .1 to .9 volts
.1 volt is Lean, too much oxygen in exhaust
.9 volt is Rich, too little oxygen in exhaust

Computer calculates OPEN Time for each fuel injector based on KNOWN fuel pressure, injector size, engine displacement, RPMs and throttle position.
After fuel is burned it gets feedback from O2 sensor in the form of .1-.9 volts
Computer then changes OPEN time to fine tune O2 sensor voltage to about .3/.4 volts
If OPEN time doesn't vary more then 15% of what computer calculated then all is well
If it does then computer will set a code

Lean code means computer is having to OPEN injectors more than 15% longer to add extra fuel to get O2 to read .4volts

Lack of O2 switching-lean means O2 stayed at .1 for a few seconds even though computer was adding extra fuel.

All of this happens in milliseconds, the adjustments.

The fact you are getting physical reactions like a misfire means system is really out of balance.

Assuming no previous engine problems that could damage an O2 sensor they generally last about 100k miles before they "go Lean", O2 sensors fail Lean as they run out of chemicals, they use a chemical reaction to detect Oxygen, so they do wear out.
If you don't know when O2 sensors were last changed and you have over 150k miles then I would change both upstream O2 sensors on spec, it WILL improve MPG over next 100k miles, for sure

If O2 is giving a false Lean then engine is actually running Rich on that bank which can also cause a misfire
What did the OLD spark plugs look like on that bank?
Dark brown or blackish would mean Rich running.


Vacuum leak on the lower intake on that bank could cause Lean issue
Exhaust leak on the exhaust manifold on that bank could as well

A MAF sensor issue or upper intake vacuum leak would cause Lean on BOTH banks of the engine
As would lower fuel pressure

It is possible that a fuel injector on that bank is failing, #3 , but longshot

Compression test would be worthwhile for all 6 cylinders
This is done to take compression issue off the table, much time and money can be wasted on non-fixes if it's a compression issue.
Compression is mechanical, so Black and white, either good or not good, so once tested it can be taken off the table or found to be the problem.


EDIT:
Just to make sure
Check firing order on Coil pack and that each spark plug wire goes to correct cylinder
Engine is numbered this way
3 6
2 5
1 4
front

Coil pack has only 3 coils inside so has MATCHED pairs
They are
3-4
2-6
1-5

So spark plugs 1 and 5 share the same coil on the coil pack
as do 2 and 6, and 3 and 4

Picture here: http://www.fordrangerforum.com/3-0-v...plug-coil.html

You can reverse 3 and 4 wires, or 1 and 5, as long as they are kept on the same coil in the pack, and this can be used as diagnostic tool, reversing the wires

 

Thanks for the detailed reply and diagnosis, RonD. Here's an update...

Replaced the MAF Sensor. Reset battery. Check engine came on again, this time threw up 2 codes, but same ones... P0303 (cyl. 3 miss) & P0171 (bank 1 sys too lean O2).

What if I replaced the coil pack? That's my next stop, but my knowledge isn't great on these things.

 

Check all vacuum hoses for leaks. If you can't find anything spray (heavily) with a squirt bottle a stream of water around the intake manifold gasket in the #3 area.

If those don't show anything you might try inspecting the fuel injector for #3.

 

Check to see if your injectors are supplying fuel. Turn on the truck and put a long screwdriver to a working injector so you know what the ticking sounds like. Compare to the injectors 3 & 5. Double check with a voltmeter to see the ohms. They should be around 16.5 ohms.