My 1994 4.0 v6 Ranger had been running with a severe lack of power for a while. it was surging and hesitating badly but now things have gotten better. since ive been gone i took my truck into work(at an auto shop) and had the pros look at it. we checked for vacuum leaks,bad sensors, anything that might cause it to run badly. we replaced plugs,wires,coil pack, fuel pump, MAF sensor, IAT sensor, and coolant temp sensor. we aslo checked for clogged cats and couldnt find anything. after all of this we got it running better(MUCH better) but it still isnt quite happy yet. it still has a slight lack of power,an occasional surge, and runs a bit rough at idle. it is throwing a koeo code 137 for bank 2 running rich. this could be caused by several things. (bad injectors, bad o2 sensor, faulty wiring, etc.) any ideas on where to go with this? even the pros thought this was starnge.(but they arent ford guys anyways)

 

Computer controlled engines aren't a Ford thing.

TFI and Pulse Vacuum Hubs are Ford things, bad things, lol.


Plugs, wires and fuel filter are normal long term swaps.
Like air cleaner and oil, oil filter are short term swaps.

Replacing sensors is generally a waste of money, volt/ohm meter will test pretty much any sensor or control.

Tighten lower intake manifold, they loosen up on 4.0ls

What kind of mileage on the truck?
O2 sensors are good for about 100-150k, you will notice slight drop in MPG as they start to wear out, O2s tend to fail "rich", then replace them both(Bank 1 and Bank 2)
You will save the cost of the O2s on the MPG increase over the next year, lol.
Don't forget to unhook battery during, or after, cleaning or changing ANYTHING on a computer controlled engine, this reset computer so it will relearn parameters.

Some 4.0ls had EGR system, does yours?

I would invest $25 in a vacuum gauge, works on all gas engines, then run the tests.
Read here: Technical Articles: Engine testing with a Vacuum Gauge - at Greg's Engine & Machine

Where you see "carb" think fuel injector, for "points" think coil.

An engine is still an engine no matter how many extra parts you add, lol.
The electronics actually make things more reliable, not harder to work on.

Your '94 only uses the computer for fuel, spark is done by the EDIS-6 module on the rad support, it sends pulses to computer(PCM) to synchronize injectors and does get spark advance updates from computer, but the EDIS can run spark all on it's own.

 

i tried to only replace the cheapest sensors first just for good measure. the MAF sensor was actually going bad anyways and it made a lot more power.

We sprayed throttle body cleaner around the intake to see if there was an intake leak but only got an occasional rpm change that wasn't consistent. we even hooked up a smoke machine and couldnt see anything. what are the torque specs so i can tighten it anyways?

this truck has about 159k on it and has no egr valve that i can see. it would be on the upper air intake right???

electronics arent my strongsuit but im learning. i prefer an engine with a carb like my nash any day.

 

Lower intake torque starts at 3ft/lb then 3 rounds to get to 18ft/lbs, start in center and work your way out, just like heads.

After engine warms up and is idling unplug IAC valve, RPMs should drop to 500 or so, or engine may stall, either is OK it means no vacuum leak.

If idle stays high then you have a leak.

Pull a spark plug on the passenger side and the drivers side, see if drivers side(bank 2) is really running rich, plugs should show darker brown or black compared to passenger side plug.
If they look the same pull all 3 plugs on drivers side, just to be sure.
If all look OK then new O2 sensor could be the issue, or it's wiring or.......computer

If spark plug does show rich burn then O2 is OK and you may have a sticking injector.
Fuel injectors rarely fail electrically, but you can test them with an OHM meter, look for about 14ohms but check at least 3 injectors to get an average.

'94 will be Batch fire and on a 4.0l that means all 3 injectors on Bank 2 open at the same time, so not sequential injection.
Mechanical failure is more likely, injector opens then sticks open every other time or ??, so that bank runs rich.
If engine has a rough idle you can often find a sticking injector by tapping on it to free it up temporarily and idle will smooth out.
These are plastic injector housings so care should be taken, place a socket extension, or similar, against injector housing and tap on it, see if idle changes, test all 3 on that side.


If you were around in the carb days and you think it was less trouble then you would be wrong, lol, in my opinion of course.
The reason most think it was easier/better is because they had to learn over time to tune a carb(or reset points and adjust spark timing), the reason you HAD to learn it was because you ALWAYS had to do it, they never stayed "adjusted", and like anything if you have to repeat it all the time you learn it.
Today's electronics are much more reliable and in most cases self adjusting, so we don't have to learn it until we need to, which is hard, same as it was the FIRST TIME you had to adjust a carb or set points.
Same as if I dropped a Chinese shop manual in front of you and said "here read it, answers are in there", you would say "this would be much easier if was in English", lol, yup, because you LEARNED English, but not over night, it took a few years, so be patient.
The fundamentals are all the same and common sense still rules.

 

the 02 sensors are good(basically new) and unplugging the idle air valve dropped the rpm as you said. i have not checked plugs or injectors quite yet so ill get to it tonight. it needs to go to emissions by friday so im picking up the pace.