ok, I fixed my tach and took out lights for 4 low and 4 high so no more blinking. fired her up and she chugged a little more than usuall. my question is 2 part, with the running lean code, I understand its the 02 on both sides seeing too much oxygen in exhaust but is it possible the air is being sucked in at back of cats where I know I have a tiny leak or does it have to be before the 02 sensors, second, can this code be causing the pcm to keep trying to give more fuel to where she is actually running rich. when I fired her up I thought the exhaust which is near rear axle smelled a little rich? she seems to be a slow learner, anytime I disconnect battery for a length of time to work on her, she seems to take a while to relearn how to run good. any ideas on this would be appreciated

 

The PCM is adaptive, and has a KAM(keep alive memory, like clock and radio presets)so yes when you unhook battery, PCM has to relearn engine/system requirements, so wandering idle is possible and a Richer fuel mix when cold, should only take 2 or 3 drive cycles to relearn
1 drive cycle is full warm up, at least 15 minute engine run time, then full cool down, 1 hour cool off

PCM(computer) is setup with tables for fuel calculations based on engine displacement, i.e. 5.0l engine will pull in 5 liters of air at wide open throttle every 2 RPM.
And a liter of air has a specific weight at sea level
14.7:1 is the mix WEIGHT ratio of air and gasoline
So 14.7 POUNDS of air needs to be mixed with 1 POUND of gasoline to get the best burn ratio in the engine.
PCM also has the fuel pressure of that years vehicle programmed in, 35psi pressure will inject XX amount of gasoline if injector is opened for .XXX milliseconds.
Or 1998 and newer used 70psi pressure which would be programmed in to PCM.

The PCM can run the engine without any sensors, except for timing sensor, Crank Position(CKP) sensor on distributorless engines or TFI pulse on engines with distributors, this pulse is needed for spark and injector timing.

The other sensors are added to make the engine more efficient, i.e. better than carb and vacuum advance spark timing.

MAF(mass air flow) sensor gives PCM a more accurate weight of incoming air, i.e. higher elevation air weighs less than air at sea level, so less fuel should be added.
O2 sensors measure "after burn" oxygen in the exhaust.
As you said too much oxygen is a Lean burn, too little oxygen is a Rich burn
PCM can fine tune fuel injector Open Time, to increase or decrease injected fuel, this happens in milliseconds, so engine is never running Lean or Rich
Lean or Rich codes are warnings about computers calculations, not actual running condition.

MAF sensor can work anytime.
O2 sensors can not, they need to be above 600dgeF to read oxygen levels accurately.
So on cold starts the PCM runs in Open Loop
Open Loop means PCM is running on pre-programmed air/fuel mix based on RPM and throttle position, and any Learned data from the KAM
As a vehicle ages the fuel pressure might be 28psi now, which is fine but PCM needs to be aware of it and that is what KAM does, there is no fuel pressure gauge for PCM, but it Learns that the engine needs more fuel that calculated for some reason so it Learns that.
It does have limits pre-programmed in, when PCM gets to that limit it will set a code, Lean or Rich depending on which limit it passed.

The engine is never running Lean or Rich, the limit is a "computer line" set to inform driver that there may be a mechanical issue, i.e. low fuel pressure, vacuum leak, injector leak, ect.....

A cold gasoline engine must be run Rich or it won't start and will stall, laws of physics
So PCM runs cold engine in Open Loop and also in Choke mode
Choke mode is Rich mix, high idle(1,100+) and advanced spark timing
PCM will slowly reduce idle as engine warms up, ECT(engine coolant temp) sensor tells PCM engine temp
As engine warms up PCM tests O2 sensors, under 600degF the O2 won't switch voltages much, as it heats up the voltage will start to change several times a second, when that starts to happen the PCM will switch to Closed Loop
Closed Loop is running engine entirely on sensor inputs/readings, so most efficient MPG and lowest emissions.

The upstream O2 sensors are the main sensors for fuel economy
The downstream O2 sensor(after Cat) is there to test if Cat is working, cleaning exhaust.
It plays a small part in air:fuel mix, wouldn't cause a Lean code.

Dirty MAF sensor can under report air, so Lean code
Vacuum leak means "extra air" unreported, so Lean code
Low fuel pressure, less fuel, so Lean code

These are common causes if BOTH banks report Lean

If only 1 bank is Lean then could be misfire, no oxygen burned so it all gets dumped in to exhaust, or lower intake manifold leak.
But it could also be the O2 sensor itself
O2 sensors are good for up to 150k miles, they use a chemical reaction, like a car battery, so the chemicals do wear out.


Simple test for vacuum leak is to warm up the engine and let it idle, and then unplug IAC Valve, it will close and idle should drop to 500 or engine may even stall, either is good, no leaks.
If idle stays high then you do have a leak

 

Hey thanks Ron, I'm guessing your a mechanic or you really love reading up on cars. Here is the funny part, I forgot the downstream 02s are after cats. My exhaust is adaptors that are welded on by myself and I know there is a small leak on one side. The driver side exhaust pipe next to oil pan needed modified to not hit the double sump pan I used. a guy that worked with me did that weld but its possible it could have a tiny leak and if so maybe that's what is causing lean on both sides? the driver side forward 02 is new, the wire got torn during swap but the pass side is old one, no telling how many miles on it. I will be doing that little IAC valve test this weekend..thanks again for reply. and now I know that if I disconnect battery to give it more run time and cool down time

 

ok so I did the 3 cold starts and run it a good 15 min or so, shut it down and cool for an hr. on the 3rd run it was probable only 5 min or so and driving around block check engine lite came on. checked the code..its 0174 I think, running lean on bank 2. didn't do the IAC motor unplug thing yet but I'm actually hoping it might be the 02 sen. easier to fix,lol. she starts now and goes into a low..about 600 rpm idle, after maybe a min she goes up to maybe 800 rpm then just sits there and purrs. at times the idle may drop and stumble but I tap gas pedal and she seems fine

 

Automatics usually have a 750-800 rpm idle then a 50rpm kick up when put into gear, similar to AC kick up

Manuals use 650 rpm warm idle, but also depends on the engine and emission standards of that year.
Idling engines have bad emissions numbers, so car makers often set them as high as practical to get passing marks.
Computer, of course, is what sets the idle on fuel injected engines, using IAC valve.

Tapping the "air pedal"(used to be gas pedal) shouldn't change the idle computer returns to.
I would check throttle linkage to see if it is sticking open a bit, and maybe test TPS(throttle position sensor) Voltage

If throttle plate is not closing all the way every time then computer sets IAC valve for say 750rpm idle for that throttle plate position
Then when throttle plate does close all the way the RPMs would drop down, tapping air pedal would get throttle plate back to that partially open position and 750rpms would be restored
Just guessing


Yes, just P0174 could mean drivers side upstream O2 is getting old

 

Thanks, I kinda figured that idle speed was a bit low, but I like it better than her trying to stall, my theory is she had 140,000 on her when I got her, I did a complete gasket job, timing chain and so its possible I missed something not being a mechanic. but I used everything from 5.0 as far as sensors, so who knows how many miles on them...mass air flow is new, intake air temp is new, egr, driver side upstream 02, .....its bank 2 or drivers side that's lean, I might have wrote that wrong...if I do get it fixed and light stays out then I plan to have the truck run on emission machine to see if she will pass? been told it wont but I want to see, ya know

 

In most states with emissions testing you have to use same year or newer engine when you do an engine swap, i.e. 1998 Ranger would have to get 1998 or newer 5.0l.
And the year of the engine will set what emissions standards you have to meet, i.e. a 1999 5.0l would have to meet 1999 emissions levels

 

Yes, But in the ultra stupid state of Pa I'm told they want only the engines that the factory would put in that vehicle also..as in the 2.3, 3.0 or the 4.0 and then of course the same year or newer which I admit I didn't think about at time I was looking for a donor. I looked at explorers but found nothing but v6 vehicles, a guy had a vehicle on craigs list so I called to check on it and he told me to look for mountaineers because he felt they had the v8s...first one I saw I jumped on it. my guess is that majority of swaps are done by guys who don't care about or need emission test. they use them for fun in woods or racing.