Hello:

Fairly recently, my truck (99 Ranger 4.0 2wd manual extended cab) started throwing "too lean" codes for both banks. My trusted mechanic also looked at it, and agreed something is wrong. When fuel trim shows 10+%, you can cleary smell the gas in the exhaust, so the trim is not actually needed, but for some reason the engine computer is calling for it.

we tried replacing the mass airflow sensor (MAF) with a purchased used but good part ("good" not independently verified) from a reputable used part dealer, and saw no change whatsoever with the problem.

Watching numbers and paying attention to ODB data (thanks Torque app!), I have found the following to be true:

Fuel trim climbs on both banks when:
Idling under 1200RPM
Outside air temperature is 25F or less

The Intake air temp is usually higher (often 10+F higher); it does not appear to matter if the engine is warmed up or not (as determined by coolant temperature diag value). It does seem to happen more often with a higher descrepency between intake air and actual outside air temp. I don't have a good source of local air temp, so its mostly based on signs in town saying 7-12F and the intake reporting 35F. I don't know how much the air is being heated under the hood before it gets used...

MAF and O2 sensors all move around in what my mechanic reports as "reasonable" variations, and there are no error codes related to them.

Is it normal for the intake air temp to be so much higher than actual air temp?

What could cause this problem? Any other suggestions?

Thanks!
--Jim

 

Welcome to the forum

+ or - Fuel trims are based on Computer's calculation, which is 0 fuel trim

Computer gets RPM and throttle position, and knows it is for a 4 LITER engine(4.0l), and it knows how much a Liter of air weighs at sea level.
So it can calculate the 14.7:1 air fuel ratio based on that.

It also uses the MAF sensor to fine tune that calculation.
Warm air rises, so is lighter than cold air
Thin air rises, so is lighter than thick air(sea level)
MAF sensor is a heated wire, air flowing past it cools it down

MAF sensor reports warm air/cold air, thick air/thin air by the amount it cools down the heated wire, pretty simple setup.
This modifies the Weight of a Liter of air for computers calculation

IAT(intake air temp) does as well BUT..........not as much as MAF, IAT is more for comparison than actual real time air:fuel mix adjustments

ECT(engine coolant temp) sensor is used for Choke Mode, the engine needs 30% fuel vapor to start and run, carbs and fuel injectors squirt in liquid gasoline, in a cold engine very little of this changes to a vapor, maybe 10%, spark plug can not ignite liquid gasoline only vapor.
So when cold the computer must squirt in more fuel to get to 30% vapor point.
Good engine compression heats up the liquid gasoline for more vapor, and engine starts.

Computer also "times" the ECT sensor, first it must show warming up after startup(RPMs above 400) and second it must show above 170degF in at least 10minutes or you will get a code

The upstream O2 sensor is what sets Fuel trims
O2 sensors only work above 600degF so engine needs to warm up 3 to 5 minutes before you see actual fuel trims
Computer runs 0 fuel trim thru engine and then watches O2 sensor voltage
If O2 reports too much oxygen in exhaust, Lean, then computer goes to +2 fuel trim and tries that, if still too much oxygen computer tries +4, and so on until O2 reports "good burn"

All engines run at least +7 at idle, have to or engine would over heat, so +10 is fine

-10 to +10 should not set any code, Lean or Rich, very normal for any fuel injection system to run in these areas.
+15 might but would need to be constant for a few days
+20 will set a code if it happens in the same higher(above 1,000) RPM range a few times

On "V" engines if BOTH upstream O2 sensors report Lean, then first stop is an air leak
MAF sensor is under reporting air flow causing computers calculated 0 to be off, 0 is not adding enough fuel so Lean burn.
Since BOTH show Lean then it is not an O2 sensor problem

Because engine intake runs at -18 air vacuum even a small leak can pull in alot of extra unreported air.

Easy test for that is to have engine warmed up and idling, automatic trans will idle at 750, manual trans 650, warmed up
Unplug the 2 wire connector on the IAC(idle air control) valve
Valve will close and engine RPMs should drop to 500 or engine may even stall, either is good, it means no vacuum leak.
Idle stays at 700 or higher then there IS a leak

Another possibility is fuel pressure, computer expects 65psi pressure at the injectors on a '99 Ranger
If you were at 45psi then computers calculations for 0 fuel trim would be wrong, less fuel is flowing in at 45psi than would at 65psi, so Lean condition and fuel trims go up.

Fuel trim is actually the "open time" for the fuel injectors
Say 0 fuel trim at 2,000rpms is calculated at 300ms(milliseconds) per injector
O2 shows Lean so computer goes to +2, say 320ms
O2 show Rich so -2 and 280ms for fuel injector open time.

Also when you watch fuel trim they should go up and down frequently even at steady RPMs above idle, that is needed to keep the Cat converters working, so hopefully your Cat converters are working, because if they are not then you WOULD smell gasoline in exhaust.
Point of Cat converter is to get hot enough to burn off gasoline that wasn't completely burned.
If you were dumping too much unburn gas into exhaust then working Cat would get VERY VERY HOT, start making pinging and popping noises as it overheated, you would hear that easily after shutting off the engine, hot metal expanding.

 

Thank you for that information! That was very helpful in my understanding of how it all works.

I wasn't watching all the correct sensors, but now I know better what to watch. I got one drive cycle since then, and noticed that my O2 sensors were in fact reporting too lean, but my post-CAT O2 sensor was reporting steady lower values, so hopefully my cat is good still.

I'll see what I get with more drive cycles and watching the right sensors. I did notice one website suggested that post 1996 vehicles should replace their O2 sensors at 100,000 miles, and my vehicle is at 96k, even though its a '99. However, both bank 1 and bank 2 O2 were reporting similar values, so I'm guessing their likely to be working correctly.

--Jim

 

Yes, 100k is specified as change period for O2 sensors, they use a chemical reaction to detect Oxygen in the exhaust so chemicals do get depleted over time, much like a car batteries chemicals get depleted over time and will no longer hold a charge.

And they do start to Fail Lean, MPG will drop a bit so time to change them, cost of O2 sensors pay for themselves in MPG gain over the next 100k miles, if they were failing.

But do the IAC Valve test for vacuum leaks before changing O2s

STFT = short term fuel trim, instant reading of injector open time as calculated from O2 data
LTFT = Long term fuel trim, averaged STFT and saved in computer memory to maintain proper operation over vehicle and engine as they get older

LTFT is used by the computer when O2 sensors are not working, during cold start or restart, O2s have to warm up before they can report accurate oxygen levels
And LTFT is also used to bias computer calculations during normal operations, so +3 LTFT will be used to calculate 0 fuel trim
As vehicle gets older there WILL be small air leaks, slightly lower fuel pressure and other issues, just the facts of life for aging and quite normal, LTFT is used to keep engine running well as systems get older