Lean codes.....temp sensor?


Effieman

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Got the old P0174 code along with P0171 pending code on my 98 4.0 again.
LTFT both stay around 25 with STFT at +/- 10 while at idle and at freeway speeds. MAF seems correct @ 3.9 g/s at idle. O2s seem normal.
The only thing that seems to be off is the intake air temp, was at 109F when engine was 188F and been running for an hour.
Can't find the chart I've seen before for what the temp sensors should read (resistance) for what temp... And keep forgetting to check the computer for air temp when cold.
Does the intake temp make that big a difference on fuel trims?
Any other help appreciated......
 


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Intake air temp is always higher then ambient. Yes the intake temp matters because it factors into the weight of the air (hot air is lighter than cold air). Fuel trims are based on air WEIGHT vs fuel weight but fuel weight is constant.

However it doesn't matter enough to drag the trims 20% out of whack. It's more of a fine tuning thing.

You could try cleaning the maf but my money is on a vacuum leak wich is the cause of most dual bank lean codes.
 

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If you have the ability to monitor your fuel trims in real time, look at what the short term trims do at WOT. If they come back closer to zero you have an air leak.

If they stay super high you have an air metering issue.

There isn't going to be one single right answer to what your IAT reading should be, because it is going to change based on things like engine temp, outside air temp, vehicle speed, engine speed, etc. The best test is to see if it matches the ambient temp with the engine cold.
 

RonD

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+1 to the above

But............LTFT at +25, and STFT at +10, thats a +35% correction of injector open time

LTFT is used so computer doesn't have to adjust for small air leaks and fuel pressure differences as the vehicle/engine and parts get older.
So when it starts cold, no O2 sensors, it uses tables in memory for the air/fuel mix then adds the LTFT to that
Once O2 sensors can be used(when they are above 600degF), then computer can calculate air/fuel mix on the fly
It adds LTFT to that and then does STFT based on O2 sensor feedback

You can test for a vacuum leak by unplugging IAC Valve
Engine must be warmed up all the way
Unplug IAC Valve 2 wire connector while engine is idling
idle should drop down to 500 or engine may stall, either is good, no leaks
If RPMs stay up then there are leaks

Exhaust manifold leaks, these suck IN air, so you get a "false" Lean, O2 sensors only see oxygen, so it sees that extra air as lean burn

Fuel pressure, in 1998 Ranger should be 55psi, IF its down to 35psi, computer would see that as Lean, because less fuel is flowing in from injectors
Computer doesn't have a fuel pressure sensor
So dirty fuel filter or weak pump can cause higher LTFT and then high STFT

You can use a fuel pressure gauge to test for that
 

Effieman

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Thanks for the info so far.
After reading your replies, forgot to mention the rest......
I did clean the MAF (seemed correct before cleaning too), couldn't find any air leaks on intake or exhaust. Fuel pressure with 'key on' is 30 psi, running is 60, dropping only slightly when revved quickly. Fuel filter only has about 6K miles on it.

Gonna have to try disconnecting the IAC though.. Hopefully AFTER remembering to check cold temp...
 

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REV it and hold RPMs at say 2,000 or so, watch if pressure is slowly dropping, it shouldn't
 


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