Bank 1,2 lean
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From: Creston IA
Bank 1,2 lean
so I have a 1998 Ford Ranger 4x4 4.0 that has been throwing bank 1,2 lean. Codes p0171, p0174. I am going to auto mechanic school and we’ve been trying to diagnose this for 2 months and now it’s Christmas break. Vacuum fuel pressure, fuel volume, injectors, are all good we’ve done more stuff but I’ve forgotten. Burns a little antifreeze so I’m hoping not a cracked head or gasket. Any info would be appreciated!
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Joined: Aug 2017
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From: xxx
How much anti-freeze is it loosing?
Lean codes doesn't mean engine is actually running lean, same for Rich codes
What these mean is that the Computer's air/fuel calculations are not coming out right
The computer calculates how long to open the fuel injectors to get the correct 14.7:1 air:fuel ratio, for example, lets say injectors are open for 100ms(milliseconds)
That is 0 Short term fuel trim(STFT)
Computer then gets feedback from O2 sensors, .1 to .9 volts, .1 is too little oxygen(rich), .9 too much oxygen(lean)
Lets say it is "lean"
Computer then opens injectors for 102ms, +2 STFT, or +2%, adds more gasoline
(If it was "rich", then computer would open injectors for 98ms, -2 STFT, or -2%)
Computer gets feedback from O2 again, and its still "lean"
So computer opens injectors 104ms, +4 STFT
Then 106ms, 108ms, 110ms, 112ms, ect.....until O2 shows average .45volts, good burn, this all happens in less than 2 seconds
When STFT gets to +15 or higher computer will set "Lean" code for that bank, -15 Rich Code
This means burn is GOOD, but computer's calculations are not.
Your Computer already "knows" it is operating a 4 LITER engine, so it "knows" how much air should be coming in at any give RPM and throttle position.
Computer is also "told" fuel pressure and injector size for that year vehicle, so it "knows" how much gasoline will flow out of each injector if it is open for 100ms
What computer doesn't know, is the WEIGHT of the incoming air
air:fuel ratio is WEIGHT, not volume, 14.7 POUNDS of air to 1 POUND of gasoline is the ratio
14.7 grams of air to 1 gram of gasoline
The weight of air changes, at sea level and at 2,000ft for example
Colder air is heavier than warmer air(hot air rises because it is LIGHTER than colder air)
So there is a MAF(mass air flow) sensor to help computer to fine tune for air WEIGHT
MAF sensors only sample about 10% of the air flowing in, so they need to be clean and accurate, a little off is A LOT OFF when you are only sampling 10%, lol.
When you have codes on BOTH banks of a V6 or V8 engine that means there will be a common cause, not just one cylinder leaking, so wouldn't be caused by head gasket or cracked head issue
So a vacuum leak would have to be on upper intake or throttle body to effect both banks.
Also the air plenum from MAF to intake needs to be air tight, this would be a PORTED Vacuum leak, won't show up on vacuum gauges.
Easy, quick test for vacuum leak is to warm up the engine, and let it idle
Unplug the 2 wires on IAC Valve and it will close all the way
Engine RPMs should drop to 500 or engine may even stall, either is GOOD, no leaks
If RPMs stay high then there is a leak, or someone has messed with "anti-diesel screw", thinking it was an idle screw, fuel injected engines can't use an idle screw, no Jets.
That's the air side
Fuel side is the fuel pressure, computer has no way to test pressure
In a 1998 Ranger computer would expect 55-65psi fuel pressure(Returnless system), 1997 and earlier used 30psi, just FYI
So if your fuel pressure was 35psi, then less gasoline would flow out of an open injector, so 100ms goes to 116ms pretty quickly and you get "lean" codes on both banks
Long Term Fuel Trim(LTFT) is just what it says, it is a LONG TERM average of STFT.
This is used to bias computers calculations as an engine/vehicle gets older, compression goes down a bit as does fuel pressure as a vehicle gets older, MAF gets a little dirty.
If LTFT gets +/-15% then you will also get codes
And these take a few weeks to clear, LTFT wouldn't be much good if you could manually reset it like other settings, so even after you fix a "lean" code or codes, you need to drive it a week or two and then clear codes or at least look to see if LTFT is getting closer to 0
What these mean is that the Computer's air/fuel calculations are not coming out right
The computer calculates how long to open the fuel injectors to get the correct 14.7:1 air:fuel ratio, for example, lets say injectors are open for 100ms(milliseconds)
That is 0 Short term fuel trim(STFT)
Computer then gets feedback from O2 sensors, .1 to .9 volts, .1 is too little oxygen(rich), .9 too much oxygen(lean)
Lets say it is "lean"
Computer then opens injectors for 102ms, +2 STFT, or +2%, adds more gasoline
(If it was "rich", then computer would open injectors for 98ms, -2 STFT, or -2%)
Computer gets feedback from O2 again, and its still "lean"
So computer opens injectors 104ms, +4 STFT
Then 106ms, 108ms, 110ms, 112ms, ect.....until O2 shows average .45volts, good burn, this all happens in less than 2 seconds
When STFT gets to +15 or higher computer will set "Lean" code for that bank, -15 Rich Code
This means burn is GOOD, but computer's calculations are not.
Your Computer already "knows" it is operating a 4 LITER engine, so it "knows" how much air should be coming in at any give RPM and throttle position.
Computer is also "told" fuel pressure and injector size for that year vehicle, so it "knows" how much gasoline will flow out of each injector if it is open for 100ms
What computer doesn't know, is the WEIGHT of the incoming air
air:fuel ratio is WEIGHT, not volume, 14.7 POUNDS of air to 1 POUND of gasoline is the ratio
14.7 grams of air to 1 gram of gasoline
The weight of air changes, at sea level and at 2,000ft for example
Colder air is heavier than warmer air(hot air rises because it is LIGHTER than colder air)
So there is a MAF(mass air flow) sensor to help computer to fine tune for air WEIGHT
MAF sensors only sample about 10% of the air flowing in, so they need to be clean and accurate, a little off is A LOT OFF when you are only sampling 10%, lol.
When you have codes on BOTH banks of a V6 or V8 engine that means there will be a common cause, not just one cylinder leaking, so wouldn't be caused by head gasket or cracked head issue
So a vacuum leak would have to be on upper intake or throttle body to effect both banks.
Also the air plenum from MAF to intake needs to be air tight, this would be a PORTED Vacuum leak, won't show up on vacuum gauges.
Easy, quick test for vacuum leak is to warm up the engine, and let it idle
Unplug the 2 wires on IAC Valve and it will close all the way
Engine RPMs should drop to 500 or engine may even stall, either is GOOD, no leaks
If RPMs stay high then there is a leak, or someone has messed with "anti-diesel screw", thinking it was an idle screw, fuel injected engines can't use an idle screw, no Jets.
That's the air side
Fuel side is the fuel pressure, computer has no way to test pressure
In a 1998 Ranger computer would expect 55-65psi fuel pressure(Returnless system), 1997 and earlier used 30psi, just FYI
So if your fuel pressure was 35psi, then less gasoline would flow out of an open injector, so 100ms goes to 116ms pretty quickly and you get "lean" codes on both banks
Long Term Fuel Trim(LTFT) is just what it says, it is a LONG TERM average of STFT.
This is used to bias computers calculations as an engine/vehicle gets older, compression goes down a bit as does fuel pressure as a vehicle gets older, MAF gets a little dirty.
If LTFT gets +/-15% then you will also get codes
And these take a few weeks to clear, LTFT wouldn't be much good if you could manually reset it like other settings, so even after you fix a "lean" code or codes, you need to drive it a week or two and then clear codes or at least look to see if LTFT is getting closer to 0
Last edited by RonD; Dec 13, 2017 at 10:19 PM.
When we see the name computer we think of our PCs or laptops but that is not what automotive computers are.
They are compute "ers", large number calculators like the original compute "ers" invented back in the 1940's, used to break codes and calculate artillery distances
They calculate air:fuel mix "on the fly" and are very good at this one thing.
But the downfall of all computers is....."garbage in/garbage out"
They can't "think", they just calculate
Error Codes(DTCs) were added so users could be informed that the computers calculations(which are written in stone) are not accurate according to other feedback(sensors)
Computer itself is never in error(as far as computer is concerned, lol) so there must be something else wrong in the real world, garbage in/garbage out
And 99.9% of the time the computer is NOT the problem, a peripheral system or sensor is the problem.
They are compute "ers", large number calculators like the original compute "ers" invented back in the 1940's, used to break codes and calculate artillery distances
They calculate air:fuel mix "on the fly" and are very good at this one thing.
But the downfall of all computers is....."garbage in/garbage out"
They can't "think", they just calculate
Error Codes(DTCs) were added so users could be informed that the computers calculations(which are written in stone) are not accurate according to other feedback(sensors)
Computer itself is never in error(as far as computer is concerned, lol) so there must be something else wrong in the real world, garbage in/garbage out
And 99.9% of the time the computer is NOT the problem, a peripheral system or sensor is the problem.
Last edited by RonD; Dec 14, 2017 at 02:55 PM.
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Joined: Dec 2017
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From: Blaine
I just had mine fixed for this same issue. Cold start issues is when I really noticed this problem escalate.It ended up being the intake manifold gaskets, at least I hope that is all there is to it. In theory sucking cold air through gaps when engine was frozen and then the gaps would re-seal when warm. Only have about 50 miles on it since repaired so have not had a really good chance to test it yet.
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