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Lean Condition - did I cover all bases?


rangerpilot

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Ok...so, I have troubleshooted so much the past few weeks regarding a P171 & P174 code. I am wanting to make sure I have covered all bases before I pull the intake. Here is a brief rundown:

1999 4.0 V6 OHV 4x4

1) Valve Cover Gaskets (and ECT sensor) replaced a few months ago
2) Not sure if they did the upper intake gaskets or not (I would only hope)
3) Checked all lines/hoses for vacuum leaks
3a) Vacuum Pressure Gauge (reads 21 in Hg. and drops to 5 psi with quick WOT)
3b) Starter fluid test
3c) Propane test (as far as I could reach around critical parts)
3d) Homemade Smoke test (using electronic fog machine on brake booster line)
4) New PCV valve
5) New O2 sensors (all three)
6) New Exhaust Gas Pressure Sensor and associated lines (I broke the old one)
7) Cleaned MAF sensor best I could
8) Did a voltage test on all leads to the MAF with no problems indicated)
9) Replaced Fuel Filter
10) Fuel Pressure holds a solid 40psi at idle and under high RPM

What else can I check? Could the fuel regulator or fuel pump be going bad and not delivering enough fuel to the rail? Any way to test EGR and related components?

It is both banks so I believe I can rule out the injectors as being the issue. I have driven like this for several months and can't narrow it down. I do have an occasional rough start (both cold and warm engine). It is very random.

I am out of ideas and really don't want to pull the intake if I don't have to.

Unrelated: I did a Block Test for Coolant Leaks (this is a totally separate issue however no problems noted there. Will be doing a compression check tomorrow as well as changing the plug wires.

Any more ideas?
 
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Rock Auto 5% Discount Code: F9A1A579ACFAD1: October 1st, 2021

RonD

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You have covered alot of bases.

P171 & P174 means the Short Term Fuel Trim(STFT) numbers are consistently high, +20

I assume you have reset computer because those codes won't clear themselves, and it should take 20-30minutes of driving before they come back if problem is not fixed.

After reset, assuming cold engine, P171 & P174 can't come back because computer can't use O2 sensors until they warm up, so 5 to 10 minutes.

Computer uses MAF sensor to get the weight of the air coming into the engine, it then calculates the 14:1 ratio for the weight of the fuel needed, 14lbs of air gets 1lb of fuel, or more precisely 14.7lbs of air gets 1lb of fuel.
That 14:1 calculation = 0 for STFT
After fuel is burned computer watches O2 sensor voltage, too much oxygen in exhaust is Lean, too little oxygen is Rich, O2 can't see fuel just oxygen.

If there is too much oxygen, computer increases the Pulse Width of the fuel injectors so they are open longer, adding more fuel, this shows up as +1 STFT
When cruising at steady RPM STFT numbers switching from -5 to +5 is normal.
At idle +3 to +9 is normal, idle is suppose to be richer.

When STFT is consistently above +17 to +20 on both banks when cruising computer will set P171 & P174, engine is never actually running Lean, computer is just giving the driver a "heads up", the computer is notifying you that it's 14:1 calculations are wrong, it is having to add too much fuel for the reported air.

This is why a vacuum leak is the first thing to look for, unreported air is coming in.
But any air leak from MAF sensor to O2 sensor is unreported air, so check MAF's air tube to intake.
An exhaust manifold leak would suck in air so false lean, lower intake vacuum leak would suck in air, but both of these would only cause one bank to get Lean code not both.
And having failed O2 sensor would also also only effect one bank, both failing at the same time would be.........very very very long odds.

Low fuel pressure would mean higher STFT numbers because computer needs injector to be open longer to get same amount of fuel it did with higher pressure

MAF sensor is a common issue for getting both P171 & P174 because it sets 0 for STFT, a little off and both banks get +20's

Computer failing would also cause it.

I would get an OBD II scanner that you can use in real time to monitor STFT.
The bluetooth units work with a smartphone and are under $60, these work on any vehicle made after 1995 so a good long term investment if you plan on owning and driving for a few more years :)

You can also monitor MAF sensor and IAT(intake air temp) sensor
 
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rangerpilot

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Thanks ... Yes I forgot to mention the STFT/LTFT. I have a scanner and have been monitoring it with every troubleshooting step I take. Up until today the STFT was consistently in the high teen low twenties. LTFT will max out at 25

Today after changing the PCV, I saw STFT in the 30s and 40s which really blew me away (I did reset everything by removing the negative cable prior to driving).

The LTFT was high on idle and would settle down under load ... Hasn't had time to max out yet as I only drove it for about 15 min).

Do the I/M monitors help me at all as it tends to throw the code after the first drive and upon and second start up with only the "E" or "EV" monitor coming on line with the rest still flashing before the codes return?

Can the voltage test on the MAF show correct ohm readings but still be at fault in some other way.


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RonD

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Ignore LTFT, it is only used to bias fuel trims before O2 sensors come on line.

Disconnecting neg. battery terminal is for OBD I, your scanner is needed to reset OBDII.
But disconnecting battery cable while working on an engine is a good practice.

For MAF sensor you need to test voltage with engine running.
pierce MAF signal wire(light blue/red wire) with a sewing pin, also pierce MAF Return wire(tan/light blue wire) with a pin, far enough apart so they can't short together.

Set Meter for DC volts
Red probe to light blue wire and Black probe to tan wire

Start engine
700 RPM should show under 1 volt, .7v is ok
2,000 RPM will be about 1.5volts
3,000 RPM 1.7
4,000 RPM 2v

You should be able to see this voltage on the OBDII scanner
If it has a Grams Per Second(GPS) read out that's from MAF sensor, 4.0l will have 4 grams per second at idle
 
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rangerpilot

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I have cleared the codes with the scanner but the only way I could get the fuel trims to reset to zero was to remove the battery cable.

Your MAF test is a little different than the one I read elsewhere. I will try this version tomorrow.

My scanner only show air volume (lb/min) on the MAF. It doesn't read the voltages. It was a $100 unit from AutoZone.


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RonD

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If you have LB per minute, then switch to metric, which is grams per second

The high fuel trims at idle and "normal" trims while cruising is a sign of unreported air, since that effects lower RPMs more than higher.

But switch to gm/sec and see if it is 4
 

rangerpilot

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On average, at idle the MAF is 4.5 g/s and will occasionally increase to as much as 7.0 g/s when AC compressor kicks on.


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RonD

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seems OK, I assume the idle is above 750RPM and then 850 or higher with AC

gm/sec at 650rpm idle should equal engines displacement in liters
So 4.0l should read 4 gm/s at 600-650rpm
3.0l would be 3 gm/s at 600-650rpm

Obviously higher RPM mean higher gm/s

If gm/s gets down to 2 then MAF has a problem

So it looks OK
 


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