P0171 again

[Rizzoj2000]

Firstly I want to apologize for starting another of the 171 threads. Vehicle has been running great for 2 weeks no codes since I changed the secondary oar check valve. Last night come to a light and feel the idle surge. Looked at the tach and revs stuck on 1000. A couple seconds later motor starts to lope like it has a racing cam. Then stalls. While driving revs keep fluctuating and sometimes engine sounds starved for fuel. Then throws a 171 code. I collected the following data from my scanner:
ABSLT TPS 28.2%
ENG SPEED 1399 RPM
CALC LOAD 58.8%
MAP 17.7 HG
COOLANT 201 DEGREES F
IAT 126 DEGREES F
IGN ADV 25.5 DEGREES
SECOND AIR ATMOS
ST FTRM 19.5%
LT FTRM 5.5%
VEH SPEED 34 MPH
FUEL SYS CLOSED
FUEL SYS 2 N/A
I love this vehicle but now sits driving me crazy. Any ideas. Maybe O2 sensors?

 

[Ed H]

Does it have full power? I ask because my P0171 was the result of a clogged cat. After replacement, it felt like I gained 100HP.

 

[IllogicTC]

Do you get O2 voltage readouts on your scanner? I'd monitor sensor 2 (downstream) after the engine's warmed up, it could tell you a really good (or bad for your wallet) story if it is indeed the cat.

 

[Rizzoj2000]

That's the only data I get from my scanner. I'm waiting for a wifi obdii dongle so I can run Dashcommand.

 

[bobdec]

Keep an eye on those trims.. The LT FTRM (Long Term Trim) says the PCM was adding 5.5% more fuel to fueling calculation and the ST FTRM (Short Term Trim) says the final burn as read by the O2 was 19.5% lean. The DTC P0171 is thrown when the trims are > 14% lean. Could be an incorrect O2 reading (defective O2) or a real lean conditioned usually caused by low fuel pressure. If the O2 lean reading is in error and incorrect then the PCM is adding fuel in error, a rich burn will result causing fouled plugs and/or cat. If it were mine, in order of low cost to high... I would verify fuel pressure , if that's OK then I would replace the upstream O2. After that look into the cat as it can mess up the O2 readings.
To test ,try this... Reset the DTC take it for a ride and see what the trims look like under mild loads, the LT & ST trims at 0% is a perfect burn, hardly ever achieved.. positive percentages indicate lean..., negative percentages indicate rich. A -5% to +5% variation is normal it's the average that the PM looks at. ALSO look at the rear S2 O2 sensor voltage, let us know what it is, it s/b steady with not a lot of fluctuation if the cat is doing it's job.

 

[IllogicTC]

Keep an eye on those trims.. The LT FTRM (Long Term Trim) says the PCM was adding 5.5% more fuel to fueling calculation and the ST FTRM (Short Term Trim) says the final burn as read by the O2 was 19.5% lean. The DTC P0171 is thrown when the trims are > 14% lean. Could be an incorrect O2 reading (defective O2) or a real lean conditioned usually caused by low fuel pressure. If the O2 lean reading is in error and incorrect then the PCM is adding fuel in error, a rich burn will result causing fouled plugs and/or cat. If it were mine, in order of low cost to high... I would verify fuel pressure , if that's OK then I would replace the upstream O2. After that look into the cat as it can mess up the O2 readings.
To test ,try this... Reset the DTC take it for a ride and see what the trims look like under mild loads, the LT & ST trims at 0% is a perfect burn, hardly ever achieved.. positive percentages indicate lean..., negative percentages indicate rich. A -5% to +5% variation is normal it's the average that the PM looks at. ALSO look at the rear S2 O2 sensor voltage, let us know what it is, it s/b steady with not a lot of fluctuation if the cat is doing it's job.

My scanner has two STFT readings? One right next to LTFT, and another independent one right next to the O2S1 voltage reading which appears to fluctuate directly in comparison to current voltage. The two STFTs show different numbers. I wonder which one his is showing.

 

[bobdec]

IllogicTC, A very long post..skip the parts you already know..

BEFORE CAT O2 scanner display info closed loop operation....
Scanners and software loggers confuse things when it comes to O2 reporting . Basically the O2 generates a voltage when it reaches 600* F. The voltage is dependent on % of oxygen at the sensor compared to outside air % of oxygen. The outside air enters through a small gap in the wire insulation and the tested air enters the sensor in the exhaust flow. The O2 feeding the PCM is a NARROW BAND sensor. that means it switches at a narrow frequency bandwidth, in the micro second range. So as the exhaust changes between each cylinder exhaust stroke hitting the sensor it switches between 100 MV (no cylinder exhaust) to 800 Mv (puff of exhaust passes the sensor). At the same time the PCM converts the O2 analog voltage to a digital byte (called trim) and then determines the AVERAGE trim value. A trim of 128 is a perfect burn (called Lamba 1 or Stoich) and at that time the average analog O2 voltage will be around 450 Mv. Be carful with the negative switch in numbers as a rich or high O2 voltage will produce a LOWER trim value. And a lower lean O2 voltage will produce a higher trim value. An trim average greater than 128 is lean (low O2 voltage) and a trim average less than 128 is rich (higher O2 voltage). Now for an easier human scanner display the software then converts the trim value to a PERCENTAGE. EG: a trim average of 120 would be (128-120)/128 = 6.25% (burn is 6.25% rich) . A lean trim of 136 or (128-136)/128 = -6.25% (burn is 6.25% lean)

So a scanner may display one or all of the following.
1- actual switching O2 voltage that moves between 100 and 800 Mv sub-second, which is impossible w/o an oscilloscope, so that voltage is a software generated sampled display.
2- The average trim value in a decimal byte value between 000 and 256 with 128 being perfect AFR (Lamba 1).
3- A trim percentage generated for display only starting at 0% for trim of 128, and going negative for trims below 128 and positive for trims above 128.

LT ST differences...
To complicate things the PCM manages the trims display into two values Called the LT trims (Long Term) and ST (Short Term) trims. The LT trims are used to feed the PCM calculation for fueling AND are adjusted UP and DOWN periodicity by the PCM on a sub-MINUTE time frame they do not move rapidly but will change to correct fueling. The ST trims are the ACTUAL average O2 readings. Here is how the process works as soon a closed loop (O2 active) is entered.. Assume the PCM was reset and all LT trims are reset to 128..starting off the PCM reads the O2 - converts the average voltage to a decimal byte ST trim- if ST value is higher than 128 the PCM increments the LT trim value - the LT trim used for fueling calculation is now a higher value and the PCM adds fuel- The PCM then looks at the new ST trim value if still high the process is repeated until the ST equals drops to 128- the LT trim value then stays at it's value from then on.. So on a scanner the LT is the value feed to the PCM to help get the ST near 128 or 0% trim. An you can then use the LT trim or % as a display determine if the PCM is pulling fuel or adding fuel and what percentage to hold the burn correct. If all is good the ST will be close to 128 or 0%. Note the PCM cam usually adjust the LT trims +/- 15% to hold the burn within tolerances. However a LT over +/-5 % is signaling a problem and a bad ST signals the PCM is not correcting.

More conusion
The PCM manages and remembers about 30 sets of LT trim values for various engine running conditions. So everything above is repeated X 30 . Also in open loop (cold engine and no O2 feedback) the PCM uses the LT trims w/o O2 correction until the system enters closed loop. A PCM reset will reset all LT trims back to 128 otherwise they are remembered and used at all startups. One reason why a car (in open loop) with a bad O2 may run good after a PCM reset. But then craps out after entering closed loop.

In answer to you question , the downstream (after CAT) O2 sensor is NOT converted to any trim values in the software as the after CAT readings are not (directly) used for fueling. So you can only have one ST trim value in a 4.2. Is that value you are seeing a voltage/number or percentage? The PCM just averages the downstream voltage and it's only used for emissions monitoring. It's usually stable between 200-500 Mv depending on the vehicle and if the voltage varies then a DTC is thrown. In the 4.2 service manual the upstream O2 is called O2S1 and the downstream is called O2S2, but be some scanners can get confused. In V8's or V6's the second bank upstream O2 is usually called O2S2 and the downstream designation of S3 S4 or something else, depends on the firmware/software and will change if they have multiple cats.

 

[IllogicTC]

I have percentages, two of them both reading STFT.

One is sitting next to the LTFT, and makes adjustments now and again (as expected) and over time the LTFT seems to adjust based on what this is reading.

Then there's another STFT percentage reading right next to my upstream voltage reading, and it oscillates in direct accordance with what the current voltage is. So while the voltage is low the number is in the + range somewhere and while the voltage is high it's in the negatives.

It's almost like it's a reading of exactly what trim would be expected based off the O2, and then the other STFT (the one next to the LTFT) is the actual final value after doing whatever other calculations are done.

Sample time on my scanner seems to be about 3 times or so a second, so I know it's pretty far off from what an oscilloscope could show in terms of actual voltage (or at least some fancy thing which could log voltages and this STFT value I see in something in the millisecond range for more accurate comparison)

This is an Actron CP9575 by the way.

 

[bobdec]

I'm guessing here, looked up the scanner users manual and they don't give a lot of info on the display. However I believe that there are a couple of ways the PCM reports STFT's . There is "STFT (SAE)" which is the standard ST trims derived from the O2 voltage . And then there is a plain "STFT" which is an AVERAGE of the (SAE) PID readings generated in the PCM (can't find any info on how it's calculated, # of samples, time window /ETC,) could be your scanner is displaying both PID's. Usually the (SAE) STFT is the default scanner display the SAE coming from the J1930 SAE standard. This stuff is confusing with little public info available..