P0171 Driving me Insane!!!

[Ecomike]

Well that's something good at least, although I thought I read somewhere LTFT should be less than 10%?

If the data looks good on gasoline and it's an LPG specific problem would that rule out faults that would be noticeable/common to both fuels such as manifold leaks???

Yes it might rule out manifold leaks, but a manifold leak, might be more obvious with one fuel than the other, as the LPG is not under control of the O2 sensor / PCM after all, from what you were told! Seems the PCM can not control the LPG with O2 sensor data as it is not set up to do so.

IIRC I read one site that +/- 5% STFT and +15% LTFT was normal, but I have not read up in detail on that!!!! The +15% LTFT was in a link a posted below, but I posted other good site info I have not had time to read cover to cover!!! YET! I do know, that I need to read more on LTFT, and what it really is and what it really means!!! Idid get the impression that LTFT was a long term number, like from cold start to hot operation, so it would be a bugger number, while you want STFT to minimal, which is +/-5% it seems?

 

[four_shot]

Stft is the adjustment that the pcm makes to counter fast (near instant) changes in fuel mix.

Ltft is the adjustment that the pcm makes due to less volitile parameters (intake air temp, engine temp, air density, etc).

the short term #s are always going to be low.

the long term #s are the ones that give the indication as to the general state of the engine. I am in the mind that anything over 10% is poor. Many systems will give you up to 15% before setting a code, but in my experiance, a good running engine wont stray over 7% in either direction. (your results may vary)

 

[Ecomike]

Stft is the adjustment that the pcm makes to counter fast (near instant) changes in fuel mix.

Ltft is the adjustment that the pcm makes due to less volitile parameters (intake air temp, engine temp, air density, etc).

the short term #s are always going to be low.

the long term #s are the ones that give the indication as to the general state of the engine. I am in the mind that anything over 10% is poor. Many systems will give you up to 15% before setting a code, but in my experiance, a good running engine wont stray over 7% in either direction. (your results may vary)

I reread the link I posted, 5% STFT and LTFT is best, so his 15% LTFT is indicating a problem.

If the computer is calling for fuel and O2 sensor is seeing too much fuel, does that not indicate that the computer is not seeing the correct O2 sensor data? Or did I get something backwards?

.88V is rich IIRC, and +15% LTFT is adding more fuel, right?

From the math I read, I can't see how LTFT can be 15% if STFT is +/-5% steady after a long while? I would expect LTFT to go to zero at a steady 10 minute hot idle. At 2500 rpm LTFT might be +15% for the first minute or so, then should drop off!!!!

Long story short, I suspect the PCM is only seeing part of the voltage from the O2 sensor, I suspect there is too much resistance in the wire connections from the O2 sensor to the PCM. Maybe a 5 to 10 ohm loss, making it think .65 volts is .45 volts. That would give the petrol data sets we are seeing, except for the shift from 1500 to 2500 rpm where the O2 sensor data gets worse, unless the signal loss gets worse at 2500 rpm, somehow.

 

[gradon]

Just to throw it out there: If you had installed a freer flowing intake, bbtb, ported/cleaned up intake manifold, and a freer flowing exhaust, I'd expect the pcm to be adding 10+%LTFT fuel to make up for the additional o2 flow available(so depending on the circumstances/mods, I wouldn't count >10%LTFTs bad). I would count +/-20-35% to be an issue in most cases.

 

[Ecomike]

IIRC he does have a non OEM exhaust on it that he replaced the cracked (?) exhaust with!!!!

His LTFT is not +/-, but is + only, specifically +15%!!!!!!

 

[WB9YZU]

Mike, He has an OBD2 unit. Map sensor is mounted on the TB. Is OEM...

Previous post on heater is also bad. Heater voltage is monitored by PCM. Also bad heater in O2 causes severe stumbling and hesitation on transition to closed loop mode until O2 reaches operating temp.

I didn't get a chance to see if upstream O2 has been replaced yet. Those were just the bad info from the last page.

 

[XJSteven]

Mike, He has an OBD2 unit. Map sensor is mounted on the TB. Is OEM...

Previous post on heater is also bad. Heater voltage is monitored by PCM. Also bad heater in O2 causes severe stumbling and hesitation on transition to closed loop mode until O2 reaches operating temp.

I didn't get a chance to see if upstream O2 has been replaced yet. Those were just the bad info from the last page.

Hi Zuki-Ron, can you please explain what you mean by the heater is also bad. I have replaced the upstream 02 sensor twice with the same results on all three.

 

[Ecomike]

Hi Zuki-Ron, can you please explain what you mean by the heater is also bad. I have replaced the upstream 02 sensor twice with the same results on all three.

I think he is just currently lost in this very long, very detailed, complicated, non typical problem solving thread, and I would ignore his last post. Non of it made any sense to me (half of what he said was obvious and known to us?), and it told me he was posting based on a partial read of a few posts, that may have had bad info? He did not say the heater was bad, he said the info was bad, but did not say what heater info was bad (no quote or thread reference?), which is not help full in a long thread like this.

I think your O2 sensor is working just fine. I think there is either a mechanical problem beyond the control limits of the PCM to control the air fuel mixes, and or a signal loss from the O2 sensor to the PCM. Only way to prove that would be to ohm test the signal wires from the O2 sensor to the PCM (isolating both ends first). If the PCM is getting a the exact data the O2 sensor is sending (meaning no connector or wiring loss of some of the signal), then it must be a hard to detect mechanical issue. I would suggest pulling the fuel injectors and having them cleaned and flow tested at this point for the petrol side, but it looks like the LPG side has worse issues, and we now know the LPG side does not care about the O2 sensor data.
Can you get an FSM like document on the LPG mods?

I wonder if the PCM is recording LTFT attempts for petrol while using LPG? How does the PCM know when it is running LPG? Does it know to ignore the O2 sensor when using LPG? Or is there just a shut off to kill the fuel pump when switched to LPG?????

 

[Ecomike]

Mike, He has an OBD2 unit. Map sensor is mounted on the TB. Is OEM...

Previous post on heater is also bad. Heater voltage is monitored by PCM. Also bad heater in O2 causes severe stumbling and hesitation on transition to closed loop mode until O2 reaches operating temp.

I didn't get a chance to see if upstream O2 has been replaced yet. Those were just the bad info from the last page.

I have seen heater voltage supply problems to O2 sensors that the OBD systems missed (no code thrown) and that did not cause the stumble-hesitation. We already knew it was OBD-II and the where the MAP sensor was located, have known that for quite a while in this thread?

PCM can see voltage from the relay to the O2 sensor heater, but that does not mean the O@ sensor sees the voltage. It can be interrupted before it gets the heater in the O2 sensor! I prefer to verify the voltage at the O2 sensor, and the ohms resistance reading of the O2 sensor itself. The PCM can be reading a wiring harness partial short, and think it is a working O2 sensor heater....When PCM data makes no sense, it is time to pull out a DMM, AMM and test sensors, and wires, etc.

 

[Ecomike]

Hi Zuki-Ron, can you please explain what you mean by the heater is also bad. I have replaced the upstream 02 sensor twice with the same results on all three.

Steven,

Right now may be a good time to write an update summary of where we are, starting from ground zero, year, make model, dual fuel history and so on, for those who may find reading or reading the entire thread too long and confusing now?

 

[XJSteven]

I think there is either a mechanical problem beyond the control limits of the PCM to control the air fuel mixes, and or a signal loss from the O2 sensor to the PCM. Only way to prove that would be to ohm test the signal wires from the O2 sensor to the PCM (isolating both ends first).

I removed the air-box, took off the black cover on the PCM and disconnected the 3 connectors. I tested the continuity of a whole bunch of wires at the PCM including the upstream O2 sensor, MAP sensor, ECT sensor, IAC and TPS - and all had a low resistance reading of approx 0.9 ohms so the wiring appears to be ok between the sensors and the PCM.

 

[XJSteven]

I think he is just currently lost in this very long, very detailed, complicated, non typical problem solving thread...

That's understandable, I keep getting lost in it myself.

Steven,

Right now may be a good time to write an update summary of where we are, starting from ground zero, year, make model, dual fuel history and so on, for those who may find reading or reading the entire thread too long and confusing now?

Sounds like a good idea when I get some spare time - and work out where to start!!!

I'm booked in to see a mechanic today. He doesn't fix LPG systems but does a lot of work on Jeeps so I'll see what he can find. Fingers crossed...

 

[Ecomike]

Was .9 ohms the internal impedance loss of the meter, or the wire? I always like to zero my equipment when getting down to readings around 1 ohm (not all meters have a zero). If the reading probe to probe is say .8 and the reading on the wire is .9, then the wire is .1 ohms which is OK. 1 ohm on the O2 sensor wire might be getting a little high. I know 5 ohms would be an issue. It was an issue for my high idle with a ground wire at 6 ohms and a 5 volt feed to the TPS.

But sounds like it would be a marginal issue, not a large issue like you have. My DMM has about .6 ohms internal loss just using the probes, probe tip to probe tip.

As you can see, even I am getting lost on what all has been tested and results on this thread, LOL.

 

[XJSteven]

we now know the LPG side does not care about the O2 sensor data.
Can you get an FSM like document on the LPG mods?

I don't think this is entirely correct Mike - probably due to my limited understanding and the lack of details I received with the very fast explanation over the phone of how LPG systems work. What I was told is the PCM doesn't control the LPG mixture but as far as I know the LPG system does still use the 02 sensor data.

An FSM for LPG would be great. I might contact the manufacturer to see if such a thing exists.

 

[XJSteven]

Was .9 ohms the internal impedance loss of the meter, or the wire? I always like to zero my equipment when getting down to readings around 1 ohm (not all meters have a zero). If the reading probe to probe is say .8 and the reading on the wire is .9, then the wire is .1 ohms which is OK. 1 ohm on the O2 sensor wire might be getting a little high. I know 5 ohms would be an issue. It was an issue for my high idle with a ground wire at 6 ohms and a 5 volt feed to the TPS.

I have no idea about this. All I did was hook my little cheap Ebay multimeter up and take a reading. I remember reading somewhere what the correct voltage range should be and it seemed to be in the correct range at the time. I'll recheck the FSM or find where I read it from.

 

[Ecomike]

I don't think this is entirely correct Mike - probably due to my limited understanding and the lack of details I received with the very fast explanation over the phone of how LPG systems work. What I was told is the PCM doesn't control the LPG mixture but as far as I know the LPG system does still use the 02 sensor data.

An FSM for LPG would be great. I might contact the manufacturer to see if such a thing exists.

Then you also need to test the O2 sensor signal at the LPG controller if that is true, which I doubt?

I thought I read your post about what the LPG guy said, that the LPG does not use O2 sensor data at all, but uses a feed ratio of LPG to air based on vacuum pressure, with some kind of pressure flow control gadget that is set at idle?

 

[WB9YZU]

Sorry to have butted in. Mike obviously knows more than I do.

 

[gradon]

The problem is 99.99% of us don't have experience with LPG. When switched to LPG mode, what prevents the fuel injectors from squirting? The o2 will read the AFRs on the LPG mixture and then start adapting/storing FTs. Then you switch to gas and the pcm has to adapt/relearn/unlearn FTs. How does the pcm tell the LPG system to lean or enrichen the mixture if it's not using the o2 sensor to adjust?

 

[Ecomike]

The problem is 99.99% of us don't have experience with LPG. When switched to LPG mode, what prevents the fuel injectors from squirting? The o2 will read the AFRs on the LPG mixture and then start adapting/storing FTs. Then you switch to gas and the pcm has to adapt/relearn/unlearn FTs. How does the pcm tell the LPG system to lean or enrichen the mixture if it's not using the o2 sensor to adjust?

I have been thinking, and wondering about that myself, and hinted at that possibility with an earlier question for his LPG gurus. I think it is still a valid question and concern!

Zuki-Ron, don't run off, I don't know it all, and if I did, my memory is not what it once ones, I still make mistakes. Hopefully Steven can write a new complete update summary here soon. I have always been curious about alternate fuel feeds!!!

 

[jeeptech1]

That's understandable, I keep getting lost in it myself.


Sounds like a good idea when I get some spare time - and work out where to start!!!

I'm booked in to see a mechanic today. He doesn't fix LPG systems but does a lot of work on Jeeps so I'll see what he can find. Fingers crossed...

Finally! i know people hate us mechanics but with the time and money you wasted on sensors you could have had it fixed all ready! just my 2 cents.