P0171 Driving me Insane!!!

[XJSteven]

I agree with Mike's diagnosis in post 135. .8V is rich for gas(have never read up on lpg, so dunno what ratio is stoich or how the o2 reads on it). I'm not sure the o2 should be reading .8V at 2500 with gas under light throttle--my 96 doesn't go into OL until 3100rpm under light throttle(I wish it went in OL earlier to be richer than that 14.7) With LPG, should the o2 be oscillating around the stoich point in closed loop/low rpms?

I would have thought so but this is all new to me.

So just to make sure I'm understanding this...the 0.880V 02 sensor reading on LPG is showing a rich condition up until 2000rpm so the PCM is subtracting approx -32% fuel (the maximum it can reduce it by) but considering I'm still getting bad fuel economy the 32% cut isn't enough. There is a super-rich condition up until 2000rpm. Once it passes 2000rpm the 02 voltage is oscillating again so there is no longer a rich condition, however the PCM adds the max fuel possible (+32%) because it overshot the mark earlier?

I don't understand why the P0171 error code says "System Too Lean" when it was clearly running too rich from the start - or could the P0171 only be thrown over 2000rpm when the PCM is adding max fuel back?

On the gasoline side of things...the 02 voltage seems fine until 2500rpm but the PCM is adding 15% fuel from idle so something is causing a lean condition from the start (opposite problem to LPG). At 2500rpm the 0.8V 02 voltage is showing a rich condition but the PCM is still adding (18%) fuel, which I don't really understand this???

Am I on the right track?

What ever controls the LPG flow hardware wise (not the PCM), that is comparable to a fuel injector is what I would look at next.

From what I gather this would be the 'stepper motor'.

 

[McQue]

Not knowing anything about LPG systems, I am just trying to break this problem down to the very basics. Your STFT numbers and 02 sensor seem to be reading correctly at idle when using petrol. I would try to acheive the same results using LPG. From what I have learned from reading through this thread and the link you provided about LPG systems, it seems like, at idle, the 02 sensor is telling us the LPG idle mixture is incorrect, assuming no other mechanical problems are hosing us over.

 

[Ecomike]

I would have thought so but this is all new to me.

So just to make sure I'm understanding this...the 0.880V 02 sensor reading on LPG is showing a rich condition up until 2000rpm so the PCM is subtracting approx -32% fuel (the maximum it can reduce it by) but considering I'm still getting bad fuel economy the 32% cut isn't enough. There is a super-rich condition up until 2000rpm. Once it passes 2000rpm the 02 voltage is oscillating again so there is no longer a rich condition, however the PCM adds the max fuel possible (+32%) because it overshot the mark earlier?
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YES
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I don't understand why the P0171 error code says "System Too Lean" when it was clearly running too rich from the start - or could the P0171 only be thrown over 2000rpm when the PCM is adding max fuel back?

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You need to distinguish between what the O2 sensor sees, and what the PCM thinks it sees, and what the PCM is trying to do as a result of what it thinks it sees (add or reduce fuel rate), and whether or not the PCM fuel rate control effort is actually doing anything, and if so what it is trying to do, and whether it is having any success (which gets back to what the O2 sensor is seeing). If there is a leak, or mechanical problem, it mucks up the O2 data versus what the PCM is doing, or trying to do)
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On the gasoline side of things...the 02 voltage seems fine until 2500rpm but the PCM is adding 15% fuel from idle so something is causing a lean condition from the start (opposite problem to LPG). At 2500rpm the 0.8V 02 voltage is showing a rich condition but the PCM is still adding (18%) fuel, which I don't really understand this???

Am I on the right track?

From what I gather this would be the 'stepper motor'.

NO! The stepper motor only controls, or limits air flow at idle. It is does nothing except limit air flow at idle to cotrol idle rpm!

Air/fuel ratio is controlled by PCM controlling fuel feed rate only (or at least it tries to control it).

 

[Ecomike]

I agree with Mike's diagnosis in post 135. .8V is rich for gas(have never read up on lpg, so dunno what ratio is stoich or how the o2 reads on it). I'm not sure the o2 should be reading .8V at 2500 with gas under light throttle--my 96 doesn't go into OL until 3100rpm under light throttle(I wish it went in OL earlier to be richer than that 14.7) With LPG, should the o2 be oscillating around the stoich point in closed loop/low rpms?

Since the O2 sensor reads oxygen and not fuel, it does not matter if it is gasoline or LPG. I would assume that the O2 control point is the same for both fuels.

IIRC deceleration should read saturated rich (high voltage) for a while till the PCM catches back up (seconds), and read lean (low voltage) during acceleration (till PCM catches up in a steady state cruise rpm). And read rich due to OL operation at WOT.

 

[Ecomike]

Not knowing anything about LPG systems, I am just trying to break this problem down to the very basics. Your STFT numbers and 02 sensor seem to be reading correctly at idle when using petrol. I would try to acheive the same results using LPG. From what I have learned from reading through this thread and the link you provided about LPG systems, it seems like, at idle, the 02 sensor is telling us the LPG idle mixture is incorrect, assuming no other mechanical problems are hosing us over.

X2 on that!

I found another good thread on O2 sensor diagnostics today:

http://www.autodiagnosticsandpublishing.com/feature/o2-sensor-testing.htm

http://www.aa1car.com/library/o2sensor.htm

I also found a great site on Fuel trim!!!!!!

http://forums.nasioc.com/forums/archive/index.php/t-1401070.html

 

[Ecomike]

You were correct Ecomike. Replacing the TPS with my old one has fixed the fluctuating rpm problem. I'm not very impressed with the Standard Motor Products brand from Rockauto.com. This new TPS was a warranty replacement because the first one they sent me only lasted a few weeks before a fault caused my auto transmission to get stuck in 1st gear. I'm back to using the original TPS that was on the vehicle when I bought it 2 years ago (despite attempting to replace it twice now!). Will have to try a better brand next time.

Anyway, the most recent data I posted for STFT and LTFT remains correct because I have confirmed it again with the 'old' TPS re-installed.

The hardest diagnostics jobs are those that have 2 or more problems, defects, bad parts, etc. People frequently make the mistake of looking for a single cause to multiple symptoms, and odd data. They also frequently make the mistake of assuming a new part is GOOD!!!!!

 

[Ecomike]

Hi McQue,

I haven't played with this myself but the first thing I did 6 months ago when I noticed the poor fuel economy was take it back to the LPG installer for adjustment. He hooked some sort of 'gas analyser' to the exhaust pipe, took some readings and said the LPG mixture was fine and didn't need adjustment. When I questioned the poor fuel economy and P0171 error code he said he could hear an air leak at the exhaust manifold gasket and that the PCM was probably adding more fuel to compensate (I now know this made no difference because I've since fixed that problem).

I presumed this meant the LPG system was ok and started looking elsewhere for the problem. What also confirmed it for me was when I cleared the codes and drove around on gasoline/petrol the P0171 code reappeared.

I won't be very happy if it does turn out to be a fault with the LPG system because as I said that was the first thing I got checked out months ago! Are you saying it looks like it could be from the fuel trim data I've posted?

If all he did was an exhaust gas analysis, after the CAT converter, fire him!!!! All the data is before the CAT converter, and not at the end of the exhaust pipe.

 

[Ecomike]

I don't understand why the P0171 error code says "System Too Lean" when it was clearly running too rich from the start - or could the P0171 only be thrown over 2000rpm when the PCM is adding max fuel back?

The P0171 code is what the PCM thinks is going on, and its response would be to add more fuel. If the PCM is not seeing the added fuel, it continues to add fuel to the FT limits, thus making it too rich.

It still looks like the PCM is not always seeing the true O2 sensor data. But at other times it seems OK, which is why I also suspect a tuned exhaust leak that changes rates of drawing in fresh air as the RPM changes.

I am not positive of this, but based on my knowledge of fluid mechanics, it seems to me that as the rpm changes, the amount of outside air sucked in by a leak can change on a per pulse basis. On the Intake side the vacuum rate changes from deceleration to acceleration. On the exhaust side the scrubbing (sucking rate, vacuum pulse between opening of exhaust valves) and exhaust rate changes as the exhaust valve opens and closes, which can create a variable pulse (sucking rate variability) for manifold leaks!!!!!!

But I would try and fix the excess fuel rate on the LPG at idle next, then rerun the tests.

 

[Ecomike]

Re: Data Confirmed - Updated

Same results today except for the changes in red:

FUEL TRIM ON LPG

At Idle - STFT: -33.59% (lean) / LTFT: -33.59% (lean)
At 1500rpm - STFT: -33.59% (lean) / LTFT: -33.59% (lean)
At 2500rpm - STFT: +32.81% (rich) / LTFT: +32.03% (rich)

The above readings were taken in PARK but if I go for a DRIVE with the vehicle moving I get the following results (regardless of rpm):

At 1500rpm - STFT: +32.81% (rich) / LTFT: +32.03% (rich)
At 2500rpm - STFT: +32.81% (rich) / LTFT: +32.03% (rich)


FUEL TRIM ON GASOLINE/PETROL

At Idle - STFT: +/-5% / LTFT: +15% (rich)
At 1500rpm - Unable to get an accurate reading due to rpm jumping up and down every second between 1500rpm-1600rpm, LTFT constantly changed from -33.59% (lean) to +26.56% (rich) - with a few figures in between.
At 2500rpm - STFT: +/-5% / LTFT: +18% (rich)

The above readings were taken in PARK but if I go for a DRIVE with the vehicle moving the LTFT constantly jumps around between +15% (rich) and +32.03% (rich).


02 SENSOR ON LPG

At Idle - 0.880V constant
At 1500rpm - 0.880V constant, and stayed constant up until 2000rpm
At 2500rpm - Normal swing (0.1-1.0V)

02 SENSOR ON GASOLINE/PETROL

At Idle - Normal swing (0.1-1.0V)
At 1500rpm - Normal swing (0.1-1.0V)
At 2500rpm - 0.7V to 0.8V range


Thoughts anyone?

Looking at this data again, just on the LPG, it sounds like the LPG feed rate regulator is not working, like the PCM is not able to control the feed rate? Alsmost like it is stuck open too far at idle, and can't open further when needed at 2500 rpm?

 

[Ecomike]

Fuel trim site:

http://forums.nasioc.com/forums/archive/index.php/t-1401070.html

So is the fuel trim data on just one cylinder that you are posting, or an average? Time to look at each separately, and see if any injectors are funky!!!!

http://books.google.com/books?id=Oz...v=onepage&q=ltft stft obd ii rich lean&f=true

Just read an interesting point showing how a miss fire can show lean to the O2 sensor (O2 was not burned, not used, thus is in excess!!!!), while being rich due to unburned fuel!!!!!!!:doh:

That had never actually acurred to me before! But it should be obvious! Nice that OBD-II can sense a miss fire and throw a code for it if it happens often enough to trip the counter!!!!

 

[XJSteven]

Thanks for all the suggestions so far, it's given me a lot to think about. I might take it into a local LPG repairer this week who also does Dyno Tuning (see below for the service they provide). Hopefully these guys will get to the bottom of it for me...

Power Run:
A power run comprises of three (3) runs on the dyno.
This will measure how much power the vehicle has at the wheels and also the Air Fuel ratio, manifold pressure and taco.

Diagnostic Run:
This comprises of as many runs that are required to find the problem within the vehicle.
A comparison run is then performed once the fault has been rectified.
Printouts will be supplied to the customer.

Dyno Tuning:
Operate the dyno and tune the vehicle based around the customer’s specific requirements and needs, whether it is a track car or daily vehicle.

 

[Ecomike]

I would ask the LPG guys for details on the LPG flow rate regulator first. Get a spec and maintenance sheet on it and any controller that interfaces it to the Chrysler PCM. See what makes it tick. Is it something you can replace like a petro fuel pressure regulator or fuel injector. My guess is it is some kind of throttle body fuel injection system, but I am not sure how the Chrysler PCM knows how to control it, since it was designed to operate 6 independent petrol fuel injectors, and not a throttle body single injector?

If your only problem was with LPG, they might be helpful, but I am not yet convinced it is the only problem. I am convinced it is the current major problem at idle to 2000 rpm. But if it is just a stuck LPG regulator, why not replace it yourself?

I would ask them how they measure the air / fuel ratio?

Taco????

I am not sure if a dyno will be any help at all at this point.

Have you tried a stethoscope to listen for manifold leaks?

You might also check the idle O2 readings, cold, versus hot to see if a manifold leak rate is changing (cold to hot at idle), once it gets to closed loop, which reminds me, you have not mentioned checking the OBD-II data on whether or not, and when it is showing open loop versus closed loop operation?

Also has the scanner shown any anomalies in the CTS, MAP, MAT or TPS data, just to cover the old stuff once more?

 

[cemeyer]

Can you take the lpg setup off of where it goes into throttle body and run tests again? I'm curious if that will change a few things.
Curt

 

[XJSteven]

Hi Mike, the OBD-II data only ever shows "Fuel System Status - Closed Loop Control using Oxygen Sensor - Normal", this is when hot or cold. I checked the 02 sensor reading this morning when it was cold and it was oscillating at idle! I then drove for 20-30 mins and checked it again when warm and it was back to the constant 0.880V. The 0.880V stayed constant up until 2000rpm and began oscillating again from approx 2100rpm onwards.

The scanner doesn't show any error codes for the CTS, MAP, MAT or TPS. I'm even having difficulty getting the P0171 code to show up again (for the freeze frame data) because I can drive around for days without it reappearing. By the way, the last tank of LPG was back to having poor fuel economy again.

I spoke to the LPG repairer I mentioned above and found out the PCM doesn't control the LPG mixture at all. He said "the gas computer shuts the fuel (gasoline) off" and the LPG mixture is set on the Converter - one bolt is the Idle and the other is the Air/Fuel. He also said the best way to check the mixture of LPG is by using a "sniffer" which they stick up the rear end of the exhaust - so maybe I shouldn't be so quick to fire the first LPG installer! They can't fit me in for another 3 weeks so I might attempt to adjust the converter myself yet.

No I don't have a stethoscope, what sound would I be listening for exactly?


Curt, I will have a look at where the LPG goes into the throttle body. Are you suggesting I turn the LPG tank off, disconnect the pipe from the throttle body (and block the hole up?) and then run the tests again on gasoline only? What things do you expect it might change?

 

[four_shot]

He's looking to isolate the petrol system from the lpg.

(one system is easier to diagnose than two)

I second that idea.

 

[cemeyer]

I'm suggesting doing just what you said. I wonder if there is a leak in the lpg system letting lpg in when on gasoline and extra air in when on lpg. I'm shooting in the dark here, but like four shot said one is easier to diagnose than two. With the lpg completely out of the loop, you know it's not contributing to the problem.

 

[Ecomike]

Hi Mike, the OBD-II data only ever shows "Fuel System Status - Closed Loop Control using Oxygen Sensor - Normal", this is when hot or cold. I checked the 02 sensor reading this morning when it was cold and it was oscillating at idle! I then drove for 20-30 mins and checked it again when warm and it was back to the constant 0.880V. The 0.880V stayed constant up until 2000rpm and began oscillating again from approx 2100rpm onwards.

That one is baffling to me. Unless you have a thermal failure of injectors, failing to seal, or close, or an LPG leak while using petrol, when hot!

The scanner doesn't show any error codes for the CTS, MAP, MAT or TPS. I'm even having difficulty getting the P0171 code to show up again (for the freeze frame data) because I can drive around for days without it reappearing. By the way, the last tank of LPG was back to having poor fuel economy again.

I spoke to the LPG repairer I mentioned above and found out the PCM doesn't control the LPG mixture at all. He said "the gas computer shuts the fuel (gasoline) off" and the LPG mixture is set on the Converter - one bolt is the Idle and the other is the Air/Fuel. He also said the best way to check the mixture of LPG is by using a "sniffer" which they stick up the rear end of the exhaust - so maybe I shouldn't be so quick to fire the first LPG installer! They can't fit me in for another 3 weeks so I might attempt to adjust the converter myself yet.

Not sure what good a sniffer is after a cat converter? Not sure what an LPG converter is, or what it does? Another fuel control computer, Hmm. I wonder how it knows when it is to be in control, and when not?

No I don't have a stethoscope, what sound would I be listening for exactly?

Listen for a local hiss sound. You can als listen to the fuel injectors open and close, a tick/clicking sound

 

[XJSteven]

Another fuel control computer, Hmm. I wonder how it knows when it is to be in control, and when not?

Probably when I press the button mounted on my steering column that switches fuel system between LPG and gasoline?

I spoke to the 1st LPG installer again today and asked him why his exhaust gas analyser didn't pick up any problems with the LPG mixture (when it was using 25% more LPG when I filled up) and whether I should try adjusting the mixture on the Converter. He said adjusting the Converter won't help me because that's only for idle, something about it being only for 1/8th of the throttle and "after that the stepper motor takes over". He said the LPG computer should still try and lean it out if it was running too rich.

I'm suggesting doing just what you said. I wonder if there is a leak in the lpg system letting lpg in when on gasoline and extra air in when on lpg. I'm shooting in the dark here, but like four shot said one is easier to diagnose than two. With the lpg completely out of the loop, you know it's not contributing to the problem.

OK thanks for that cemeyer (and four_shot). I gave this a try today with not much change so at least the LPG is not contributing to the problem. Results as follows:

FUEL TRIM ON GASOLINE/PETROL (with LPG disconnected)

At Idle - STFT: +/-5% / LTFT: +15% (rich)
At 1500rpm - STFT: +/-5% / LTFT: +13% (rich)
At 2500rpm - STFT: +/-5% / LTFT: +18% (rich)

02 SENSOR ON GASOLINE/PETROL (with LPG disconnected)

At Idle - Oscillating (0.1-1.0V)
At 1500rpm - Oscillating (0.1-1.0V)
At 2500rpm - 0.7V to 0.8V range

 

[Ecomike]

Probably when I press the button mounted on my steering column that switches fuel system between LPG and gasoline?

I spoke to the 1st LPG installer again today and asked him why his exhaust gas analyser didn't pick up any problems with the LPG mixture (when it was using 25% more LPG when I filled up) and whether I should try adjusting the mixture on the Converter. He said adjusting the Converter won't help me because that's only for idle, something about it being only for 1/8th of the throttle and "after that the stepper motor takes over". He said the LPG computer should still try and lean it out if it was running too rich.


OK thanks for that cemeyer (and four_shot). I gave this a try today with not much change so at least the LPG is not contributing to the problem. Results as follows:

FUEL TRIM ON GASOLINE/PETROL (with LPG disconnected)

At Idle - STFT: +/-5% / LTFT: +15% (rich)
At 1500rpm - STFT: +/-5% / LTFT: +13% (rich)
At 2500rpm - STFT: +/-5% / LTFT: +18% (rich)

02 SENSOR ON GASOLINE/PETROL (with LPG disconnected)

At Idle - Oscillating (0.1-1.0V)
At 1500rpm - Oscillating (0.1-1.0V)
At 2500rpm - 0.7V to 0.8V range

All of that data looks normal and good, except for the 2500 O2 sensor data. From what little I know and have had time to read, the fuel trims look OK, especially the short term fuel trims.

 

[XJSteven]

All of that data looks normal and good, except for the 2500 O2 sensor data. From what little I know and have had time to read, the fuel trims look OK, especially the short term fuel trims.

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???