P0171 Fuel Trim too Lean

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PeterJJ21
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P0171 Fuel Trim too Lean

Post by PeterJJ21 » Sun Apr 01, 2018 12:51 pm

I have the P0171 fuel trim system too lean code on my wifes 2008 1.2 polo which is running fine without any issues of roughness, staling or poor starting.
I have run the VCDS test for MAP and Lambda sensors which check out fine, I have checked for any vacuum leaks both visually and with a smoke test all of which are Ok with no leaks.
One thing I have noticed in VCDS 01 engine, measuring blocks, block 32 field trim at idle is high at 14% to 18% and understand that this should be below 10%, but when running at about 1500 revs the figure drops to zero to 1% so don’t think it’s a fuel or injector issue
The check engine light comes on anywhere between 1 and 5 days after a code has been cleared with the same P0171 fuel trim system too lean code.
I have done the usual trick of using carburettor cleaner and smoke test with no leaks found, new servo hose tried and the issue is still there at idle and I have replaced the seal on the throttle body and recalibrated and all tests OK.

Any advice would be welcome as I’m running out of ideas as to where to look now.

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alexperkins
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Re: P0171 Fuel Trim too Lean

Post by alexperkins » Tue Apr 03, 2018 1:42 pm

Have you had the timing checked? Things like this can show when the valves get out of time in the sequence

BrentV
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Re: P0171 Fuel Trim too Lean

Post by BrentV » Sat Apr 21, 2018 11:51 pm

I have the exact same issue with my Polo 9N3 1.2 from 2008 with engine code BBM. (Does your car have the same engine code?)
I have had this issue for the last 2 years and I am unable to fix it.

Tried multiple things
- relearn throttle body
- replaced lambda probe
- replaced front half of exhaust (flexible part was leaking due to rust)
- replaced carter ventilation
- replaced intake manifold seals
- replaced injector seals
- replaced MAP sensor
- ultrasonic cleaning of the injectors
- checked fuel rail pressure and replaced fuel filter

Also replaced but not for the error:
- replaced spark plugs
- replaced coilpacks
- replaced cam sensor

Car drives perfectly normal with 6l/100km fuel consumption.
One advice I can give you (from own experience), do not drive too long without getting this fixed because it will damage the catalytic converter.
Due too this fault, mine is broken as well. I have a new one already here but I am not going to install it without first solving this fault. Otherwise I will also wreck my new catalytic converter. (€600+)

Hope we can find the solution together.

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Re: P0171 Fuel Trim too Lean

Post by RUM4MO » Sun Apr 22, 2018 2:38 pm

BrentV, that is a lot of time and money you have put into this engine trying to resolve it, but, like Alex has suggested, have you checked the static timing of the cam shaft to the crank shaft?

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Re: P0171 Fuel Trim too Lean

Post by BrentV » Sun Apr 22, 2018 10:36 pm

Thanks for your reply, no I haven't checked the timing yet.
How can I check if the timing is still OK? Or is this someone that can only be done with professional equipment?

Searching around on the internet, there are a lot of 3 cyl 1.2 VW engines with timing chain problems. So I think that this might actually be the case on my Polo as well.

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Re: P0171 Fuel Trim too Lean

Post by RUM4MO » Mon Apr 23, 2018 9:09 am

BrentV, I have never owned a VW Group car with the 1.2 3 cylinder engine, so what I "know" about this is only what I have read on these forums. I think that checking the static timing of the cam shaft relative to the crank shaft is very straight forward and easy for you to do as it only involves removing a cover of a couple of rubber plugs from the chain drive cover and checking that when the crank shaft is turned so that its pulley lines up with the correct marking - the cam shafts pulleys are in the correct position. Hum, on reading the Haynes workshop manual for the 2002 9N Polo it does not mention doing that! Sounds more like turning the engine until it has reached TDC on cylinder 1 then checking down through where the camshaft position sensor was located.

Remember same engines were fitted to Skoda Fabia SEAT Ibiza - if checking other forums for information.

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Re: P0171 Fuel Trim too Lean

Post by BrentV » Thu May 10, 2018 8:14 am

Yesterday I got the timing chain checked by a professional garage.
They used a PicoScope and measured the time between the pulses of the camshaft and crankshaft sensor. Then they compared these results with a known good engine from the PicoScope database.

The time was exactly the same so my timing should be right. Timing chain has not jumped some teeth or is not stretched.

They suggested checking if there's an update available for the ECU. This because the engine runs perfectly and has more then enough torque for a 60HP engine.

Next Monday they are going to contact Volkswagen and let them check if an update to the ECU is necessary.

Thanks for the support over here! I will keep posting updates when I know more so maybe this thread can help someone else.

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Re: P0171 Fuel Trim too Lean

Post by RUM4MO » Thu May 10, 2018 8:24 am

I hope that you get to the bottom of this problem, I would like to own PicoScope, but as a DIYer I can't really justify buying a kit!

Sounds like you have taken your car into a place that should be able to sort it out - and not just a place that will change another few parts.

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Re: P0171 Fuel Trim too Lean

Post by PeterJJ21 » Thu Aug 23, 2018 8:26 am

Hi All, has anyone had any joy resolving the P0171 Fuel Trim too Lean issue at idle, as at my wits end trying to resolve this problem.

One thing I have noticed recently is at start up in the morning the idle revs go up to bet 1500 for about 15 seconds then go back to normal after that and that when the AC is switched on the fuel trim goes back to normal levels at idle, otherwise the car runs fine, no misfire lack of power or stalling.

Any ideas?

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Re: P0171 Fuel Trim too Lean

Post by alexperkins » Thu Aug 23, 2018 8:30 am

Hi Peter

What have you tried so far?

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Re: P0171 Fuel Trim too Lean

Post by BrentV » Thu Aug 23, 2018 9:48 pm

PeterJJ21 wrote:
Thu Aug 23, 2018 8:26 am
Hi All, has anyone had any joy resolving the P0171 Fuel Trim too Lean issue at idle, as at my wits end trying to resolve this problem.

One thing I have noticed recently is at start up in the morning the idle revs go up to bet 1500 for about 15 seconds then go back to normal after that and that when the AC is switched on the fuel trim goes back to normal levels at idle, otherwise the car runs fine, no misfire lack of power or stalling.

Any ideas?
Hello Peter,

I am also looking for a solution for the exact same issue on the same engine in my 2008 Polo. No lack of power, no misfires, no stalling, starts right away, and not rough running for a 3-cyl. You can read my previous posts on this topic to see what I already tried, maybe it can be a solution for you.
Last week, I made a DIY smoke machine and smoke pressurized the intake. I didn't see smoke escaping anywhere, so I presume the intake does not leak and suck in extra unmetered air. Also, there was no ECU update available at the vW dealership.
On a really cold start, my engine goes to 1500 rpm for 1-2 seconds (definetly not 15) and then goes to 800-900rmp.

I haven't really looked at the fuel trims because I don't really understand them, and don't know what to look for. I do have VCDS so I can check if it can be useful for anyone.

If anyone knows something I/we can try, I would be very gratefull.

Thanks!

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Re: P0171 Fuel Trim too Lean

Post by RUM4MO » Fri Aug 24, 2018 9:26 am

BrentV

This is just my experiences when dealing with a car issue that has been corrected in the past by VW Group by applying a S/W fix:-

1) 2002 VW Polo with BBY petrol engine - many illogical fault codes being logged including "bank 2 inlet timing errors", that engine does not have a second cylinder bank, handed that car into a VW main dealer when it was maybe 6 and then when it was 8 years old, and told them what was happening, and I also said that I had earlier noted that some of these engines had had an engine ECU update, so check that and apply a S/W update - when I went back to collect that car all I got was "no updates available or needed"! So as this was my second attempt to sort this out, I gave up and lived with needing to clear some weird logged fault codes usually during December or January each year!!

2) 2009 SEAT Ibiza SC with the later version of 1.4 16V Petrol engine - I was looking after this car while my daughter was working abroad, she normally used it every day are at least every other day during work holiday periods so did not experience this issue, which was, battery discharging if car left unused for more than 5 days! Now searching on VW Group car forums, I discovered that this was a rare but well known issue with some early 6J Ibiza SC with the 1.4 engine only it seemed and was said to be due to incorrect coding or incorrect management program in the BCM. So, after investigating every other possible reason/failure with that car, after another 9 months, I gave in and handed that car in for investigation/sort, making sure that they knew the complete history of what I had already done to try to dismiss any other possibilities other than a S/W in the BCM. After 10 days, the garage reported that they had finally found the root cause of this problem, it was the steering sensor under the steering wheel, now I'm not trying to be clever, but I questioned this and was told that it was definitely the root cause, this part was replaced and coded to the car and I got a bill of £385! Initially being positive I thought that all was well, so to prove it was now okay, used that car to travel to and back from our local airport when going on holiday, but as back up, I left a starter battery pack in the boot. On our return 7 days later the battery was heavily discharged so I started it using the booster starter pack and drove home. I logged the battery voltage over the following few months and it always discharged quickly, so back to SEAT dealer, No Fault Found!! Then that dealership closed as there was another one in my area, so while the first one said to get in touch with SEAT UK as I was what was classed as a "good loyal customer", they would handle things to get a resolution, I did not bother trying to do that, instead I booked it into the other, "not so well thought of" SEAT dealership explaining what had already been done, the service receptionist immediately agreed that it was probably a S/W issue with the BCM as they had recently had a traded in a 2009 VW Polo 1.4 with the same problem, checked for S/W updates, none found, logged that problem with VW and were advised where to find the S/W update. So this car was booked back in, No Fault Found, and no S/W available, they logged that problem with SEAT and some days later were advised that a S/W fix was available, that was applied to that car and now it can be left for 4 to 8 weeks and still start okay.

Now, what I was trying to point out there is, VW Group workshops, it seems only have access to recent S/W updates on their online "help" system, if that update was from a few years ago it does not seem to be visible to the workshop, so if they think that there is really an issue, they log a help request from, in your case VW and that normally ends up with a full check being carried out against VW's update records. So maybe request that your dealer does this for you.

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Re: P0171 Fuel Trim too Lean

Post by BrentV » Tue Sep 18, 2018 5:10 pm

Yesterday I went to a VAG specialist with the car. He connected a diagnostics tool and did the following measurements:

On idle, lambda probe 1 (before catalytic converter) is switching like it should between 0.1 and 0.9V and lambda probe 2 (behind catalytic converter) has a static output voltage of 0.75V. This is all like it should. (See Screenshot idle)
With the engine at 2000 rpm, lambda probe 1 is switching like it should, and lambda probe 2 is doing exactly the same as probe 1. Thus telling us that the catalytic converter is not working properly. (See screenshot 2000 rpm)

Then the VAG specialist took the following conclusion:
The catalytic converter is probably worn out and cannot convert/handle the extra exhaust gasses compared to idle. The ECU is measuring on lambda probe 2 that the exhaust mixture after the catalytic converter is not OK and is trying to make a better mixture by injecting less fuel. Lambda probe 1 is measuring that there is not enough fuel in the mixture and is setting the 'P0171 - System too lean' trouble code.

The solution would be to replace the worn out catalytic converter .

What do you guys think about this diagnosis ? Thanks!
Attachments
Screenshot idle.png
Screenshot 2000 rpm.png

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iichel
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Re: P0171 Fuel Trim too Lean

Post by iichel » Tue Sep 18, 2018 5:19 pm

Makes sense to me.

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Re: P0171 Fuel Trim too Lean

Post by Willni » Tue Sep 18, 2018 9:40 pm

Yep same as iichel, makes sense. Should be alright to get a used one if you want to avoid paying for a new one.

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