Hi. My fabia 2003 1.2 12v (AZQ) have been getting code p0171 for a while. I've looked fuel trims and when check engine light is on, short fuel trim shows 0% and long fuel trim -4.7%. When codes cleared, short fuel trim is 39.8% and long fuel trim the same -4.7%. This happens only when engine is half warm. When engine is warmed fully up, fuel trims are normal. Code appears on open loop condition, but it seems that ecu doesn't change to closed loop even with warm engine. Fuel consumption is sometimes 15L/100km and sometimes 7L/100km on steady speed. Also sometimes theres a huge power loss which comes with increased fuel consumption.

Whole intake has been inspected for leaks but none is found. The car has been modified for E85 with eflexfuel ethanol kit, but i think it's not the problem because it does the same with pure gasoline. Any suggestions for next steps?

16555 (P0171) - Fuel Trim; Bank 1: System Too Lean

 

1 hour ago, Tiisseli said:

The car has been modified for E85 with eflexfuel ethanol kit, but i think it's not the problem because it does the same with pure gasoline.

 

Can the modification be taken out of circuit?

1 hour ago, Crasher said:

16555 (P0171) - Fuel Trim; Bank 1: System Too Lean

 

 

Can the modification be taken out of circuit?

Yes and I will test taking it out next time I fill up.

 

I took a test drive after work and again with cold engine there was lean mixture, but when it got to working temperature OL-condition changed to CL-condition and everything was back normal.

I would replace the pre-cat lambda probe if the car won't operate on closed loop properly. If it's never been replaced then it's 20 years old and not to be trusted.

Can you see if there is a specific G62 temperature when it goes from misbehaving to being OK?

1 hour ago, sepulchrave said:

I would replace the pre-cat lambda probe if the car won't operate on closed loop properly. If it's never been replaced then it's 20 years old and not to be trusted.

Spot on, the absolute obvious first port of call.

1 hour ago, sepulchrave said:

I would replace the pre-cat lambda probe if the car won't operate on closed loop properly. If it's never been replaced then it's 20 years old and not to be trusted.

Both lambda probes has been replaced a year ago with bosch sensors. On closed loop it works perfectly but on open loop it doesn't

 

1 hour ago, Pete_Ex-Wino said:

Can you see if there is a specific G62 temperature when it goes from misbehaving to being OK?

At around 86°C it starts working as should, but this happens only if it changes to closed loop, sometimes stays on OL for longer time. On open loop it keeps system too lean.

 

If I have understood right, on closed loop ecu uses sensors to create right air fuel ratio, but on open loop it use only program?

 

 

What measure are you using to distinguish between open and closed loop operation?

6 minutes ago, Pete_Ex-Wino said:

What measure are you using to distinguish between open and closed loop operation?

Looked from generic OBD tool, not sure if its accurate. But it seems to be only thing which distinguish right and wrong engine behavior.

So does this tool literally say "open loop operation" or "closed loop operation"?

Or are you looking at pre-cat oxygen sensor voltage?

 

3 minutes ago, Pete_Ex-Wino said:

So does this tool literally say "open loop operation" or "closed loop operation"?

Or are you looking at pre-cat oxygen sensor voltage?

 

On Freeze frame/live data there is "FUELSYS1" and after that "Open Loop" or "Closed Loop"

Can you see the output voltage of the pre-cat sensor?

1 hour ago, Tiisseli said:

Both lambda probes has been replaced a year ago with bosch sensors. On closed loop it works perfectly but on open loop it doesn't

 

At around 86°C it starts working as should, but this happens only if it changes to closed loop, sometimes stays on OL for longer time. On open loop it keeps system too lean.

 

If I have understood right, on closed loop ecu uses sensors to create right air fuel ratio, but on open loop it use only program?

 

Ok then you must change the ECT sensor because the ECU is not reading the temperature correctly.

 

Open loop is basically on choke, as soon as the engine is warm enough the ECU should switch to closed loop.

20 hours ago, Tiisseli said:

Yes and I will test taking it out next time I fill up.

 

I took a test drive after work and again with cold engine there was lean mixture, but when it got to working temperature OL-condition changed to CL-condition and everything was back normal.

 

I have to admit I and probably most mechanics in the UK have little or zero experience with E85, we have only just got E10 and that is already causing some odd issues with the fuel trim window being exceeded by 10% and causing an error. When you are back on E10 (I assume) I will be able to help better.

21 hours ago, Pete_Ex-Wino said:

Can you see the output voltage of the pre-cat sensor?

Ok, took the car to another test drive and this time everyting was great, all symptoms were gone. At cold start pre-cat sensor showed first around -0.800mA and it settled to around zero in couple of minutes. Lambda value first was around 0.880 and settled around 1.000. Post car sensor is switching between 0.750-0.900v. On OBD tool open loop condition changed to closed loop, when engine temperature got 60°C. Fuel trims kept at zero whole OL time. When OL changed CL fuel trims went around -2.3%.

I didn't save the freeze frame when problem was there (stupid me) and o2 sensor data is missing. I do remenber though the lambda value were whole time 1.299 which is maximum lambda value, when short fuel trim was 39.8%. In my opinion engine is not even able to run with values like that, so I started thinking if the pre-cat probe is not sometimes functioning as should or ECU is living its own life.

I will check the specs for the precat sensor in a little while, I think the expected number is quite strange, and your 1.299 is actually lower than it should be.

Here you go:

Note 15 says 'for AZQ engine'

Edited 16 March, 20224 yr by Pete_Ex-Wino

33 minutes ago, Pete_Ex-Wino said:

I will check the specs for the precat sensor in a little while, I think the expected number is quite strange, and your 1.299 is actually lower than it should be.

 

22 minutes ago, Pete_Ex-Wino said:

Here you go:

Note 15 says 'for AZQ engine'

Thank you!

Went to get some groceries and problem was there again. Pre-cat sensor was reading from 2.300mA up to 3.900mA and lambda was again 1.299. Post-cat sensor vas reading up to 0.999V. After driving long time I was able to get these number back to normal again. If internet has taught me right I would say its running more rich than lean? At least by smell of gas after idling...

I wonder how I can measure pre-cat sensor voltage? On OBD I am only able to get milliamps. Training is still in progress...

 

VCDS Lite with cheap FT232 cable.

Then you get numbers that look like workshop manual stuff.

I guess the ECU thinks the engine is very cold, try disconnecting the ECT sensor to see if it improves.

The open loop of the lambda probe is when engine is working at full throtle at 100% load. At that point lambda probes dont regulate mixture, it already controls ECU, and it needs more fuel. You have lean mixture at open loop. First thing to do, change fuel filter 3bar, check fuel pressure, do readeness code with diagnostic device

Everything seems to be normal now! I replaced the pre-cat lambda probe, cause its values were so much off. These cars seems to use o2 sensors straight from start and adjust the mixture, even my obd showed "open loop" condition after cold start.

Thanks everyone for helping!

Well done. Let's hope this sensor lasts longer than the last one.  What did the removed one look like? Any obvious damage/dirtiness?

 

4 minutes ago, Pete_Ex-Wino said:

Well done. Let's hope this sensor lasts longer than the last one.  What did the removed one look like? Any obvious damage/dirtiness?

 

It was a little reddish, nothing else obvious.

Have you been using any fuel additives? Everything you need to know about lambda sensors - Garagewire

 

"Fuel/Engine Additive Contamination

Similar to anti-freeze, the sensor nose will be contaminated with white or red deposits."