Troubleshooting P0171 System too Lean - Interpreting Fuel Trim Data

[incomming]

Hi. I've got a P0171 code (system too lean) that I've been struggling to figure out for my LX450. I made some plots of the OBDII sensor data trying to narrow down the problem and I wondered if folks here could help me interpret it.

I'm attaching a couple of screen grabs that plot fuel trim and engine loading together under a range of conditions including idle, acceleration, cruise, and coast. By my research, it seems the basic "families" of problems that cause this condition are quite simply either too much air or not enough fuel.

It looks like I don't really have a problem at idle, which in conjunction with the fact that I've done a basic vacuum leak check using carb cleaner and didn't find anything seems to rule out a vacuum leak.

Peeling the onion a little, it seems that the fuel trim seems to get out of whack at significant changes in engine loading - either stepping on the throttle, or releasing it to coast. When I'm at constant throttle, the fuel trim seems to stabilize.

Because the problem seems to correlate with changes in throttle, my gut tells me it's a fuel delivery or perhaps more specifically a fuel delivery control issue. I'm not sure where to go from there though.

I've changed the fuel filter and am half way through running a tank with BG 44k and I'm not seeing an improvement.

Any suggestions of what to check next?

 

[Box Rocket]

I had a lean condition and injector cleaner didn't seem to help much (didn't surprise me since the injectors had been flushed not that long ago). I replaced the fuel pump because I was also having a bit of a power drop as well. Fuel trim numbers on my Ultragauge were very high (~40 combined). The lean condition was partly to blame for the loss in power but also was causing higher than normal temps. After replacing the fuel pump the temp issues were fixed and trim levels on my UG were much better (typically between 0-5 combined), but still had some power loss which ended up being clogged cats.

Edit: I had no trouble codes. Just higher temps and power loss symptoms that motivated the fix.

 

[landtank]

First off this code doesn't mean that your engine is running lean. The system being referenced is the fuel system in that the fuel being delivered from the base injector timing is off by more than 20%. Which would be indicated by the LTFT value.

On the input side of the circuit you have the MAF sensor which could deliver a lower value than what it should (dirty). This would give you a base injector timing that would fall short on delivering enough fuel and the ECU would then compensate for that by increasing the LTFT.

On the duty side it could be a fuel delivery problem where the amount of fuel that is expected to be delivered falls short and the ECU increases the LTFT to compensate for that.

On the feed back side you could have a problem with providing accurate AFR reading to the ECU, telling the ECU the engine is running lean and causing the ECU to increase the LTFT which would actually make the engine run rich. Think exhaust leak pre O2 or bad O2.

Each situation takes a different approach to diagnose.

If you want to throw something against the wall and hope it sticks I'd suggest a new Fuel Pump Relay.

 

[incomming]

If you want to throw something against the wall and hope it sticks I'd suggest a new Fuel Pump Relay.

I was hoping to avoid throwing more parts at it. I was hoping the sensor plots could help narrow down the most likely culprit. I could also chose some other parameters like air flow, short term fuel trim, etc and post those plots if it would be helpful. It just seems to be an awfully characteristic trend that the trim tends to go out of spec only when the load/throttle changes, then it stabilizes to an in spec value generally within a couple of seconds. Maybe I'm reading too much into that though...

 

[Tools R Us]

How old is the intake to air filter tube? They are known to crack and can be intermittent, the crack open with engine movement, acceleration, etc.

 

[musemech]

Hi. I've got a P0171 code (system too lean) that I've been struggling to figure out for my LX450. I made some plots of the OBDII sensor data trying to narrow down the problem and I wondered if folks here could help me interpret it.

I'm attaching a couple of screen grabs that plot fuel trim and engine loading together under a range of conditions including idle, acceleration, cruise, and coast. By my research, it seems the basic "families" of problems that cause this condition are quite simply either too much air or not enough fuel.

It looks like I don't really have a problem at idle, which in conjunction with the fact that I've done a basic vacuum leak check using carb cleaner and didn't find anything seems to rule out a vacuum leak.

Peeling the onion a little, it seems that the fuel trim seems to get out of whack at significant changes in engine loading - either stepping on the throttle, or releasing it to coast. When I'm at constant throttle, the fuel trim seems to stabilize.

Because the problem seems to correlate with changes in throttle, my gut tells me it's a fuel delivery or perhaps more specifically a fuel delivery control issue. I'm not sure where to go from there though.

I've changed the fuel filter and am half way through running a tank with BG 44k and I'm not seeing an improvement.

Any suggestions of what to check next?

View attachment 1844800View attachment 1844801View attachment 1844802

Did you ever resolve this issue? Have something similar going on. Thx.