P0171: System too Lean (Fuel Trim)

[TeCKis300]

Working through this and could use the hive minds support. P0171: System too Lean (Fuel Trim). Also P0300: Random/mulitple Cylinder misfire detected. Multiple specific cylinder misfires P030x.

Symptoms: In diagnostics after getting home. LT fuel trim on both banks at idle are high. +15 and +18. LT trims are only incrementally elevated at higher engine speeds/loads (+4/+4 at WOT), so trim is inversely proportional to load. Initial suspicion is a vacuum leak, but didn't find any in cursory troubleshooting. Cleaned MAF and is not the issue. LX570 is a 2009 with 140k. Note I reset all codes after initial issue below, and they have yet to re-appear but presume they will when the engine is really heat soaked idling.

First noticed the issue on a off-road run on a 95 degree day. Idling over 30 minutes, on a really steep 30 degree incline with A/C running, 3/4 tank fuel. Stepped away with kid asleep to help spot buddies over obstacles. Came back to the car noticeably stumbling at idle, with CELs above, barely power to climb to a safer spot, before troubleshooting. Stopped engine, let cool for 10 minutes, restarted and reset codes, and ran fine. Up the trail, and idled probably 10 minutes on another occasion on flatter ground and minor codes came back. Reset. Avoided extended idling through rest of trip and was fine.

Since this issue manifests in both banks, it's likely something that affects both. Could still be a vacuum leak that I haven't found as the symptoms support that. Not MAF, not PCV. I need to work isolating vacuum leaks more systemically but it's a straightforward vacuum system and I didn't see or find any with propane trying to affect fuel trims. There's not much on these boards to show any systemic issues on the 200-series.

I have a new fuel pump in hand and may attack that but wanted to see if anyone had any ideas?

TL;DR - Repair here

DIY Fuel Pump Replace

Replaced a fuel pump recently as part of resolving an idle fuel pressure issue. Diagnostic thread here: https://forum.ih8mud.com/threads/p0171-system-too-lean-fuel-trim.1289672/ Didn't find much documentation on this site so here's my attempt for future reference. Couple strategies to do...

forum.ih8mud.com

 

[gaijin]

Sounds like you may be getting localized "boiling" in the fuel line somewhere upstream of the engine causing air pockets which interrupt normal fuel flow.

Just my US$0.02

 

[Moby]

I haven't looked at this in the 3UR but I assume positive trim values are adding fuel?

As you say, your vacuum leak theory matches the symptoms (high vacuum at idle pulling in more unregulated air and creating a lean at low rpm condition). I think that I've seem "smokers" for finding vacuum leaks. Inject through the intake... I'd definitely work towards completely eliminating a vacuum leak issue.

Aside from vacuum - fuel pressure regulator or fuel pump (since you have one) might be worth a shot? But I would think that this would be the opposite of what you are experiencing. I would expect it to fuel OK at low rpm/load (can generate enough pressure for short injector duration) and have trouble at higher rpm or load.

Could it be downstream? O2 reporting lean causing overfueling at idle and higher rpm can consume the extra fuel? That's a bit of a stretch and also does't really match your intermittent symptoms but it might be worth confirming that it really is running lean, not just what the ECU "thinks" that it sees. Either by smelling the exhaust, with an independent tailpipe wideband o2 or even a smog shop visit.

Even more of a stretch but I assume this has happened on different tanks of gas?

I'll think on this some more.

 

[a200]

Is it possible that air found its way into the cooling system at that angle and allowed things to get a bit hot? I would like to see this replicated on level ground.

 

[Sandroad]

You’ve described such an extreme circumstance that it’s hard to say anything is amiss. I’ll guess it won’t happen again unless you happen to idle for a half hour in 95F heat with A/C running on a 30 degree slope! (It was still recovering when it reoccurred on the same trip.) That said, put in the new pump anyway.

 

[bloc]

One note about the fuel system in these trucks.

The idea that a bad pump would present as adequate flow at idle but not enough at higher engine loads is complicated by the fact that our fuel system steps down fuel pump supply voltage at low engine loads. It is plausible that the stepped down voltage may be enough to get the pump into trouble, especially with a ton of heat soak in the fuel system.

I did verify that the fuel pump module is at the rear of the tank so it shouldn’t have been uncovered and let the bucket get low as long as you were pointed uphill

 

[Moby]

One note about the fuel system in these trucks.

The idea that a bad pump would present as adequate flow at idle but not enough at higher engine loads is complicated by the fact that our fuel system steps down fuel pump supply voltage at low engine loads. It is plausible that the stepped down voltage may be enough to get the pump into trouble, especially with a ton of heat soak in the fuel system.

I did verify that the fuel pump module is at the rear of the tank so it shouldn’t have been uncovered and let the bucket get low as long as you were pointed uphill

Very good data, thanks!

 

[grinchy]

Fuel pump is definitely in a little plastic bucket.

 

[TeCKis300]

Thanks guys and helps me take a fresh thought perspective.

Surely it could be a fluke created by temps and situation, and I was curious if others had seen such a thing. The extreme nose up situation could create all sorts of funny issues. I do think there's something going on as there's seems to be subtle idle fluctuations even outside of those conditions.

For the moment, I'm ruling out items that are associated with specific banks like O2 sensors, exhaust or leaking manifolds. And also cylinder specific issues like coil packs and misfires. The fuel trims (ECU adding fuel) are a bit more elevated than I would expect for a stock car (generally +/- 10 is reasonable), which is why I'm chasing this.

I did some vacuum testing today by isolating PCV, fuel system/evap. Checked intake plenum. Cleaned throttle body. Doesn't seemed to have changed anything.

Going to dump some fuel cleaner in the tank tomorrow, and fill with fresh fuel later this week. Also have a fuel pressure testing coming in. If there's idle fuel pressure issues, that would be a sure sign. At which point could be the pump or pump ECU?

Anybody change the fuel pump on their own? I hear dealerships drop the tank, but working at home, I'd rather go in through the body if there's sufficient access?

 

[grinchy]

Thanks guys and helps me take a fresh thought perspective.

Surely it could be a fluke created by temps and situation, and I was curious if others had seen such a thing. The extreme nose up situation could create all sorts of funny issues. I do think there's something going on as there's seems to be subtle idle fluctuations even outside of those conditions.

For the moment, I'm ruling out items that are associated with specific banks like O2 sensors, exhaust or leaking manifolds. And also cylinder specific issues like coil packs and misfires. The fuel trims (ECU adding fuel) are a bit more elevated than I would expect for a stock car (generally +/- 10 is reasonable), which is why I'm chasing this.

I did some vacuum testing today by isolating PCV, fuel system/evap. Checked intake plenum. Cleaned throttle body. Doesn't seemed to have changed anything.

Going to dump some fuel cleaner in the tank tomorrow, and fill with fresh fuel later this week. Also have a fuel pressure testing coming in. If there's idle fuel pressure issues, that would be a sure sign. At which point could be the pump or pump ECU?

Anybody change the fuel pump on their own? I hear dealerships drop the tank, but working at home, I'd rather go in through the body if there's sufficient access?

I exchanged the fuel pump (and the rest of the stuff, including the float) for the dual tank capable version. I did it thru the top of the tank, by removing the middle row and using the inspection hole.
Details on the sub tank thread. You do have to do very minor surgery on the inspection hole to widen to extract the pump. Well within your diy range.

You’ll need a fuel pump plastic ring remover.

 

[bloc]

Personally unless you have rust I believe dropping the tank is easier and less intrusive than doing augmentation surgery on the body hole under the seats to get the module out.

It is surprisingly easy once the tank is mostly empty, and there are a couple ways of accomplishing that.

 

[TeCKis300]

I'm mentally preparing for the challenge/fun of changing the fuel pump. I've done this many times in other cars for boost. But first time on the 200-series.

There's some good youtube vids of the process. Going in from the top looks to be more work, but reasonably straightforward. From the bottom, looks quicker, but working from underneath can be a PITA. Though I do have a 2-post lift and hydraulic table. Caveat is my lift is rated for 6k, and the LX is probably 6400lbs as it sits. Maybe removing the wheels and spare will buy me enough margin.

Still keeping my options open and will wait for the fuel pressure tester to come in tomorrow.

 

[grinchy]

If you do decide to drop the tank I'm interested in how long it takes you to undo the tank vent fitting from the filler vent tube. That clamp was super unwieldy for me, ended up having to customize a pair of pliers to get the buttons depressed while allowing to pull at same time.

 

[bloc]

I'm mentally preparing for the challenge/fun of changing the fuel pump. I've done this many times in other cars for boost. But first time on the 200-series.

There's some good youtube vids of the process. Going in from the top looks to be more work, but reasonably straightforward. From the bottom, looks quicker, but working from underneath can be a PITA. Though I do have a 2-post lift and hydraulic table. Caveat is my lift is rated for 6k, and the LX is probably 6400lbs as it sits. Maybe removing the wheels and spare will buy me enough margin.

Still keeping my options open and will wait for the fuel pressure tester to come in tomorrow.

From the top would be easier if the existing hole in the floor was large enough to fit the module through, but it isn’t.

And all of that said I haven’t dealt with the specific fitting grinchy refers to, but to avoid cutting the floor of the rig I’ll definitely modify some tools.

 

[grinchy]

It’s a thing of beauty

 

[MoJ]

Any fuel odor when it happened?
Any substantial pressure build up when opening the fuel cap?

 

[TeCKis300]

Any fuel odor when it happened?
Any substantial pressure build up when opening the fuel cap?

No fuel odor so I didn't suspect anything around the cap or look into it. I'll say I didn't do much troubleshooting during the incident other than shutdown, reset, restart and was going again. Was focused on getting everyone else in my group over obstacles that I didn't bother telling them, as they all had more stressful off-roading navigation issues to worry about.

Taking @a200 's thought of trying to reproduce it, I left the car idling for 20 minutes in an open garage after getting home today. Nothing seemed to be amiss other than the LT trims were still elevated, but not as much (+14, +12.2). It's a pretty cool day about 78*F. Idling saw coolant temp drop from 198 to 191. LT trims were rock solid through the test. One thing that stood out however was intake air temperature (IAT). When first parking, was at 92 degrees. This kept creeping up, landing at 154 degrees 15 minutes in and stabilizing. That I found pretty eye opening. Can only imagine being heat soaked, fuel and all after off-roading a few hours on a hot day, slight elevation (4k ft), parked with extreme nose up, and hot air radiating up and saturating the engine bay and air intake in the fender. Wonder what IATs could look like in those circumstances? Maybe @gaijin and @Sandroad are onto something.

I added a bottle of fuel cleaner this morning and will see if that effects any change. I should have the fuel pressure tester tomorrow.

 

[MoJ]

No fuel odor so I didn't suspect anything around the cap or look into it. I'll say I didn't do much troubleshooting during the incident other than shutdown, reset, restart and was going again. Was focused on getting everyone else in my group over obstacles that I didn't bother telling them, as they all had more stressful off-roading navigation issues to worry about.

Taking @a200 's thought of trying to reproduce it, I left the car idling for 20 minutes in an open garage after getting home today. Nothing seemed to be amiss other than the LT trims were still elevated, but not as much (+14, +12.2). It's a pretty cool day about 78*F. Idling saw coolant temp drop from 198 to 191. LT trims were rock solid through the test. One thing that stood out however was intake air temperature (IAT). When first parking, was at 92 degrees. This kept creeping up, landing at 154 degrees 15 minutes in and stabilizing. That I found pretty eye opening. Can only imagine being heat soaked, fuel and all after off-roading a few hours on a hot day, slight elevation (4k ft), parked with extreme nose up, and hot air radiating up and saturating the engine bay and air intake in the fender. Wonder what IATs could look like in those circumstances? Maybe @gaijin and @Sandroad are onto something.

I added a bottle of fuel cleaner this morning and will see if that effects any change. I should have the fuel pressure tester tomorrow.

I asked as the scenario in which it happened (high ambient temps, altitude, low speed) sounded a lot like what can trigger a saturated charcoal filter and/or tank pressure on some 80’s and 100’s. The exact cause has never been fully established. Some report ethanol-free fuel helping. Others believe it’s related to the charcoal canister/filter needing replaced. Generally doesn’t throw a code but with the 200’s more sophisticated system it’s possible I guess.

 

[TeCKis300]

Got the fuel pressure tester (w/ 7.89mm/SAE 5/16 hair pin connector)

And idle fuel pressure shows...

~30PSI. That's well under the expected 41/42 psi per the repair manual. So now question is whether it's the fuel pump or fuel pressure controller? 80% sure it's the fuel pump.

 

[Moby]

Nice work!