P0171 Lean Code Diagnosis

[fR3ZNO]

Hey guys, I've been starting to have the CEL come on for P0171 (lean bank 1) these past few days and trying to figure out the issue. Also have P0411 incorrect SAI flow, but I can't see how that's related.

This is a 1.8T with an APR Stage 1+ which uses TT225 injectors (386cc), only other engine mods are a Forge DV and 034 HFC.

Things I've checked:

1. Vacuum is good, boost gauge is reading -19 to -20 inHg. Checked vacuum lines in engine bay, couldn't hear/find any leaks as well.

2. Did a pull to redline in 2nd and 3rd while logging MAF values (g/s) and got a maximum value of 130 g/s which seems very low considering APR tune is rated at 235hp and rule of thumb is that max g/s value should be 80% of rated HP, or about 188 HP.

3. Logged short term and long term fuel trims. Short term looked normal, but long term slowly creeped up to +24% at cruise, confirming the P0171 code that's being set.

4. Max boost during pulls was about 13 psi. Normally it spikes to about 20 and tapers to 18.

Need to check:

1. Boost leaks. However, I would be running rich and not lean if there was a boost leak, correct?

2. I disconnected the MAF but forgot to reset the fuel trims, didn't notice a difference and boost still seemed low (7 psi).


Does the ECU limit boost if the LTFT are very lean? I would imagine if it is pulling as much fuel as possible, then it would limit boost as well.

When I reset the fuel trims, I noticed that I was able to get about 15psi of boost.

The MAF currently installed is an "OEM" one from an online vendor, but has no Bosch markings or anything on it. I installed it about 2 years ago, after a misdiagnosis that the original was bad.

I'm thinking the current MAF is going bad, but wanted to verify before swapping it out blindly.

[Psquared]

When the ECU see lean fuel trims it will limit boost. I had this issues and my intake came off right at the turbo. Everything was fine at Ilde, but once the engine moved it sucked in a lot of excess air.

[fR3ZNO]

When the ECU see lean fuel trims it will limit boost. I had this issues and my intake came off right at the turbo. Everything was fine at Ilde, but once the engine moved it sucked in a lot of excess air.

Makes sense. Would explain why boost wasn't limited as much right after I cleared codes and reset the fuel trims.

[MacFady]

Sounds like unmetered air entering somewhere as suggested above. I've only had lean codes on previous non-turbo cars, was always a hole somewhere between the MAF and the intake manifold despite the dealer diagnosing bad MAF.

[BARRY]

confirm all your evap check valves are in good working order. i believe if a check valve is bad it could cause this error.

[fR3ZNO]

Thanks for the suggestions. I'll take a look.

[MetalMan]

With engine idling, you can individually pinch rubber lines before they hit check valves. If there's a problem with one of them, you may notice the engine sound change or notice a change in MAF reading @ idle.
Definitely boost leak test! I bet you'll find something.

[old guy]

I suspect you are going to find a leak somewhere between the MAF sensor and the turbo. With a leak there you will see full vacuum at idle (which you do). When you start driving you will still be able to build boost but your LTFT will max out since the turbo will be sucking all of that unmetered air (sounds like your symptoms). One it gets bad enough the ECM will start reducing the N75 duty cycle to protect the engine (also sounds familiar).

Edit: And while you are sucking all that unmetered air you will get a lower than expected MAF g/s which is what causes the high LTFT. (also sounds familiar).

[fR3ZNO]

Found this before heading to work this morning. Looks like the headlight cap clip is rubbing on the TIP. I couldn't tell if it broke all the way through or not. I'll take a closer look tonight.

[MetalMan]

Luckily headlight removal is only a few screws!
Speaking of screws, the ones with speed nuts you sent showed up yesterday. That was fast!

[fR3ZNO]

Edit: And while you are sucking all that unmetered air you will get a lower than expected MAF g/s which is what causes the high LTFT. (also sounds familiar).

Just to play devil's advocate here; if the MAF itself was faulty and was giving a lower than actual air flow reading, wouldn't the LTFT be high as well, since the ECU is thinking its getting more air than it's expecting?

Luckily headlight removal is only a few screws!
Speaking of screws, the ones with speed nuts you sent showed up yesterday. That was fast!

Yep! I'm glad headlight removal doesn't require taking the bumper off too.

That's priority mail for you I won't tell anyone you have Passat screws in your S4

[BARRY]

glad to hear you've figured out your issues.

[fR3ZNO]

I'm not sure if there's a hole in the TIP yet, but that's not good regardless.

[old guy]

Just to play devil's advocate here; if the MAF itself was faulty and was giving a lower than actual air flow reading, wouldn't the LTFT be high as well, since the ECU is thinking its getting more air then it is expecting.

Easy enough to check. Unplug the MAF and take it for a spin. If the problem is the MAF sensor you should see a significant drop in your long-term fuel trims.

[fR3ZNO]

So I just got home from work with the MAF unplugged. I cleared fuel trims before leaving. LTFT when I got home was ~9.4%. Compared to yesterday driving home with it plugged in and LTFT was at ~14%.

[old guy]

So I just got home from work with the MAF unplugged. I cleared fuel trims before leaving. LTFT when I got home was ~9.4%. Compared to yesterday driving home with it plugged in and LTFT was at ~14%.

In your first post you indicated that you were getting LTFT readings of +24%. Is that no longer happening?

[fR3ZNO]

In your first post you indicated that you were getting LTFT readings of +24%. Is that no longer happening?

The +24% was after a few days of driving with the lean code.

+14% was when I cleared the fuel trims and drove home.

[jon_b6]

I had the p0171 code as well in went away when I replaced the combi valve and that solved


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[old guy]

So If I am understanding you correctly the longer you drive with the MAF plugged in the higher the LTFT go until they ultimately max out.

Unplugging the MAF gave you a reading of below 10% which isn't all that bad considering the ECM is going to run you on the rich side for protection. but with the MAF unplugged the ECM doesn't care if there are any leaks between the MAF and throttle body since it is no longer metering air flow.

At this point you really need to perform a boost leak test. A change from 9.4% to 14% is a lot different than a change from 9% to 25%. With those readings I could build a case for either a leak or a bad MAF. Eliminate a leak as a possibility first.

[old guy]

I had the p0171 code as well in went away when I replaced the combi valve and that solved

That's a good point. A leaking combi valve will allow extra air to enter the exhaust stream and consequently the front O2 sensor is going to read it as a lean condition.

[fR3ZNO]

Boost leak tester hooked up at 10psi.

Thankfully I’ve got a spare SJP.

[old guy]

Unfortunately that isn't a big enough leak to explain your performance issues. If that's all you found I would say you probably have a defective MAF sensor.

[fR3ZNO]

I didn’t find any other leaks. I’ll check again when I put the new SJP in.

[fR3ZNO]

Put the new SJP in, couldn't find any more leaks. Took the car out and ran a few logs, reset the fuel trims first.

After going for 20~ minute drive, group 32 showed values of -5.1% adaptation (idle) and +13.3% adaptation (partial).

Earlier I was just using my generic bluetooth OBDII scanner that just showed STFT and LTFT.

Peak MAF value for a WOT run to redline was about 160 g/s, from previous logs when I was troubleshooting an issue unrelated to the MAF, I was getting about 175 g/s peak. But the previous logs were from over two years ago and during the summer. So taking those with a grain of salt.

Specified and actual boost values seemed to be okay as well, so verifies no boost leaks.

+13% still seems high to me, so I'm kinda stuck. Only thing I can think of is the MAF is on its way out.

[Razgriz227]

A few months ago my P0171 was fuel filter related. Replaced and no more issues.


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[fR3ZNO]

I replaced my fuel filter back when I first bought my car, since it was so clogged the fuel pump would whine constantly.

[fR3ZNO]

I had the p0171 code as well in went away when I replaced the combi valve and that solved


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That's a good point. A leaking combi valve will allow extra air to enter the exhaust stream and consequently the front O2 sensor is going to read it as a lean condition.

While I was checking for boost leaks yesterday, I tried blowing through the lower SAI hose which leads to the combi. I couldn't blow through it. So combi is good.

With engine idling, you can individually pinch rubber lines before they hit check valves. If there's a problem with one of them, you may notice the engine sound change or notice a change in MAF reading @ idle.
Definitely boost leak test! I bet you'll find something.

Still need to try this, will pick up hose pinch pliers later. I remember hearing a very slight hiss near the SJP, just couldn't pinpoint it until I did the boost leak test.

confirm all your evap check valves are in good working order. i believe if a check valve is bad it could cause this error.

I tested the evap check valve near the airbox. Only allows flow toward the engine. Forgot to check the one under the manifold.

[fR3ZNO]

Put another MAF in (used, known to be good). Still seeing 160~ g/s peak. Logged some 3rd gear pulls after resetting fuel trims. Got back home and partial adaption on block 32 was 13.3~%. So now I’m lost. Maybe something in the PCV system?

[Gin+]

160g/s sounds on par for stage 1+

[Johnny O]

Could be the pump in the tank, that would cause the code too

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[old guy]

When was the last time you looked at your air filter and snow screen?

[fR3ZNO]

When was the last time you looked at your air filter and snow screen?

A few months ago when it was replaced. Not sure about snow screen.

160g/s sounds on par for stage 1+

I had been getting around 175 g/s peak.

Could be the pump in the tank, that would cause the code too

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True.

[fR3ZNO]

Okay, so I've been doing some research. Keeping in mind I have been getting this P0171 code and P0411 (SAI incorrect flow) this post has me wondering. However, it's not really a lean at idle issue.

[fR3ZNO]

With the car idling, I can hear air rushing near the SJP which was replaced and didn't produce any EXTERNAL leaks with a boost leak test.

If I squeeze the rubber hose leading from the SJP to the intake manifold (this one), the rushing air sound STOPS. However, I don't really notice a change in idle speed, etc.

At idle, that hose is under vacuum, so air is being pulled through the SJP, is it normal to be able to hear that?

[MetalMan]

If it sounds like there's flow through that hose, then that could be a problem I'd think.
The SJP attaches to the breather system and brake booster.

Naturally there ought to be some flow through the breather system, but this should be limited by the PRV.

Brake booster shouldn't have any flow.

Try pinching more hoses? Are your check valves looking old?

[old guy]

With the car idling, I can hear air rushing near the SJP which was replaced and didn't produce any EXTERNAL leaks with a boost leak test.

If I squeeze the rubber hose leading from the SJP to the intake manifold (this one), the rushing air sound STOPS. However, I don't really notice a change in idle speed, etc.

At idle, that hose is under vacuum, so air is being pulled through the SJP, is it normal to be able to hear that?

If you have an '03 or newer the SJP has a fresh air source coming from the TIP just after the MAF sensor. So the air is being metered by the MAF. Pinching the hose really won't do much to the A/F ratio or idle speed. There may be one little blip when you first pinch it but that's it. And yes it should be pulling air. The air flow is what creates the vacuum amplification for the brake booster.

[fR3ZNO]

If you have an '03 or newer the SJP has a fresh air source coming from the TIP just after the MAF sensor. So the air is being metered by the MAF. Pinching the hose really won't do much to the A/F ratio or idle speed. There may be one little blip when you first pinch it but that's it. And yes it should be pulling air. The air flow is what creates the vacuum amplification for the brake booster.

Yep, I have an '04. Makes sense now after looking at this:

If it sounds like there's flow through that hose, then that could be a problem I'd think.
The SJP attaches to the breather system and brake booster.

Naturally there ought to be some flow through the breather system, but this should be limited by the PRV.

Brake booster shouldn't have any flow.

Try pinching more hoses? Are your check valves looking old?

Check valves look okay, but need to test them more thoroughly.

I removed and tested the one for evap near the airbox.

[fR3ZNO]

Since the additive trim at idle is much lower than the multiplicative trim, wouldn't that indicate it's something other than a vacuum leak?

Or is it possible that an issue with a check valve, etc in the breather system would cause more issues off idle?

[old guy]

Faulty check valves are going to cause a lot more issues at idle than when driving. Simply because the additional amount of air would be a much greater percent of the total air volume at idle. So yes, a vacuum leak will be more apparent in the additive (idle) trims rather than the multiplicative (partial) trims.

[fR3ZNO]

Looking at Ross Tech's Overview for this code

16555/P0171/000369 - Fuel Trim; Bank 1: System Too Lean

Possible Causes
Mass Air Flow Sensor (G70) defective - Assuming current MAF is good, then this is not the cause
Intake System Leak(s) - Seems possible, but boost leak test has not shown any obvious leaks

Possible Solutions
Check Mass Air Flow Sensor (G70) - Checked, values seem okay (160-165 g/s)
Check Fuel Pressure Regulation - B6 w/ returnless system, FPR is integrated with fuel filter. Fuel filter is roughly 3 years old at this point.
Check Fuel Pump - This could be a possibility. Need to check pressure at rail with fuel pressure gauge. Will try to pick one up at Harbor Freight tonight
Check Injectors - Not sure how to check these aside from pulling them out and looking at spray pattern. I consider this a last resort.
Check Intake System for Leaks - No leaks present on TIP when boost leak testing.
Check Exhaust System for Leaks - Leak before pre-cat O2 leaves exhaust manifold gasket or turbo to cat gasket as possible leak source. Both appear to be fine, a no leaks are heard when running.
Check Secondary Air System for Leaks - Combi valve is not leaking, confirmed by blowing through hose from SAI to combi.
Check Vacuum Pipes for Leaks - SJP was leaking has been replaced. Other areas to check?