P0171

Hello,

I have a 1999 Tacoma 2.4L and I cannot beat this P0171 code. I know there's a ton of info available on this code, but I feel like I'm chasing my tail so I wanted to see if anyone could provide more clues based on what I've done and what I'm seeing..

I've done the following:

• Vacuum - checked at shop (no leaks) & home w/ carb cleaner (no idle revving)
• MAF sensor - replaced
• EGR Valve - replaced
• Air intake filter - replaced
• Fuel filter - replaced
• Upstream O2 sensor - replaced
• Downstream O2 sensor - replaced
• Vacuum switching solenoid - replaced
• Spark plugs - replaced
• Master cylinder - replaced
• Fuel injectors - cleaned w/ seafoam

I've been using a BlueDriver OBD2 scan tool to watch the (limited) live data and I'm getting 19.5% for short-term fuel trim bank 1 at idle. That number starts to go down when I accelerate and stays between 0% and 10% for the most part while I'm driving, but always goes back to 19.5% at idle. I'm not sure if that 19.5% is significant or if that's just the limit of the OBD2.

Also, when I disconnect the battery and the computer resets, the truck runs very sluggish until I drive long enough for the the trim readings to kick back in. I suppose that means the truck is in fact running lean and the O2 sensors weren't giving me false readings... so I probably didn't need to change them. Not sure if that helps eliminate anything else.

Here's what's left on my list... if anyone has thoughts on (1) which would be more or less likely to be the issue, (2) which I could easily replace myself, or (3) suggestions for anything else I should check it would be much appreciated.

• PCV valve
• Fuel pump
• Fuel injectors
• Ignition coils
• Intake manifold gasket
  
• Brake booster/hoses
• Exhaust leak
• Catalytic converter

Thanks.
Jared V

 

Strange.
Typically when fuel trims get better as you accelerate, that indicates a vacuum leak.

 

Thanks yeah I'm not sure I trust the shop's results, but I also sprayed everything under the hood with carb cleaner and didn't hear any changes. Are there internal vacuum-related parts that could be malfunctioning but might not show up on the pressure or spray tests? I changed the vacuum switching solenoid but I'm mostly just shotgunning based on what I've learned online... I really don't know how these systems work.

 

Pop a coil and see if the tip is dry or oily, maybe.

 

Screams vacuum leak to me, pinch off the hose going to the brake booster and see what happens to your fuel trims.

 

So I had this issue a year or so ago, and that PO171 code plagued me for like a year and a half. I checked everything just like you, seemed like the possible issues were as long as a CVS receipt. But one of the issues that aren’t brought up about that code is the possibility that the ECU is going out. Long story short, after all my research and 3 shops later, an older mom and pop shop mechanic told me he had one tacoma ever come in with this issue and it happened to be the ECU. So back to researching, I couldn’t ever find anything extensive on the subject unfortunately. Found a guy on YouTube that worked on them and shot him an email and he asked if I would send in a picture of the internals of it, so I did and he told me the issue was a capacitor was leaking on to the chip board and it just so happened to be the portion of the chip that was the fuel control module.
Remedy: bought a 97 ECU with the same numbers as my OG Ecu and fixed everything. I could not believe it!!!

This is my first post ever. I just joined, but have scoured this website for over 4 years now for all my research needs. I wanted to share my personal experience with this gremlin of a code because I was seriously considering selling the truck if I couldn’t figure it out and I’m very glad I didn’t.

 

Thanks @South_Ga Taco, I'll have to take a look at the ECU caps soon if I can't figure this out.

Update: I replaced the PCV valve (& grommet) and the trim was reading normal levels while driving and idling, but then it became intermittently lean while idling. The readings are now normal whenever I'm driving, but slowly climb back up to 19.5% each time I stop and idle (both in gear and in park). I also replaced the valve cover gasket but saw no improvements.

Anyone have any theories for why a new PCV valve would have made the readings appear ok at first but then begin failing again? I removed the throttle body to get to the idle air control valve & throttle position sensor... does replacing those seem like a reasonable next step?

• Valve Cover Gasket - replaced
• PCV Valve - replaced (seemed to improve the issue but not fix it)
• Vacuum - checked at shop (no leaks) & home w/ carb cleaner (no idle revving)
• MAF sensor - replaced
• EGR Valve - replaced
• Air intake filter - replaced
• Fuel filter - replaced
• Upstream O2 sensor - replaced
• Downstream O2 sensor - replaced
• Vacuum switching solenoid - replaced
• Spark plugs - replaced
• Master cylinder - replaced
• Fuel injectors - cleaned w/ seafoam

 

Did you do the pinch test on the brake booster hose that I mentioned above?

 

@Dm_93, Yes I did, thanks... I guess I should've included that in my list. So if nothing changed would that rule out the brake booster?

 

Yea that would rule that out as a vacuum leak.

Still positive fuel trims at idle and normal trims driving really suggest a vacuum leak, can't explain the PCV temporarily fixing it as PCV air should be metered. Unless you moved around something that was leaking in the process of changing the valve and it temporarily sealed.

What is your MAF reading at hot idle?

 

How is the LTFT behaving? If that is significantly off (+/- 10%?) that could indicate a longer term issue/change in performance like degrading fuel pump and the ECU trying to compensate.

If you had the intake manifold or throttle body off, what was the condition of the gaskets and were they reinstalled in the correct orientation? I once had the TB gasket clocked wrong and it caused a vacuum leak that carb cleaner didn’t pick up.

Have you checked MAF readings? I know you said it’s new but wouldn’t hurt. Should be 3.3 to 4.7 g/s at idle and 12.9-18.3 at 2500 rpm no load, and calculated load at WOT should be 85% or more. (This is what I found for a V6, unsure if it’s different for you)

Your symptoms really sound vacuum related, maybe hook up the scanner and watch FT data while testing with carb cleaner to see if it picks up anything you weren’t hearing otherwise?

 

I've been down the same route you've been. My answer may not be for everyone, but my mystery vacuum leak was the exhaust-side EGR gasket. I didn't find it with a smoke test, but with a mechanic stethoscope.

 

I'm not sure what the MAF reading is at hot idle. I have the throttle body off at the moment but once I put everything back together and crank it back up I'll check.

I've reset the ECU so many times the LTFT probably isn't based on much data. I believe it's been around 38% though.... which seemed strange to me because I thought that was just a long-term average. I haven't seen the STFT above 19.5% so the math isn't adding up, but maybe the LT numbers aren't reset when you disconnect the battery? I was leaning toward the fuel pump after changing the fuel filter didn't do anything, but that didn't seem like an easy job so I don't want to attempt it if I'm just guessing. Would the high LTFT reading point more toward fuel pump than anything else I haven't tried yet?

Would it make sense to replace the idle air control valve or throttle position sensor since i have the throttle body off?

The throttle body gasket was crusty and broke into pieces when I removed it so maybe it was allowing air in. I'm about to go pick one up and re-assemble everything... I'll of course be happy if that fixes it but if I've spent all this time and money to find out it needed a $3 repair I might just drive the truck into a river

Thanks for the help!

 

Think of long term fuel trim as learned fuel trim and short term fuel trim as current fuel trim, the goal of long term trim is to keep short term trim at near 0%. Fuel trims are stored in load cells which you can think of in simple terms as a spreadsheet with one axis being throttle position and the other axis being engine RPM and each cell being a storage slot for long term fuel trim.

You add long term and short term trim numbers together to get total fuel trim, so if you have a long term trim of +38% and a short term trim of +19% that's +58% total trim which is massive (fuel trim limit on most vehicles is +/- 50%, normal range is +/- 10%), that would mean the ECM is having to add 58% more fuel than the base fuel table is calling for to keep the air/fuel ratio in the correct range.


We always want to add short and long term numbers together to get our fuel trim.

 

Did u smoke the system?

 

Oh ok thank you Dm93. I had no idea you add LT & ST but with that context the problem doesn't seem like a tiny leak somewhere that the vacuum test didn't pick up. Wouldn't it need to be a pretty significant leak to be that lean?

I'm no expert on this stuff, but if the problem isn't too much air then wouldn't that mean not enough fuel? I already replaced the fuel filter and cleaned the system with seafoam but didn't see any improvements... seems like that would point to the fuel pump, but I'm not sure how that would explain why STFT is 19.5% at idle and around 0% when driving.

Anyway, I looked at the fuel pump procedure and it'd not as intimidating as I thought so I may try it, but I'm wondering if I should replace the idle air control valve and/or the TPS while I have the throttle body out. Any thoughts?

popusas... the shop tested vacuum but I'm not sure if it was a smoke test or some other method (pressure?)

 

It would have to be a pretty substantial leak if it were a vacuum leak (I would say 1/4 in or larger hole) to be +58% fuel trim.

Would need to see total trim while driving but typically a weak fuel pump will be more lean the more load the engine is under (higher fuel demand).

That's very unlikely cause a lean condition.

Where did the injectors you put in come from?

A vacuum leak big enough to cause those kind of fuel trims I would think you would be able to hear.

I'm curious, when this code started happening was there any recent work done prior?

 

OK I finally had some time to work on this and I have some new clues that’ll hopefully make sense to somebody more experienced than me.

First, Dm93… to answer your question I didn’t replace the injectors, I was just saying I cleaned the system with Seafoam (2-3 times recently).

I replaced basically all of the vacuum hoses as well as the idle air control valve. I figured I’d try the IAC because according to my live data the STFT is 0% when I’m driving… it only jumps up to 19.5% at idle. Plus the truck’s idle RPMs are all over the place depending on whether it’s warmed up, if the AC is on, etc.

Anyway, I disconnected the battery and pulled the EFI relay when I replaced the IAC, so now I’m seeing 0% LTFT and 19.5% STFT. I’m guessing LTFT will jump once I drive it a little more and hopefully STFT will then go to around 0%. The truck currently gets around ok, but there’s definitely been a loss of power since changing the IAC and resetting the computer.

There is also a subtle behavior that might be a clue. I'm not sure if this means anything but STFT now stays at 19.5% basically any time I keep the RPMs below 2000, whether driving or idling. It only drops to 0% if I get the engine above 2000 RPMs and only after I let off the gas pedal. Does that tell anyone anything? Any other ideas?


· Vacuum - checked at shop (no leaks), home w/ carb cleaner (no idle revving), & home with a vacuum gauge (normal)
· Vacuum Hoses – replaced most
· Idle Control Valve - replaced
· PCV Valve - replaced
· EGR Valve - replaced
· Valve Cover Gasket – replaced
· Throttle Body – cleaned thoroughly; replaced gasket between it and the intake manifold
· MAF Sensor - replaced
· Upstream O2 sensor - replaced
· Downstream O2 sensor - replaced
· Air Intake Filter - replaced
· Fuel Filter - replaced
· Vacuum Switching Solenoid - replaced
· Spark Plugs - replaced
· Master Cylinder - replaced
· Fuel injectors – DID NOT REPLACE but cleaned w/ seafoam

Thanks,
Jared

 

Since you have pretty much ruled out a vacuum leak I really think we need to check fuel pressure next.

That's normal behavior, the ECM turns off the fuel injectors during a decel and goes into open loop so fuel trims will go to 0 since they aren't used in open loop.

 

@jredv31

do you have any updates on this? Following