Another P0171 thread...

I’m chasing a P0171 CEL. Yes, I know there are a lot of threads about just that. I’ve read through all that I can find, and I think based on my circumstances I can reasonably eliminate some of the potential causes folks most typically list. Here’s the situation. I did a valve cover gasket replacement, which many of you know involves the disconnection/removal of a lot of items related to CAI, vacuum, electrical, etc. I finished the job and had no issues for around 40-60 miles and then the P0171 hit. I checked over all of the work from the gasket job and didn’t find any glaring issues or misses. I checked vacuum and it was right at the ‘upper’ end of spec. in the 19”-20” range. Since the code popped up shortly after the valve cover gasket work I think it has to be related to that since it would have to be quite a coincidence for it to be related to the fuel pump or injectors. And for the record, I replace the fuel filter every 30k miles.

I continued to drive the truck and after about a week the CEL mysteriously went off and around the same time my truck started idling noticeably higher than normal. Not knowing what to try next I ordered a new mass air sensor. Before the sensor came in, the high idle went away, and the CEL came back on. I swapped out the mass air sensor anyway and that didn’t fix the code. I don’t want to get to the point of just throwing parts at it, so I figured I would check here. Since the high idle went away, I do not notice any changes or issues with performance or fuel economy. What else can I look for or test?

 

A very important bit of info for the P0171 are your short- and long-term fuel trims. Can you give us the values at say:

• Idle
• 10mph
• 30mph

What are you seeing for those? (I'm assuming here that you have an OBDII reader and can get these, since you diagnosed the original error code). If you don't have one, then something like this one (cheap) (more robust) and the Torque app for you phone can read the values.

 

Seems to have quite a bit of variation . . . or at least that's what my copilot (wife) told me when taking the photos of the display...

Readings are short term/long term:
Idle, cold = 0.0/38.3
10mph = -4.7/38.3, 0.0/19.5, 0.0/19.5
30mph = -3.1/14.1, -0.8/12.5, -1.6/13.3, -0.8/16.4
Idle, warm = 3.9/38.3

An interesting note, since I've mentioned sporadic-ness during this diagnosing process. When I started the truck the CEL had gone away and the idle speed was very low (750RPM). I turned off the truck and started it again and the idle was back to normal at around 1k RPM and still not CEL. After taking the truck for a drive to get the fuel trim readings at speeds, the CEL came back on while parked to get the idle/warm reading. It's all very strange...

Any thoughts based on this data? I appreciate the help.

 

That definitely confirms the code, fuel trims should stay between -10 and 10... Over 10 means you're running lean which a vacuum leak would cause. Those are some high lean trims if I'm reading it right.

You can use starting fluid or a smoke machine to try and narrow down where the leak is.

 

All of my short-term trims were between 0.0 and -4.7. The long term trims ranged from 12.5 to 38.3. So you're saying both ST and LT fuel trims should stay between -10 and 10 at all times? If that's the case, and out-of-whack trims being a result of a vacuum leak, could I actually have a vacuum leak significant enough to cause the code, and still be getting over 19" of vacuum? Not questioning you, just trying to understand what I'm up against here. Thanks.

 

It would have to be one Hell of a vacuum leak to cause trims that high. The other possibility is a fuel problem. That trim indicates it’s adding an enormous amount of fuel from base to attempt to get complete, clean combustion. Either it’s a huge vacuum leak or it’s getting insufficient fuel and attempting to compensate. Think injector or injector elec connection. Filter, fpr and fuel pressure are possibilities also. Put a long screwdriver or steth on each injector to listen for the tick. Proper trims, as mentioned should oscillate no more than 10% from zero , you won’t trip the CEL till 20 or so. Good luck

 

I appreciate all the help. I got a stethoscope rod on the injectors for 1,2,3, and 5, but 4 and 6 are buried under the intake manifold, so I can neither confirm electrical connection or no functionality. That will take some disassembly, but even in that case I can only confirm the plug is secure; is there another way to check an injector? Kind of a catch 22 since I can't check them without taking parts off, and I can't check them with those parts removed...

I'm certain the fuel filter is good as that was replaced within the past 10k miles. Sorry for all the questions, but what's the proper way to get a good fuel pressure reading? I've never done that before. Thanks.

 

if there isn’t some enormous vacuum leak and you have double and triple checked everything after the MAF, I would rent a fuel pressure test kit from local autozone and test the fuel pressure. YouTube videos aplenty. If that checks out, I would suspect an injector problem (maybe just a crappy connection) it’s not getting enough fuel or you have unmetered air. *DISCLAIMER, it took me four years to figure mine out and it’s a 2.7. Go look at some Timmy videos on YouTube and give it hell

 

Total trim (Short term + Long term) should normally stay +/- 10%

Being it gets better at higher speeds I suspect vacuum leak.
Get you a can of carb clean and spray around the intake area while watching your short term trim, it will go negative when you hit the leak.
Another way to find a vacuum leak is with water as shown in the videos below.

https://www.youtube.com/watch?v=0rQn9YBMZIg
https://youtu.be/K-on2hbYUCc?t=1057

 

The one other thing you might check is the AFR sensor (which many also call the forward O2 sensor). It could be that it is sending incorrect air/fuel readings back into the system, making the system think that it's running lean, when in fact it is not. When I had the P0171, I replaced it.

Chasing the P0171 Engine Light – ADVENTURETACO

Note that the part I used was a little different than you'll use due to MT/AT, and the one you want (if you're going to replace it) is the Denso Air-Fuel Ratio (O2) Sensor for AT 5VZFE (234-9002)

 

with your scan gauge installed watch your short term fuel trims as you slowly open the throttle. If the STFT gradually changes to 0 as you open the throttle there is a vacuum leak on the motor side of the throttle plate if they start to go the other way the problem is on the atmosphere side of the throttle plate, either false air after the MAF or the MAF. At idle, read the MAF's g/sec of air... a rule of thumb is 1 g/sec of air for each liter of displacement, so your 3.4 should show air in the 3'ish g/sec. If it is way off that is an indicator.

If there are no problems with the above then your problem is on the fuel delivery side. It all starts with fuel pressure. Fuel pumps usually make a little noise before they go but then there are those that just die. The next thing to check is the fuel pressure regulator. Pull the vacuum hose off and see if it has fuel in it, if it does the regulator is bad. I have had only one instance where the regulator was so bad, non Tacoma, that there was such low pressure in the fuel rail the engine would barley run and it too had a P0171. It had great dead head pressure but with the pressure regulator wide open it was dumping all the fuel back into the tank.

As the fuel injectors on Tacomas age the internal resistance increases and they get less and less effective so they go lean. Similar issues with the front O2 sensor.

You problem could be one or more of what I have described but with some common sense troubleshooting you will figure it out. There are no silver bullet answers in auto diagnostics anymore.

 

I have twice been able to clear P0171 codes by cleaning the PCV valve. However, I didn't have idling problems associated with it.

 

I’m surprised nobody has mentioned the possibility of an exhaust leak at the manifold or downpipe drawing in extra o2 and making the engine think its lean. I definitely had that issue, in addition to my mass airflow sensor being lazy. It was accurate at idle but at higher rpm it was under-reporting airflow. Replacing the pcv valve is also an important and often overlooked maintenance item.

 

Given the proximity to the valve cover gasket job- I’d suggest checking the three vacuum lines connected to the IAC sensor. One of them is easily accessible, but the other two are only accessible once the main wire harness is “out of the way”.



I’ve done my best to highlight them in green.

I had replaced all of my vacuum hoses prior to my valve cover gaskets, and the two lower ones under the loom were the only ones I couldn’t get to easily.

I don’t think they were leaking, but the ends of the passenger side were dry and cracked. So I feel better about changing them out, and they’ll be good for another 20 years.

You might be able to get to them without taking the intake manifold off. Possibly, just the TB and then anything holding the loom in place.

 

It actually is, people just think it’s a crummy vent but it’s designed to open and close according to vacuum conditions and God knows these friggen things are fussy about proper vacuum

 

I convinced myself that my truck was supposed to run crappy for years. You should try it.

 

I appreciate all of the help/suggestions guys. I didn't have much time to dig further in, but was able to this weekend...

@Rachelsdaddy I have not tried a fuel pressure test yet, but it's on the list.

@Dm93 I went through bottle after bottle of water trying to use that method to identify a vac leak with no success. I just can't get myself to go spraying carb cleaner around, but if get desperate I may give it a try.

@turbodb I am fairly certain it's not an exhaust sensor issue. Both the A/F and O2 have been replaced within the past 5k miles.

@zach141b and @Rastopher I replaced the PCV valve and gasket during the valve cover gasket replacement job (and have every 30k miles since buying the truck new), and @Rastopher I don't think a leak at exhaust manifold or downpipe would be the issues since the A/F and O2 sensors are not suspect.

@Andy01DblCabTacoma that is certainly something I'll need to dig into the next time I have the opportunity.

@Glamisman at idle my STFT starts around -3.1 at idle, then drops to around -7.8 at 2k rpm, then hits 0.0 at 3k rpm. So, it gets farther from zero just as I start opening the throttle, but zeroes out as it gets higher. Your thoughts? As you noted, mine is a 3.4L engine and my g/sec at idle stays in the 5.1-5.3 range.

For all, I would like to note that if it were not for the CEL I would not have any indicators of an issue. There were some odd happenings, but as of right now it idles normally, runs great, and my fuel economy has not changed. The only sign is that dreaded light. I have reset it many times and it always comes back on, so it's not a fluke bad reading.

 

The only other mystery vacuum leak possibility is the power steering air bypass valve. If it’s acting up air would be bypassing the throttle plate. Mine was screwed up in 2016. Dealer item…. You can cap them off without noticing a difference, especially w/ a 6cyl. Just increases idle when turning wheel while stopped