P0174 Learn How to Read Fuel Trims

I have a 2006 Tundra 4.0 2WD with 287K miles. I have code P0174 Bank 2 Lean. I also notice a hesitating "gallop" under hard acceleration, but this does not trigger a code. When I reset the code, it comes back in 1-2 weeks. No other codes. I plan to sell this truck after I am done with my job, and I don't want to throw the parts cannon at it (e.g. MAF sensor, fuel pump, injectors, upstream o2 sensors). I also want to learn how to read fuel trims for diagnosis, and for evaluating my next truck purchase. Here are some fuel trim numbers:

In park at idle
SHRTFT1 0.8
LONGFT1 16.4
SHRTFT2 1.6
LONGFT2 14.8

In park at 2500 RPM
SHRTFT1 (-2.3)
LONGFT1 24.2
SHRTFT2 1.6
LONGFT2 20.3

In drive under acceleration (peak numbers)
SHRTFT1 17.2
LONGFT1 29.7
SHRTFT2 10.9
LONGFT2 29.7

Are these data samples useful? I have a hundred questions, but will let those with answers lead the discussion. I could start by testing the fuel pressure and cleaning the MAF, but I really want to learn how to read fuel trims as well.

 

It adds fuel to compensate for unwanted air. Could be a vacuum leak, could be plain old H2OS sensors. They are good for about 100-150k miles, depending on brand/model.

 

I'd add, with the 1st gens, failure to use Denso's part with O2 sensors specifically can lead to weird code anomalies, especially with upstream. We've seen it all too often.

 

Yes. A rule of thumb is to use the same engine sensors brands as the brand of the ECU. I.e. Bosch/Bosch, Denso/Denso, Siemens/Siemens.

 

OK thank you. Is there a way to tell from the fuel trims that the upstream bank 2 sensor could be faulty? Is there any other way to diagnose the upstream bank 2 sensor without just replacing and hoping it fixes it? Also, any general comments on the fuel trims, how they compare to a healthy engine, etc would be great!

 

Yes. If the part numbers match when you look up your vehicle using the part finder at www.densoautoparts.com then you can swap the upstream sensors between banks 1 & 2 and see if the lean code migrates with the sensor.

 

FWIW, you didn't say what package your truck was but when I plugged in the base and SR5 model over there with your other specs, it shows the upstreams are identical. Swap bank1/bank2 upstream sensor and see if the code sticks or changes.

Whatever you do, DO NOT buy anything but Denso sensors, and DO NOT buy them on scAmazon, fleaBay, or other websites where the inventory isn't validated. The market is flooded with knockoffs, and those are two big chains helping.

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Sounds good thank you for the info!

 

Also can be caused from getting less fuel than anticipated. i.e. reduced flow/bad spray pattern caused from dirty injectors.

 

The only way to test injectors is to bench test right? Or at least swap injectors to the other bank? Is the fuel trim data indicative of the issue being isolated to bank 2 per the engine code? My concerns is maybe both banks are lean, but bank 2 just a tad more enough to trigger the code.

 

95% it is oxygen sensors, if they are still factory ones. You have compensation in both banks.

 

Is RockAuto a safe source for Denso?

 

Yes, many here have bought from them.

 

A healthy engine has STFT oscillating slightly around 1 (0.98 to 1.02, 98 to 102%, -0.02 to 0.02, air-fuel ratios depending on how the sensor reading is presented)) in its perpetual look out for the best AFR. If ECU detects STFT leaning away from 1.0 it adjusts LTFT, so STFT would return back to 1. LTFT has multipliers for low, mid and high loads. This is why it jumps instantly when you stomp on the pedal. LTFT in a healthy engine is as close to 0% as possible.

Yours LTFT readings are probably in AFR, I think? Zero deviation would be 14.7.

 

Those fuel trims can be a few things. But the numbers you have there align with potentially contaminated fuel. Not sure if you noticed issues right after a fill up. Just putting that piece of info in there since we deal with it ALOT

 

OK, thank you. I am going to replace both upstream (AFR) sensors with Denso. When I remove the old ones, I am going to look for signs of contamination. If heavy carbon buildup, I am going to be worried about bad injectors, and going to at least clean them and maybe go as far as bench test them myself. To remove the old AFR sensors, I am going to use an O2 sensor socket (crowfoot) and maybe a MAP torch or PB blaster or both? Would it be better to remove when engine hot or cold? The truck has 287K miles and I can't risk ruining the threads. Any advice is appreciated.

 

Bench testing is really the only good way to test fuel injectors. You want to see the spray pattern and also "flow" the injector to make sure it is delivering the amount of fuel that the ECU expects it to. I remember seeing a post recently (not sure if here or on another group/forum) of someone with a 4.7 that sent out their fuel injectors to get flowed and cleaned. And half of them had a bad pattern and were way under delivering on flow.

If you have a scanner that will give you freeze frame data for the DTC then post that up here. It will give the the conditions of the engine at the time the DTC was triggered. So you should be able to see the fuel trims, TPS, MAF, RPM, etc, etc.

Note this code triggers when one of the fuel trims hits +/-35%.

Here's an abbreviated version of what the service manual says to test/check for this DTC:
Note: after each step it says if the check passes then move on to the next step, if it fails then fix/replace that component and clear the codes and see if it comes back.

1. Check for any other DTCs
2. Check PCV connection and piping for leaks.
3. Check air intake system for leaks.
4. Perform active test (A/F control) <- you probably can't do this unless you have a high dollar scanner. Basically it will vary injection volumes from -12.5% to +25% and you can watch all the sensors and how they react. This can then point you to if the O2 sensors are the culprit or if it's another issue.
5. Check coolant temp value. Cold (i.e. first start of the day) you want to see coolant temps within a few degrees of ambient temp. Hot should be between 167F and 203F.
6. Check MAF values. While idling: X g/s. At 2500rpm: X g/s. (Edited to remove incorrect info) (Note: A/C: off and Shifter in Neutral)
7. Check fuel pressure.
8. Check exhaust system for leaks.
9. Check for spark/ignition.
10. Check injector injection. 304 - 364 cc/min is the spec.
11. Check O2 sensor heater resistance.
12. Check O2 sensor relay.
13. Check harness for open/short.
14. Replace O2 sensor.

Obviously there are some things that are hard/impossible to do as a DIY mechanic. Personally I would perform as many of these checks/tests as I felt comfortable doing and if all those pass then replace the O2 sensors. If it comes back after that then I would check pull the fuel injectors and get them tested and cleaned.

 

Thank you for the info on fuel trims. How do I verify if my LTFT readings are measured as a % of AFR? I am using an Autel ML619 and don't see mention of the unit of measure in the owners manual.

Also, I don't think the issue has to do with fuel quality because it the code has reappeared after several fill ups.

 

Most of the fuel trim values I've seen report in percent with 0% being the perfect or no fuel added/subtracted number. Positive numbers means it is adding fuel (running lean) and negative numbers mean it is removing fuel (running rich).

 

Thank you. That MAF benchmark sticks out to me, I think mine was reading different. Will check that and the other items that I can before proceeding. Great info!! Where did you pull that service manual? I am going to need to get one at some point

 

One more question, I looked through the misfire counter on the scan tool, and one of the cylinders had one detected misfire. Is that anything significant, and could it be related even though no other engine codes? I am not sure how the misfire counter even works.

 

I personally wouldn't worry about it unless it was happening regularly and threw a visible check engine light. However, I probably would examine the coil and plug to make sure they look healthy (i.e. no fouled plugs or carbon streaks or cracks in coil housing).

 

Agreed, I wouldn't worry about it unless it was setting a code. P0300 to P0306 is the range of DTCs related to misfiring.

As far as how it detects misfires...the ecu monitors the crankshaft and camshaft position sensors for speed fluctuations. When a fluctuation hits a certain threshold it will trigger a misfire count.

It will trigger a DTC if the misfire rate is above 2%.

 

OK, sounds good about the misfire counter. Here is my MAF reading. 0.01 lb/s at idle, 0.04 lb/s at 2500 RPM. I think that comes out to 4.5 grams/sec and 18.1 grams/sec respectively. This seems very far from the advised range. Do you agree, and if so, does that indicate bad MAF sensor?

However, if my MAF is reading higher than normal, I thought that would create a rich condition, whereas I have a lean condition.

 

That seems way off, which makes me wonder if the numbers I got are wrong. A dirty or bad MAF that results in a lean condition (aka large positive fuel trims) typically under reports the MAF value. So I would be expecting to see a smaller number than what the spec states, not a larger one.

lb/s is also too big of a unit for mass air flow, typically. Are you able to change the units on your scan tool to grams/sec?

Edit: Also want to verify this was at operating temp, A/C off, and in neutral?

 

Removing them cold could be laborious. You may start cold and use a torch on the nut, so it would expand.

These sensors have service life. 100-150k miles, depending on the model. You can go ahead and replace them routinely every 150k miles, otherwise engine power, responsiveness and MPG will all suffer, long before you get any codes.

 

If this were MAF, it should be impacting both banks I would think.

I'd get those upstream O2s swapped. Seriously. See what happens. If no change, then start digging into the deeper stuff. I think you'll find that's the issue, the upstream O2.

 

Both banks have their fuel trims off. But I think ECU would lit a CEL for MAF flow below/above what it should be for the known engine load.

But true, MAF has a limited service life as well. Another thing to check/replace.

 

I am considering buying the two upstream AFR sensors. If I was keeping the truck long term, I would for sure. However, I still want to figure out the MAF sensor. I changed my scanner to metric and received the following (engine hot, neutral, no ac):

idle - 3.85 g/s
2500 rpm - 15.64 g/s

Can someone confirm what the benchmark is, because this seems really off. Seems like the MAF might be the issue.

also I have checked exhaust for leaks with shop vac and soapy water in past and seemed fine, I also checked for vacuum leaks with brake clean and nothing stands out.

 

Seems to be very low. But need to compare against a similar engine to be sure. My 2.5l inline-5 has 14.5 g/s at idle (800 RPMs).