2002 MAZDA MPV P0171, P0174, P0151

After watching the excellent videos on fuel trim analysis I was set to find the cause of my codes. But the charts look normal to the best of my understanding.
In the fuel trim charts up to the marker at 372 seconds the engine was idle. The spike in LTFT both banks immediately after is when i briefly went WOT. Then the rest of the chart is at ~2000RPM and just before marker 484 back to idle.
On the O2 sensors voltage chart engine is idle up to second 74. Then a snap of the pedal and hold at ~2000RPM.
The freeze frame is always at RPM > 1500, with both LTFT values at 15% and different STFT values, lowest I saw 10%, highest 21% all positive of course.
Fuel pressure is good and vacuum leaks not obvious, though I haven't done a smoke test. MAF appears to read reasonable values, 0.4V @idle, 3.8V @WOT.

Thanks

Hey adrian! Do you recall what the coolant temp was when these captures were taken? Mostly wondering if there's an intake leak that's getting better with engine temperature. While we're talking about vacuum leaks, these engines are known for leaking rubber PCV elbows. They like to collapse and suck air, but don't always show on a smoke test, and may not be audible.

The MAF going to 3.8V at WOT is a bit low... Would it be possible to watch the upstream O2 sensor and trims while doing a WOT run in first and second gears? That'll tell us a ton about the fuel system and the MAF.

Hi Tyler, thanks for responding!
The engine was warm but had not been driven when I took those traces.
As far as the PCV hose and elbow they were replaced about two years ago and still look pretty healthy; at the time when they failed the symptom was a very rough idle, and now the car idles nicely. I'm not saying there's no vacuum leak, just relating past information.
I got a trace of the O2 levels and MAF while flooring the accelerator. About a second after it went in third gear I eased off the gas, see below.

Hey Adrian the o2s look good which suggest fuel system is good I see were only getting 3.5v on the maf which is a little low can you view grams on the maf signal at idle with a warm engine? It appears that the maf sensor needs a good cleaning. What engine is in this? does it have intake runners?

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Thanks for the capture! I'm with Ben, the MAF is definitely low. Cleaning is a great suggestion, follow by resetting the KAM and doing another WOT run.

hey Tyler and Ben, thanks both for answering! I am out of town for Christmas, prior to leaving I did check the MAF against the spec given in the shop manual for my MPV. They give two checkpoints, one at idle which was marginally within spec and one at 2500 RPM (no load) and that one was out of spec about - 15%. Funny thing is I did clean the mAF with isopropyl spray before taking all of these readings and graphs. Perhaps i need a replacement sensor or a better cleaning.

Happy 2018!
After cleaning with "CRC MAF sensor cleaner" the readings are within the specs from the shop manual at 2500 RPM.
I don't know how to reset the KAM, I am using a generic OBD adapter in conjunction with the FORScan software on a laptop. The DTCs have not disappeared after cleaning the MAF.
Below is the plot of the MAF, front O2 sensors, LTFT and STFT in order from top down, the WOT event is at marker 182 on the timeline.

Ah dang, that's kinda disappointing. I mean, I'm happy for the improvement! But there's clearly more to do. Just to be clear, the DTCs still come back after clearing them, correct?

As far as clearing the KAM, you can always go for the classic battery disconnect and have a cup 'o coffee. That'll reset the KAM, trims, and the idle strategy, too. So, don't be surprised if the idle is uncertain on first startup. I know the mobile version of FORScan won't reset KAM, not certain if the PC version will or not.

Are you able to back probe the MAF and perform a full intake manifold snap throttle test?

ScannerDanner has several videos demonstrating this test. I have found this test to be very telling.

Could the MAF values be substituted?

Hey guys, thanks for your help, you were right on the money about cleaning the MAF!
I apologize for not making it clear that I hadn't cleared the lean codes prior to my Monday post. After clearing the memory by disconnecting the battery -which also took care of the DTCs-- they never came back. Although I only drove 80 miles since, they would have been back, I was getting them as soon as the third drive cycle, and there are not even any pending ones.
Thanks again for the education and help.

No worries! Where did the total trims turn out after the KAM reset? Not worried, just curious. I figure you would have gotten codes back after 80 miles, so the PCM HAS to be happy at this point.

Sorry for the late reply, I took a trace right after the KAM reset on my first drive being curios to see how the trims looked; indeed they both started at 0 and began adjusting as I drove the car. Also the MAF peak reading is 3.87V, a bit higher than my previous one.
I don't know if it's my imagination but I even think there is more power to the engine than before, especially when I'm on an uphill and I hold the pedal observing how much speed it drops.
You're so awesome to tele-diagnose a car like this!

Thanks for the update. The long terms are still higher than I'd like, but at least the codes are gone! I see that the MAF max voltage definitely improved, but it still seems a bit low. I don't have any captures from a Mazda MPV, but looking at other 3.0L Duratec engines from around the same year all show the MAF going over 4.0V on WOT.

Maybe I'm just nitpicking? :silly: I can't say it's enough justification to replace the MAF, especially since the codes are now gone. Either that, or there's a genuine breathing issue, and I tend to doubt that since you note that the engine seems to have plenty of power.

Hi there again! Happy 2022!
After 4 years since the [virtual] repair you did on my car I have the Engine light again --just before the emissions inspection is due!
This time I only have the bank 2 system too lean code P0174.
I checked the MAF, low readings again, so I decided it was time for a new one. Now MAF readings are good but the P0174 code came back after a few days of driving with no codes.
Please let me know what your thoughts are and whether any readings and charts would help.

thank you!

adrian.c wrote:

Please let me know what your thoughts are and whether any readings and charts would help.

I think it's really rude of the MIL to pop up right before inspection. :lol: Could have had the common courtesy to wait until AFTER you got a sticker!

If you're willing, another test drive with scan data going would be a good starting point. Engine speed, IAT, ECT, MAF g/s and volts, all four O2's and Loop status. Freeze frame for the P0174 if it's still available.

Welcome back, by the way! Always fun to get follow up like this.