Unexplained Low LTFT on my Mazda cx7 2.3 turbocharged

Motor Vehicle Maintenance & Repair Asked by MAK on November 1, 2020

So, it started when i got CEL with code p0172: system too rich bank1. I took the car to the mazda dealer, they said you need to replace MAF sensor. I agreed and got the new sensor installed.

Now CEL is off but I still have low ltft readings specially at idle (-14%) which goes up to (-8%) when im on highway and to 0% when im driving above 80 mph.
Im feeling like the numbers are getting lower every day coz when I first checked ltft after MAF replacement it shows minimum (-7%) then -11 and now -14. Im sure the code will come again in couple days or a week. The same happened before and I refunded the MAF when same code pops up again. I need help because the dealer is not willing to check low ltft unless there is a code.. and when they clear the code it will take time to reach -16 or -18 again. So they are doing bad diagnosis and clearing code without solving the problem.

Stft is low also. Ranging +3 to -10. The ltft numbers are not getting positive at all.
I noticed that when the boost is positive the numbers improves or go to zero, and when vaccum it will go negative. Sometimes I smell gasoline around the car but not strong.

MAF reading is 3.5 g/s at idle and goes up to 200 maximum when wot at high speeds.

Also, I notice that the power is not as before. When accelerating I feel like small hesitation in rpm around 3 to 4 thousands and acceleration is not very robust. It feels like choked sometimes specially when its driven for long periods.

The car starts normally in the morning and warms up smoothly and have no rough idling.

I did some research but did not find something useful for turbocharged engines.

I just replaced turbo. I cleaned the oil control valve (sometimes called vvt) and I replaced sparkplugs.
I have no modification at all. Everything is stock and OEM parts only.

Btw, I have fuel rail pressure 1660 psi and when I checked the workshop manual it says its within range of high pressure fuel pump (Fuel_pres pid shall be approx 11.5 Mpa.)

All air intake system is checked for leakage several times and no leak detected.

O2 sensor voltage is within range 0.1 to 0.8 and is responding to throttle changes. I think that o2 is working properly because its giving rich readings and I smell gasoline.

When I turn on the cold Air condition while idle, the ltft readings go to -11% and MAF reading go up to 5.5 g/s

My question: how to diagnose if leaking injector or bad cat or pinched return fuel line or blocked exhaust pipe?? I dont want to get this same code again because when it pops up, the car will drive slow and not responding to throttling. Also, I dont want to leave it like this and maybe get bigger problems in future.

Any help or ideas are welcomed.


Update: yep, it happened. Today I noted that numbers are getting lower and reaches -16 ltft and -10 stft.. sure I got CEL on as expected… so the MAF has nothing to do with this issue.

I captured the freeze frame and code log data:

Freeze frame report:

Vehicle VIN: JM7ER193XA020****

Vehicle Manufacturer: Unknown

Vehicle Calibration ID: L3CCEA000L

Freeze frame information:

Fuel Status = 0 byte

Engine Load = 26.275 %

Engine Coolant Temperature = 99 °C

Fuel Trim Bank 1 Short Term = -6.25 %

Fuel Trim Bank 1 Long Term = -16.406 %

Intake Manifold Pressure = 4.641 psi

Engine RPM = 694 rpm

Speed (OBD) = 0 km/h

Timing Advance = 11 °

Intake Air Temperature = 46 °C

Mass Air Flow Rate = 3.04 g/s

Throttle Position(Manifold) = 12.157 %

Run time since engine start = 3,107 s

Fuel Rail Pressure = 427.861 psi

EGR Commanded = 0 %

Distance travelled since codes cleared = 1,956 km

Barometric pressure (from vehicle) = 14.359 psi

Voltage (Control Module) = 13.196 V

Engine Load(Absolute) = 20 %

Commanded Equivalence Ratio(lambda) = 1

Relative Throttle Position = 3.529 %

Absolute Throttle Position B = 12.157 %

Accelerator PedalPosition D = 31.373 %

Accelerator PedalPosition E = 20.392 %

End of report.

Fault log report:

Vehicle VIN: JM7ER193XA020****

Vehicle Manufacturer: Unknown

Vehicle Calibration ID: Not present

Current Fault Log

P0172: System too Rich (Bank 1)

Pending Fault Log

ECU reports no pending faults

Historic Fault Log

ECU reports no historic faults
Other discovered fault codes

(possibly pending, current or manufacturer specific)

ECU reports no other fault codes logged

End of report.

3 Answers

Thanks for the FFD, that helps.

Based on the data provided, your MAF sensor reading is too high. That is based upon your rpm, throttle opening, engine displacement and number of cylinders (yellow tab), in those condition your MAF should be 1.94 g/s.

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Provided the MAF has been replaced, I would check the wiring integrity especially by the MAF connector. Allow the engine to idle, make sure its in close loop, ECT above 45 degrees. Check if MAF voltage PID is aprox. 0.72V and stable. Shake and twist the harness connector, gently pull the wires one by one while watching the graph scale on your scan tool. Good result is stable voltage with no intermittent voltage drop.

There has been some connector issues especially on the CX5-KE.

Another thing that throws P0172 is high oil dilution, this is common on all Skyactive-G engines. This based on your driving habits, shorter trips and or shorter period of time will result in the engine running in a cool condition for a long period of time. Fuel will adhere to the cylinder walls, scraped back into the crankcase, mixed with engine oil resulting in the oil dilution. Simply check the oil on your dipstick and make sure its not above the F mark.

If your O2 is reading between 0.1 - 0.8 V that's fain, although proper measurement would be to raise the car to 2000 rpm and count the oscillation, you should have 8-10 oscillations within 10s time frame in that rpm. This is usually done with scope as it's much faster that ordinary scan tool.

Now to answer your question to check for leaking injectors is a little difficult without a scope, you would be looking for a spikey signal at the bottom of the waveforms while measuring the signal on your first 02.

A quick check of your cat efficiency can be done while monitoring your first 02 vs rear 02, rear 02 should be a steady signal of aprox. 0.6V under all load conditions. If your graph scale on your rear 02 copies your first 02, your cat is not doing any job and need to be replaced. This usually happens if your engine run in excessively rich conditions for long period of time. Pinched return line will result in abnormally high fuel pressure, blocked exhaust can be again check with a scope and pressure transducer or cheaper option with a vacuum gauge either connected to the first 02 (max 2 psi at 3000 rpm) or the inlet (different values applies). Lot of YouTube videos on that topic.

Answered by Adam Cimler on November 1, 2020

P0172: I strongly recommend you to check connector of MAF and MAP sensors. specially MAF sensor due to its location near Engine Air Filter, so every time you clean or replace that filter they disconnect then reconnect the MAF, usually technicians tend to pull it by wire (which can damage or loosen the connection).

Your description and readings seems normal and since this is an intermittent issue, I can tell you its due to loose connection.

Try to gently wiggle MAF wires inside connector while monitoring STFT readings, If you notice sudden change -- BINGO -- so you need to replace / repair that bad connector.

no need to change MAF if its working, specially that you already tried that !!!

Now, For the performance issues, check your VVT mechanisms and parts or filters, specially that little one inside the bolt...

good luck

Answered by Blinkey on November 1, 2020

I have the same problem, if you put compressed air can upside down on the throttle body sensor in the front it jumps fuel trims all around. When popping the hood the hot air comes out from the ac. then the throttle body sensor starts cooling off again taking the fuel trims down and further down. Try this!

Answered by RedMustang1987 on November 1, 2020

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