I have a 98 toyota tacoma with the 2.7L 4 Cyl motor. I bought this truck about a month ago, with the knowledge of two check engine lights. One was P0420, and the other was P0171. The P0420 was due to no catalytic converter, and I put one on as soon as I got the truck.
The P0171 is the one that baffles me. I have gotten a few freeze frames that set the code, they are as follows:
0x02 Freeze frame DTC P0171 -
0x03 Fuel system status Closed-loop, using oxygen sensor feedback to determine fuel mix -
0x04 Calculated load value 12.16 %
0x05 Engine coolant temperature 185 F
0x06 Short term fuel % trim - Bank 1 0.78 %
0x07 Long term fuel % trim - Bank 1 37.5 %
0x0C Engine RPM 694.75 RPM
0x0D Vehicle speed 0 MPH
0x0F Intake air temperature 46.4 F
next one is:
0x02 Freeze frame DTC P0171 -
0x03 Fuel system status Closed-loop, using oxygen sensor feedback to determine fuel mix -
0x04 Calculated load value 12.55 %
0x05 Engine coolant temperature 181.4 F
0x06 Short term fuel % trim - Bank 1 -0.78 %
0x07 Long term fuel % trim - Bank 1 35.94 %
0x0C Engine RPM 694.25 RPM
0x0D Vehicle speed 0 MPH
0x0F Intake air temperature 50 F
I have found no vacuum leaks (none that are apparent), and the mass air flow sensor has been cleaned, with no improvements. It has a stock air cleaner. Both the oxygen sensors (bank one sensor one, and back 1 sensor 2) flat line at an idle, or when using compression to slow down. Under driving, these sensors oscillate as they are suppose to.
This is all I know, perhaps you have a test I can try to pinpoint the problem.
One more freeze frame I forgot to include:
PID Description Value Units
0x02 Freeze frame DTC P0171 -
0x03 Fuel system status Closed-loop, using oxygen sensor feedback to determine fuel mix -
0x04 Calculated load value 12.94 %
0x05 Engine coolant temperature 174.2 F
0x06 Short term fuel % trim - Bank 1 19.53 %
0x07 Long term fuel % trim - Bank 1 23.44 %
0x0C Engine RPM 943.75 RPM
0x0D Vehicle speed 0 MPH
0x0F Intake air temperature 57.2 F
P0171 = System too lean, bank one…Clear the codes and see it they will stay cleared…if not, the prime suspect is the front oxygen sensor, new or not…But since this has been going on a long time, who knows??
Lesson learned. Don’t buy vehicles with the CEL on…
I have cleared the code several times, hence the different freeze frames. The front oxygen sensor is behaving as well as the rear oxygen sensor. plus the two would need to be fully functional to throw the P0420 code (catalytic converter below performance spec(due to no converter)). At the time, I figured the P0171 code had to do with no catalytic converter, seeing how so many people were talking about the O2 sensor.
I think it’s an air leak, somewhere between the MAF and the throttle butterfly valve.
While checking for vacuum leaks did you pay special attention to the intake manifold?
They can just be really hard to find. Perhaps its on to a smoke test.
However, you should also not focus entirely on air. At least check the fuel pressure - leanness is too much air - or not enough fuel.
You might want to check on those fuel injectors after running some Sea-Foam through them. Listen to them with a stethoscope and get a test light or whatever tool the manual says to test them with.
I guess I will try a smoke test, or the old starting fluid trick (spray it at various sections, and if the engine speeds up or dies, there is a leak). I will also try the injector cleaner. I forgot to mention this, but i am getting 21-23 MPG.
The front oxygen sensor is generating the signal that is throwing the code, be it correct or incorrect…It might be telling the truth, it might be lying…
“Both the oxygen sensors (bank one sensor one, and back 1 sensor 2) flat line at an idle”
Are idle speed and smoothness normal?
Your long term fuel trim percentage is really high.
Can you completely clear out the history on the ECU, and start from scratch?
BC.
Idle seems a little rough, but nothing worrisome. When I clear the computer with the engine running, it definitely gets a little rougher, but that should be expected. I will try and upload a picture of the O2 sensors graph, mass air flow sensor graph, RPM’s and any other graph that may be relevant. Due to real time monitoring, I can see all sensor readouts with my laptop computer via a ODBII to USB interface.
(ElmScan 5 Compact USB Scan Tool $40 dollars on amazon, worth every penny).
“Idle seems a little rough”
That is to be expected if it’s running lean. I think the O2 sensor is telling the truth.
Have another look at the plumbing between the MAF sensor and the throttle body.
A little air leaking in there will affect idle, when the total flow is small.
Part one of sensor data
Part two. All the sensor readouts that I can provide with the tools at hand. This hopefully sheds some light on the issue at hand. This weekend, I will look into tracking down a vacuum leak.
…
Sometimes a picture’s worth a thousand words, but
“Both the oxygen sensors (bank one sensor one, and back 1 sensor 2) flat line at an idle”
is still the strongest clue here.
Is it possible to graph O2 and TP together?
Too bad it doesn’t work with a Mac.
So I narrowed it down to the Mass Air Flow Senesor. I replaced it with a used one from the wrecking yard for 75$, that looks rather dirty, and threw it in. The engin started idling a lot better, and the light has not come on for 40 miles so far. I plan on cleaning it once I find the best way. I do not have pictures right now, but the long term fuel trim is ranging between -10% and + 15% as aposed to 30-40% like before. The oxygen sensor data is cycling back and forth between its phases at idel, in stead of flat lining.
The best way to clean a MAF sensor is with MAF sensor cleaner. Parts stores sell this in an aerosol form.
Tester