P0171 and Fuel Trim Help

[twosmoker]

Having some issues diagnosing my P0171 code. I haven't done any recent work before getting the code. I ran tech stream and my Fuel Trims are all over the place. Short FT seem to average from -5 to 5% but do spike up every now and then to +/- mid teens. Long FT are really high, averaging in the 20's but as high as mid 30's. Not seeing any consistency between the banks but both spike up on down and similarly. Car runs smooth, no signs of vacuum leaks, plugs look good and didn't detect any signs of leaks from spraying carb cleaner around the manifold and intake areas. Also replaced all the gaskets and fuel injector seals around 20k ago when doing the starter.

I plan on replacing the fuel pump this week since I'm nearing 300k but other than that I really don't want to throw parts at it. Any suggestions on things I should be checking/testing? Any suggestions on a fuel pump, the Denso model seems to be sold out from all the vendors I trust.

Here are some screenshots of my tech stream on a drive around the block. Also, unsure why I'm not throwing P0172?

 

[jerryb]

o2 sensor replacement and diagnosing - Google Search

 

[twosmoker]

If the 02 sensor went bad would I get these numbers from both banks? The car is old enough I guess both LT could be out. Any way to test them?

 

[white_lx]

If the 02 sensor went bad would I get these numbers from both banks? The car is old enough I guess both LT could be out. Any way to test them?

In Techstream under 'active test' you can add or decrease the injector volume by x%. That should force the B1S1 and B2S2 in lean and rich conditions.

The MAF signal on one of the photos is pretty high at 1916 rpm. But Techstream also reports a high load. Was this indeed a high load? Or is that calculated? Does MAF respond 'linearly' with RPM under no load? What is the MAF reading at idle? And are you seeing high trim values then as well? If they disappear under no load/idle, then it could point to the fuel pump.

 

[twosmoker]

In Techstream under 'active test' you can add or decrease the injector volume by x%. That should force the B1S1 and B2S2 in lean and rich conditions.

The MAF signal on one of the photos is pretty high at 1916 rpm. But Techstream also reports a high load. Was this indeed a high load? Or is that calculated? Does MAF respond 'linearly' with RPM under no load? What is the MAF reading at idle? And are you seeing high trim values then as well? If they disappear under no load/idle, then it could point to the fuel pump.

Thanks for the help! Excuse my ignorance but I'm still learning tech stream and appropriate values. I'm not sure I understand what I would be looking for by adjusting my injector volume? Am I trying to increase fuel to reduce my Fuel Trim value?

For the MAF, I was recording a drive so I assume it was high load? MAF is linearly through rpm at idle and not seeing numbers over 20 unless moving, idle is around 5.75 to 6.5

Long FT are high at idle, around 20% but spike up further under load. Short FT seem ok at idle.

 

[white_lx]

Thanks for the help! Excuse my ignorance but I'm still learning tech stream and appropriate values. I'm not sure I understand what I would be looking for by adjusting my injector volume? Am I trying to increase fuel to reduce my Fuel Trim value?

Your question was how to test the oxygen sensors. That is what the active test is for. The active test allows for increasing / decreasing injector volume by 10%. If the oxygen sensors are functional, it should create a rich (stuck at 0.9V) / lean (stuck at 0.1V) condition respectively. If that indeed happens, then you know that your oxygen sensors are functional. If they don't behave as expected then they are suspect. After completing this test, you can be sure your B1S1 and B2S1 are not the problem (or they are the problem and the fix is easy). B1S2 and B2S2 sensor issues would not affect the fuel trim by these extremes and are only used to monitor catalytic converter efficiency (but after ~10 s they should follow the S1 sensors).

For the MAF, I was recording a drive so I assume it was high load? MAF is linearly through rpm at idle and not seeing numbers over 20 unless moving, idle is around 5.75 to 6.5

Long FT are high at idle, around 20% but spike up further under load. Short FT seem ok at idle.

Those MAF numbers look good. The fact that problem is worse at higher rpm's / load certainly looks like a fuel delivery issue: dirty injectors, plugged fuel filter, failing fuel pump etc. You mentioned you are replacing the fuel pump. But if you don't want to fire the parts cannon, the first step would be fuel pressure check.

 

[2000WA100]

Any resolution to this? I'm tackling the same issue.

So far - P0171 code only with no noticeable performance issues. Idle around 700 RPM. Voltage on both banks O2 sensors track- s1 oscillates and s2 is steady low. LTFT runs high, in the 30s at idle. STFT seems to be normal +/-5.

I pulled and sprayed MAF sensor with MAF cleaner. 30 minutes with battery disconnected. I have maybe 50 miles since. Disconnecting MAF wire harness while running shuts the truck down.

Tomorrow I'm going to take techstream on the highway and see what she is doing under different conditions.

2000 LX, 485k

I have not cleaned throttle body. I have not checked spark plugs. Depending on results tomorrow, I'll check those next.