I am looking for fuel trim advise

[NTBarnett]

1999 1500 5.9L 160k miles. My beast has been running fine until a few weeks ago. I started noticing a minor misfire at idle and at speed. This kept getting worse, so I figured the old girl was due a tune up ... cap, rotor, plugs and wires. Now it is even worse. No check engine light. Got out the scanner and along with a P0171, it showed every single cylinder misfiring. Started looking at O2 voltage and short and long trims. In closed loop, upstream O2 is not oscillating and only changes with rpm changes, so I was suspecting that might be the issue. Downstream O2 pretty much echoes the upstream .. so now I'm wondering also a possible cat issue? But I looked and short term trim is maxed out low and long term trim is maxed out high...is that possible?!? Maybe the ECM has lost it's mind? Any help would be wonderful.

 

[Bear_Gibson]

Front O2 won't show any significant movement until the engine is warmed up. Wouldn't hurt to replace the front o2 sensor as part the tune up. Rear o2 is for emissions only

Have you checked the fuel pressure?

P0171 (M) 1/1 Fuel System Lean A lean air/fuel mixture has been indicated by an
abnormally rich correction factor.

The pcm is trying to fatten it up.

Could be a lack of fuel pressure / flow, or dirty injectors, or the o2 sensor is falsely indicating a lean condition. The pcm is trying to fatten it up.

 

[HandyCruiser]

Fuel pressure is what I am thinking too. You can often get a fuel pressure gauge kit as a loaner tool at Autozone or Advance. You can check for blocked cat with a vacuum gauge. After you've made sure the air filter is clean, when you rev the engine, if the gauge drops close to zero and then climbs slowly, you may have a blocked cat converter.

 

[NTBarnett]

Thanks for the replies. I have a fuel pressure tester and just ran out and checked it. 42lbs idle and 50lbs at approx 2500 rpm. I'll have to find my vac gauge tomorrow (haven't used it for years) and see what it shows.

I'm still curious that the short term fuel trim is showing it is trying to lean the fuel out, but the long term trim is trying to make things go rich. I have only ever seen both go the same direction...either both on the rich side, or the lean side.

Thank you again!

 

[NTBarnett]

A neighbor stopped by and suggested pulling a battery terminal for a while to clear out any 'learned' bad habits, (DOH! Why didn't I think of that?!?) so I did that. I started it and let it warm up, and the idle was smooth as could be. After 5 to 8 minutes mainly idling with a few revs to 2k now and then, It ran like a new truck! I looked at the fuel trims and they looked normal. Just slightly pulling to rich on the short term ( 5% or less ) and pretty much 0% on long term trim. I took it out and put a couple of 10 mile trips on it and it ran great until the last 4 or 5 miles. Then it started doing the same thing as before.
I can't say for sure at what exact mile or how many minutes passed when it started acting up, but it ran great for such and such a period of time and slowly I started noticing a slight miss which steadily started getting worse until just before getting home it barely topped the last small hill without having to really nail the gas. Just over about 2500 rpm it cleared out and I noticed a HUGE sudden jump in power...like when you hit the power band on a 2 stroke dirt bike. Got it home and hooked up the OBDII again and sure enough. O2 sensor 1 was not responding and was staying low and the fuel trims were like before, short term pulling full lean and long term pulling full rich.

So now it seems that something in the system is going whacky over time or prolonged heat?

I HATE throwing parts at a problem, but I am stumped. I am going to go ahead and replace the upstream O2 sensor today and see what changes, but I am growing more suspicious about the ECM. Any thoughts?

Thank you for your help.

 

[Fast69Mopar]

A neighbor stopped by and suggested pulling a battery terminal for a while to clear out any 'learned' bad habits, (DOH! Why didn't I think of that?!?) so I did that. I started it and let it warm up, and the idle was smooth as could be. After 5 to 8 minutes mainly idling with a few revs to 2k now and then, It ran like a new truck! I looked at the fuel trims and they looked normal. Just slightly pulling to rich on the short term ( 5% or less ) and pretty much 0% on long term trim. I took it out and put a couple of 10 mile trips on it and it ran great until the last 4 or 5 miles. Then it started doing the same thing as before.
I can't say for sure at what exact mile or how many minutes passed when it started acting up, but it ran great for such and such a period of time and slowly I started noticing a slight miss which steadily started getting worse until just before getting home it barely topped the last small hill without having to really nail the gas. Just over about 2500 rpm it cleared out and I noticed a HUGE sudden jump in power...like when you hit the power band on a 2 stroke dirt bike. Got it home and hooked up the OBDII again and sure enough. O2 sensor 1 was not responding and was staying low and the fuel trims were like before, short term pulling full lean and long term pulling full rich.

So now it seems that something in the system is going whacky over time or prolonged heat?

I HATE throwing parts at a problem, but I am stumped. I am going to go ahead and replace the upstream O2 sensor today and see what changes, but I am growing more suspicious about the ECM. Any thoughts?

Thank you for your help.

STFT and LTFT work together. When you are looking at the adaptive memory in the PCM the STFT can adjust between -33 and +33. The LTFT can adjust the same amount between -33 and +33.

The STFT is the fuel control for what is happening right now. The LTFT is for precise corrections when everything is working properly. If you are experiencing a lean condition from the upstream oxygen sensor your STFT will start adding fuel and it will show as a positive number like +20. The purpose of the LTFT is to correct the +20 so it will show -20 to attempt to get it as close to zero as possible. It works both ways too. If the STFT is negative the LTFT will be positive.

When an upstream oxygen sensor has a problem your fuel trims can look crazy. If your upstream oxygen sensor is not switching lean/rich, low/high then your fuel control will be off. When oxygen sensors age they lose their ability to switch. Once the oxygen sensor stops switching is may get stuck on the lean side, or low voltage, and the PCM will attempt to correct for it.

Before you condemn the PCM you should verify that the PCM can do its job. You can do this with a DVOM. First, disconnect the upstream oxygen sensor and measure the voltages in the body side connector. One pin should have 5.0V and it should show the same thing in your scan tool. Another pin should have B+ battery voltage and that is for the oxygen sensor heater.

Grab the wiring diagram from the Factory Service Manual I'm the 2nd Gen DIY forum and then check for continuity between the oxygen sensor connector to the PCM connector. If you find that to be good then replace the upstream oxygen sensor and retest.

 

[NTBarnett]

Thank you! I'll test the wires before swapping the O2 sensor.

 

[NTBarnett]

Well, I checked the wires as best I could following a youtube video on how to go about it and the wires seemed to check out ok, so I replaced the upstream O2 sensor. The truck runs really good so far, but I still do not have the rich-lean-rich oscillation on the upstream sensor like I am used to. I seems to just float around on a whim. The trims do similar. I have seen the long term trim jump full lean to full rich while the short term stays steady for quite a while...it is very confusing.

I am still suspicious of the ECM/PCM (whichever it is). If the trims are an internal function of the computer, then something whacky is going on in there as there are no longer error codes ( I have driven it enough to successfully complete the onboard tests needed for state inspection) ...which to me means the sensors are all sending values that are in the acceptable range even if they might not be correct and/or the computer might not be sending the right info through the OBDII maybe?

Oh well, I guess I'll have to just drive it till something breaks to the point it is obvious.

If I do find something, I will update this thread. Maybe it will help someone else some day?

Thanks again to all who replied.

 

[Fast69Mopar]

Well, I checked the wires as best I could following a youtube video on how to go about it and the wires seemed to check out ok, so I replaced the upstream O2 sensor. The truck runs really good so far, but I still do not have the rich-lean-rich oscillation on the upstream sensor like I am used to. I seems to just float around on a whim. The trims do similar. I have seen the long term trim jump full lean to full rich while the short term stays steady for quite a while...it is very confusing.

I am still suspicious of the ECM/PCM (whichever it is). If the trims are an internal function of the computer, then something whacky is going on in there as there are no longer error codes ( I have driven it enough to successfully complete the onboard tests needed for state inspection) ...which to me means the sensors are all sending values that are in the acceptable range even if they might not be correct and/or the computer might not be sending the right info through the OBDII maybe?

Oh well, I guess I'll have to just drive it till something breaks to the point it is obvious.

If I do find something, I will update this thread. Maybe it will help someone else some day?

Thanks again to all who replied.

Do you have access to a different scan tool? I have seen plenty of scan tools that do not have a processor that is fast enough or a refresh rate that is too slow to be able to view the oxygen sensor switching.

If you could see the complete adaptive memory table with all of the cells you would be able to get a better picture of what the PCM and the oxygen sensors are doing.

Also, does your scan tool have the ability to reset the adaptive memory? Since you had an upstream oxygen sensor proving bad information to the PCM you can expect the adaptive memory cells to be way off. With each cell being related to a certain RPM and map value the engine can run poorly or feel down on power until the adaptive memory relearns your driving habits.