What can the fuel-trim below tell you about the P0171 & P0174 codes (lean banks) on the Heritage V6 4.2?
I am not sure where to start - does this looks like a typical vacuum issue?

 

Your long term fuel trims are 28% on both Banks.... Greater than 25% throws a code.
Your engine is attempting to add more fuel based on oxygen sensor feedback, and has exceeded the limit of what would be normally able to be compensated for..by extending the injector duration.

So you are running lean more or less or right on the edge of where the computer can't compensate anymore.

No idea what RPM this is is this idle or 2000 RPM or what

You could have clogged fuel filter, malfunctioning fuel pressure regulator, weak fuel pump, massive air leak on your intake bypassing the MAF sensor... But it's most likely something that affects both Banks equally. It's unlikely it would be clogged injectors for that reason.

Another clue is does the leak rate ,ie fuel trims, get significantly better at higher speed than idle... If it's an air leak it probably will. If it's a fuel tegulator pressure problem... It may or may not. If it's a fuel pump problem or a fuel filter...it definitely won't it may get worse at higher speed than idle.

Now that said I'm going to guess you revved the engine in the middle there and that caused the fuel trims to drop to 3% almost normal..... And not the other way around..... So you have an air leak most likely, definitely much worse at idle. Starting by looking at your PCV tube and emission hoses would be smart. If you don't find anything there check rest of your Air supply and intake gaskets downstream of the MAF.

 

I have a '99 4.2l and experienced a number of codes, including PO171 and PO174 (I also had P1131, PO133, and P1132). That was back in 2017. Two repairs remedied all codes: 1) replaced the infamous elbow in the pcv system situated under the MAF (had a hole in it, new one seen below), 2) replaced both upstream 02 sensors. I may have just been lucky and I appreciate that your investigation is much more detailed and mbb's advice is professional; still I was able to fix my dilemma for about seventy bucks. Maybe a classic case of throwing parts at the wall and they happened to stick but I thought it was worth letting you know. In any event, best of luck with it.

 

Same with my 99 4.2, but mine had holes in both of the pcv lines, the elbow below the MAF and the vacuum hose located on the passenger side valve cover, in the back near the firewall.

 

It’s quite likely that typical “you need isolator bolts and an intake manifold gasket” issue.

Also, change IMRC bushings while the upper intake is off.

 

Thank you all for your advice; it is greatly appreciated!

• replaced the isolator bolts + rubbers this time around. I applied a little bit of blue locktight as some of the bolts were slightly loose
• swapped out the EGR and PCV valves, as they appeared to be in less than optimal condition
• the IMRC bushings seemed intact with no signs of deterioration or damage to the tabs, so I left them as is

I'm pleased to report that I've resolved the -28% lean situation on both banks. Below are the current fuel trim readings for a warm engine, post-repairs.

1. Question: Bank 1 is running 1.56 lean, whereas bank 2 is running 7.03 rich. Can anyone shed light on the typical values and potential causes for the large difference?
2. Question: Despite the repairs, the engine still exhibits slight "shutters" at idle, occurring roughly every 10 seconds or so. Any insights into what might be causing this? Could it be related to timing?
3. Question: The fuel trims provided were measured at idle, where the engine's idle appears somewhat unsteady on the data below. Could this instability be linked to the rich condition? Or is a fluctuating idle common for the 4.2 V6 (2004)?