Loss of power under load

My 02 V6 MT recently started losing power under load, like accelerating into highway traffic or trying to maintain speed on uphills. This is new within the past few weeks. P0171 comes and goes.

There is no hesitation or sputtering at startup or idle. Cruising on level ground is no trouble, city driving is no trouble. I just can't keep up with traffic on climbs and such. I do get a little more power when I back off the accelerator, and I find it keeps up a little better if I'm running one gear *higher* (ie lower RPMs) than I normally would.

The truck got a new factory exhaust in 2022, including lower O2 sensor (Denso).
Intake and injectors were cleaned in spring of 2023, new plugs, new PCV, and any questionable vacuum lines were replaced at that time.

When the trouble started I searched for a vacuum leak with the propane trick, finding nothing. I put in a new upper O2 sensor (Denso) and new coils and wires. The original fuel pump had developed an intermittent whine so I dropped in a new pump and strainer. MAF and connectors cleaned. So far there is no improvement.

The air filter shows 5k miles worth of dust and dirt, but nothing particularly worrisome or unusual. The fuel filter is about three years old.

Before I go any further with the parts cannon it's time to step back and get some more opinions.

I don't have a live data scanner but I know where I can borrow one.

Any ideas?

 

The symptoms suggest low fuel pressure or a MAF sensor that's under-reporting airflow.

Did you change the inline fuel filter when you did the pump?

 

That's what I was thinking too. Cleaning the MAF and connectors was the first thing I tried.

I am going to replace the fuel filter this afternoon. The current one is only about three years old, but it's such a cheap and easy thing to try so *why not*.

That said the parts cannon hasn't fixed it yet, so I'm starting to think about what comes next.

 

If the filter doesn't fix it then your gona need a scan tool and possibly check fuel pressure.

MAF VE test would be my first check though.

 

Goona left field this... the clutch isnt slipping is it?

 

It's a fair question, but I don't think so. I'm not seeing the RPMs climb when it happens.

 

OK, a new fuel filter did the trick! I was able to maintain speed and even accelerate along a 3 mile 11.5% grade, which I couldn't have done yesterday.

Lots of rusty crud came out of the old Duralast filter. I was inside the tank yesterday for the pump replacement and it looked fine, so I guess my middle-term to-do list now includes new hardlines.

Thanks, all.