4.0 SOHC way down on power

Hi,

I've had a 2003 FX4 LII since last September, and it's always seemed quite sluggish compared to my father's 2011 base model 4.0. This truck has 4.10 gears, an automatic trans, and 32-inch tires.

Today, I bought a 6x12 cargo trailer and lugged it home over some moderate hills on the highway. The trailer weighs 1350 pounds empty, and my Ranger was more or less working flat out just to pull a small, light trailer. I also used a quarter tank of gas running about 60 kilometers (40 miles) at highway speed.

So, it's time to figure out what's going on.

Here's what I've done so far:

- Tested the TPS (correct voltages)
- Tested the MAF (correct voltages)
- Had a look at a couple plugs, which look brand new (the wires were replaced right before I bought it as well)
- Checked the air filter, which was a little dusty but still fine
- Checked the air inlet hose from the MAF to the throttle body, which is fine
- Checked the throttle body, which appears to have been cleaned quite recently and is spotless

This truck has relatively high mileage (301,000 kms... 187,000 miles), but doesn't use any oil, has no rich mixture soot in the tailpipe, and has no evidence of blowby under the oil filler cap. It starts fine and idles fine... no staggering or obvious missing.

It occasionally throws a P0128 "Coolant Temp Below Thermostat Regulating Temp" code when it's being worked hard off-road. I erase the code, and it stays gone for months at a time. The temp gauge (if I can believe that thing) is normal, even on days like today where I had to flog the poor ol' girl just to get the trailer back to town.

I intend to borrow a fuel pressure gauge from a buddy tomorrow to look into that, and I'll probably swap the fuel filter as well regardless of what the gauge says.

The truck feels like it's running rich. It has a loagy, lazy feel during acceleration. But, aside from the unreasonable fuel mileage (even for this thirsty engine), I don't have any evidence that it's actually loading up.

I'm thinking maybe the cats are clogged. I fought with the collector to front cat bolts tonight, hoping that I could separate them enough to go for a noisy test drive to check for any differences, but I couldn't get the top flange bolt to move. I've got penetrating oil on it for tomorrow as well. The exhaust is in decent shape, but looks like it might be original (or at least certainly nowhere near new).

I have tightened up the throttle cable, so it's not the problem.

Any thoughts? Thanks in advance for your help.

You can pull the upstream O2 sensors out to relieve exhaust pressure, usually easier than separating exhaust pipes

Or a vacuum gauge can ID plugged exhaust.

Spec for changing O2 sensors is 80-100k, they wear out like spark plugs, just takes longer.

The running cold code could be where the issues is, I would get a Live Scanner and see what temp the engine and air are reading when engine is warmed up, computer bases it's air/fuel mix on these temps, along with O2 feedback.
Colder temp causes computer to add extra fuel.