High Negative Fuel Trims - 2000 Ford Ranger

I have a 2000 3.0L FFV that is having high negative long term fuel trims at idle, and I'm not sure what to troubleshoot next. The trims stabilize around -8 LTFT/+2 STFT when driving, but will spike to -15 after pulling off the freeway and idling.

I bought the truck 5 years ago and discovered it was misfiring badly due to low compression from bad valve seats (common 3.0 issue). A rebuilt set of heads fixed the issue and the truck ran fine. Last year, the heater hose coupling failed and I lost all coolant and warped the heads. I resurfaced the heads and truck has been running fine once again. Recently it started running a little rough at idle and I'm trying to track down the root cause.

I've performed the following troubleshooting items:

New O2 sensors - both upstream and downstream. Originals were very slow to update and were biased rich.
New idle air control valve
Fuel pressure test from fuel rail - 59 psi KOER, 50 psi KOEO. Pressure would hold with key on but would immediately leak down after turning key off. Fuel trims/O2 showed very rich on both banks, and replacement injectors fixed leakdown.
New injectors - old ones failed leakdown, cleaning didn't fix issue.
Smoke test on intake - smoke leaked from EGR valve, replaced EGR. No other leaks found.
New Intake air temp sensor - original was stuck hot
New coil pack, plugs, and motorcraft wires - original plugs were toast and old coil pack had cracks in it
Replacement MAF from Autozone - original looked slightly burnt and BARO value was slow to update

The only other issue I can think of is maybe plugged exhaust. The misfire and leaking injectors make me suspect the catalytic converters are plugged, but the truck accelerates to redline fine.


Driving at cruising speed -




After exiting freeway and sitting at idle:





WOT in 4th gear-

The first question that came to my mind when I saw FFV, is what does the Fuel Composition/Alcohol Content pid say?

The second thing I would look for is a leaking Fuel Pressure regulator. Remove the vacuum hose from the regulator and make sure it is dry. If you it is wet with fuel, it is leaking.

Fuel pressure test from fuel rail - 59 psi KOER, 50 psi KOEO.

Fuel pressure should be higher KOEO. It should be about 10 psi lower KOER. Engine vacuum allows more fuel to pass through the regulator and return to the tank, lowering fuel rail pressure. When vacuum is low, fuel pressure is high.

A user on another forum suggested the Flex Fuel sensor as well, and the value is spiked. I'll try to look for a replacement unit from the junk yard tomorrow.

As for fuel pressure - I've found this on the internet:
1998~2000 Ranger 3.0L Flex-fuel ONLY:
55 PSI (+/- 8PSI) constant
Regulated in tank
Mechanical Returnless system

My fuel pressure gauge is a cheap one from amazon so I'm not convinced of it's accuracy.