A/C works but weakens above 90F. Tested, and looking for next steps

[SmoothLC]

Welp, this certainly isn’t helping things:

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Weird that Uncle T puts a filter access door on the 99 evaporator box but then ships that model year with no cabin filters. I don’t think it’s even been opened

I’m gonna vacuum it out and re-test.

Weird yes, but in my case a bonus!

 

[Super77]

What’s the condition of the evaporator fins, are they clean and free from any obstruction?

See my post today. Just cleaned it.

my guess is the fan clutch isnt as strong as it once was, or your condenser and/or evaporator is dirty

Second part proved true.

I may have missed it somewhere, but what are your engine coolant temps during all of this testing?

Is the vehicle just in park while you are testing vent temperatures

How old is your radiator?

Yes, in park during testing, did not measure coolant temps but it's a 1-year old OEM radiator with a recent block flush and the correct coolant. But the vent temps I'm seeing stationary are the same as in motion.

So... re-tested today after cleaning the evaporator and the pressures all were within spec range for the ambient temp: 25-28L / 175-200H / 85 static @1500 RPM. But my lowest recorded vent air temp was ~8 degrees higher (44-45F) than when I tested yesterday under similar conditions but with a dirty evaporator. And, no bubbles in sight glass, which the FSM says is an overfill condition.

Condenser fan is confirmed working and hummed along nicely during this last test.

 

[BonS]

Perhaps one of your blend doors is not fully closing as it should and bleeding hot heater core water into the airstream. Some blend door systems have foam as a final seal and that could have rotted or fallen off. Your evaporator picture showed a lot of straw like material that could have migrated into a blend door seal. (Some versions of the LX470 have a cable driven heater core valve shut-off but the North American version circulates hot water full time through the heater core.)

AI says:

• Servo vs. Foam Failure: This issue is typically caused by one of two things on a vehicle of this age: either the air mix servo motor internal gears have stripped/worn out (preventing it from pulling the door completely shut), or the factory foam insulation tape on the perimeter of the plastic blend door has dry-rotted and blown away, leaving a physical gap for hot air to bypass the door even when the servo closes it completely.

Quick Diagnostic Check
With the vehicle fully warmed up and the A/C running on Max Cold, grab a flashlight and look under the passenger side dash near the center console stack:

• Watch the Servo: Change the temperature on your climate control screen from Max Cold (65°F) to Max Hot (85°F) and back. Watch the plastic linkage arm on the air mix servo.
• Check the Swing: If the servo moves smoothly but stops just short of its mechanical limit, you can sometimes gently push the linkage arm by hand to see if it seats the door further and causes the vent temperatures to instantly drop.