Hi Everyone - Thanks in advance for any help.
Long time lurker, first time poster.

I know overheating is a (usually) easy to diagnose problem as there is a standard checklist to run through...but I am stuck and could use some help before I start ripping things apart.
V6 Stock- 86K miles and driven with respect.

Problem: Overheats after warm-up, after 15-20 minutes of driving or so. Coolant starts to boil out of the overflow.

Hoses are "squishy" with no blockage
Cooling pipes are not pinched
Water Pump has been replaced
Cooling system is burped
Thermostat replaced
Fan kicks on when motor reaches temp
No coolant in oil/spark plugs (a tad of smoke on startup, but I think this is because of the valve seals)
Radiator in good condition
a bit of sludge in the reservoir, but nothing alarming. Could use a proper flush.

Now I am just smart enough to not be stupid... but I don't know what else to check.
Before typing this out - I ran through the list again to mitigate being an idiot (so much for that) and think I have one solution but was hoping for feedback.

When I kick the A/C on while driving, the condenser begins to squeal a slight amount as if it is seizing up AND when idling the motor struggles to stay above 1K rpm's. And I mean Struggles. But we still have cold air.
Could the condenser potentially seizing really be causing that much stress on the motor/not cool properly to cause it to overheat?

I think this is my last diagnosis before ripping the head off and looking for a blown head gasket or cracked head.

Again, thanks in advance.

-burtman

[This message has been edited by burtman (edited 05-06-2021).]

quote Originally posted by burtman: Water Pump has been replaced

Metal or plastic impeller? The plastic ones are reported to get loose on the spindle... and then not rotate.

quote Originally posted by burtman: When I kick the A/C on while driving, the condenser begins to squeal a slight amount as if it is seizing up AND when idling the motor struggles to stay above 1K rpm's. And I mean Struggles. But we still have cold air.

Check for retarded ignition timing; that would cause overheating AND low torque.

quote Originally posted by burtman: No coolant in oil/spark plugs (a tad of smoke on startup, but I think this is because of the valve seals)

Head gaskets can have a failure mode where water does not get into the oil, but cylinder gasses can enter the coolant passages.

This will overpressurize the cooling system and cause it to boil over.

You may not even get smoke, since the high pressure area is the combustion chamber.

That said, have you checked/replaced your radiator cap?

[This message has been edited by fishsticks (edited 05-06-2021).]

quote Originally posted by Patrick: [QUOTE]Originally posted by burtman: Water Pump has been replaced

Metal or plastic impeller? The plastic ones are reported to get loose on the spindle... and then not rotate.[/QUOTE]

Metal Impellers - I saw that being a common problem in the past.

>>>When I kick the A/C on while driving, the condenser begins to squeal <<<

You mean the compressor, right? The condenser is up front next to the radiator and looks similar to a radiator.
Do a compression test on all cylinders. As someone mentioned a blown or partially blown head gasket will pressurize the cooling system and cause overheating. I had this happen on my 67 GTO when it was only a year old. No external leakage whatsoever.

Spoon

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"Kilgore Trout once wrote a short story which was a dialogue between two pieces of yeast. They were discussing the possible purposes of life as they ate sugar and suffocated in their own excrement. Because of their limited intelligence, they never came close to guessing that they were making champagne." - Kurt Vonnegut

quote Originally posted by burtman: Yes, radiator cap has been checked and replaced just in case as well.

Are you absolutely sure you have the correct non-vented rad cap? Have a look Here.