Stymied - 96 corolla 1.8L overheat

[bgdalovitt]

my 96 corolla 1.8L 7A-FE (155K miles) continues to oveheat. I am stymied and looking for any insight.
I replaced the water pump, radiator, 2 thermostats, 2 radiator caps.
Installed a new headgasket because of 'oily' look in the radiator fluid. No evidence of water in the oil. Also replaced the valve stem seals.
The car runs WODERFULLY with much more engine power.
For the overheating.....
I can leave the car idling 30 minutes without overheat.
I can drive over 10 miles @ 75 mph before the gauge rises above the center.
If I kept driving, all of the water steams out of the overflow tube.
I pressure tested the cooling system (radiator, hoses and block) to 18 PSI and the gauge holds study.
I removed the radiator hoses and thermostat and flushed the block with a garden hose - no apparent blockage.
Also flushed the radiator with the garden hose and the radiator flow seemed fine.

OK, I am about stuck - Looking for suggestions. It's personal now to get it fixed.
Thanks in advance

[PatrickGSR94]

What about the heater core? Did you flush that out? Did you have the temp lever set on full hot when draining, flushing, and refilling? If not you could have a massive air gap in your cooling system since the temp slider must be on hot to allow coolant to flow into and out of the heater core when servicing the cooling system. Air expands and can make coolant levels rise and boil over.

Also are you using a 50/50 mixture of coolant and distilled water, or a pre-mixed coolant? You should never use anything else and you should never use water out of the garden hose in the mixture as tap water has all kinds of chemicals that can corrode the system.

[bgdalovitt]

Thanks so much for the reply.

Did I flush the heater core? - Yes
Used the garden hose. Yet, I agree that distilled water would be bette.

However, I have good news - I found the problem -
I was sold the incorrect aftermarket radiator cap! The cap fit the radiator and tightened down snuggly at the top of the radiator neck.
The problem was that the seal was not making contact against the radiator opening. Got the third (and correct) cap drove 20 miles at Arizona highway speeds (75 + margin of course) engine temp sensor stayed centered.
It was a tough road but, hopefully, it is complete