Car overheating

1998 Pontiac Sunfire 2.2L

Purchased this car off craigslist. Has several issues but am trying to resolve them myself. The car overheats but is NOT losing coolant. I have since replaced the water pump, radiator, radiator fan motor, and radiator hoses. Car still overheats. It does not seem to be rushing around in the coolant tank. Could it be blocked somewhere? I was thinking about bypassing the heater core to see if that would solve the problem?

No oil in the water, no water in the oil. Coolant does not get low. Could it be possible the head's blown? Nothing indicates that. Car runs like a dream.

Engine codes... several of them but most have been fixed I think. P0141 which was the oxygen sensor code. Replaced it but light came back back on for it. The others went away though. Replaced cam position sensor, and the crank position sensor and those codes no longer register... just p0141.

Any advice or questions?

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Speed makes a difference for 2 reasons.
If it overheats at stops or under 30-35 mph, but does not heat up at higher speeds, it indicates a lack of airflow through the radiator, i.e. a cooling fan problem.
If it heats up at highway speeds, but runs cooler at lower speeds,(lower rpm) it indicates a problem with coolant flow.

Have you checked the coolant level again to be sure the coolant is still full, and has not gone low due to the thermostat opening?

Another thing to mention; you say you checked the thermostat and it was opening in boiling water. Is it opening fully? I have had to change thermostats that were partially opening, but still causing a restricted flow and causing overheating. If it is not opening all the way, this could be your problem.

If everything checks out good, call a radiator shop and ask if they have a "block tester" that checks for combustion in the cooling system and have that checked....

Is this a "4" motor(8th digit of VIN)?

If I am going though the trouble of pulling out a thermostat, it's getting replaced(use OEM, there have been some problems with aftermarket).....

A plugged heater core can cause overheating(the core acts as the engine by pass)......Drive the car on the highway, and as the coolant temp needle reaches the 11 o'clock position, turn the selector to vent, full heat(windows open)....continue down the road.....should have about 145-150F heat from vents.....as the engine continues to move towards overheating, does the temp out of the vents drop? If it does, there is either an air pocket being formed or the core is plugged(can be verified by touching the inlet and outlet hoses to the core.....SHOULD be the same temp....if the outlet is cooler, try a reverse flush....

Coolant fan should come on at about 228F.....

Absolutely sure you are not losing coolant? Always check reservoir level when cold.....if this has a radiator cap, check radiator level cold too...if it doesn't have one(the pressure cap is on the reservoir), then just check level in reservoir.....