Hey everyone I've been trying to narrow down the reason for this and was hoping you could help.

The car drives fine in town but after 30minutes or so on the highway it slowly overheats to max(the heater helps cool it down). Once I slow down it cools back down again. The fans are on after 5min of driving and on the highway. No strange colors are coming out of the exhaust. After it overheats and I open the hood, the fans are still spinning and the overflow is boiling.

Things I've tried:
+Replaced Thermostat
+Replaced Radiator Cap
+Replaced Fan Relay
+Replaced Dashboard Cluster(was going bad anyways)
+Flushed Radiator(no effect)
+Replaced 50/50 coolant and checked hose clamps
+Toke out air conditioning radiator for better airflow(AC doesn't work)

I mean I could get a new radiator also but flushing it didn't even change anything slightly which leads me to believe it's not the problem. Any help/tips are much appreciated. Thanks

 

Any ideas?

 

What is the condition of the radiator? Are there fins missing? May have to take fans off to see. Or is the radiator covered in oil and such to reduce airflow.

Car only overheats on highway? It doesn't overheat if left parked while running for 30 minutes even when it's 85f out?

Have you ever replaced the waterpump?

 

Yeah I forgot to mention I replaced the waterpump also. I can drive it for 30minute in 85F weather but once I hit the highway for a few minutes it starts to overheat.

No fins missing but there a few bent ones. No oil or gunk on the radiator, it's a bit rusty on the inside though.

 

You pulled the fans off the radiator and looked at the radiator from the rear? Have seen many a dead radiator that looks fine from the front and even through the fans but once the fans are removed.... ugly sight.

Okay, maybe the radiator looks perfect on both sides. The passages in the radiator could be filled with crud. Think of it like a clogged heater core. Still looks pretty but doesn't radiate heat.

Replace the thermostat again, it may be faulty. Make sure it OEM 195*. That happens too.

If you can keep temps down in normal operating temps just by running the heater. It has to be the radiator. If it's OEM it's seen a crapload of tapwater which can contain calcium and rust and got knows what in 16 years.
Never put anything but engine coolant in your car. In a pinch you can buy or make distilled water.

Pull the upper/lower radiator hoses off and see if the insides are swelled.

You can get a radiator for under $50 on amazon... Do a flush-n-fill once a year.

 

A bad battery ground can cause the fans to not operate at full capacity

 

Hey thanks for the help everyone. I gave up trying to flush the radiator and bought a new one. It worked like a charm, no overheating at all. Hope this helps someone.

 

Yeah but at highway speeds there is airflow. City driving, it does't overheat and there is far less airflow.

Friend had a Dodge Caravan with the same symptoms. It was the radiator. Looked great from the front but the rear where the fan was... it was corroded and clear why the radiator was shot.
It only overheated on the highway but was fine in the city. Lower RPM's, less demand on the engine.
He had an automatic transmission which added to the heat the radiator had to dissipate. Transmissions produce more heat at 60-70 MPH don't they? Compared to stop and go driving. That's a real question btw.