My Car keeps overheating? Why?

My 2002 Honda Civic Keeps Overheating. We put in a new radiator and a new thermostat and it still keeps overheating. Any idea of what it could be?

Maybe it's the water pump.

When it's cold run the car with the Radiator or Reservoir Cap OFF. See if there are any bubbles or wisps of smoke coming out. 5 minutes is enough time for that test. If it passes, the Water Pump is more than likely the reason. The Radiator and Water Pump are the MAIN components of the cooling system. But you have to make sure no combustion gasses are getting in there.

Water pump or head gasket

Water pump for sure… Also when putting in new coolant in an empty radiator you have to start with 70% coolant and 30% water then afterward you can top off with 50/50 coolant later on if needed.

My guess is head gasket leak. You should have had the system checked for leaks instead of wasting money.

It's not the radiator or thermostat. What's left to guess about: radiator fans, water pump, head gasket, radiator cap, heater core, freezeplugs, hoses

Blown head gasket, a very, very, very, very, very common problem on this car.

It is usually the head gasket leaking compression and combustion gas pressure into the cooling system jacket from the head gasket. If you heat up the engine in the driveway by holding 2K rpm until at operating temperature with the pressure cap removed and it seems fine, then you install the pressure cap and it immediately overheats, the head gasket is leaking. There's also a chemical test but the pressure is coming from the engine head gasket in most cases and the cost of repair may be over what the vehicle is worth. My test (same as GEO mentioned) is simple and will tell you quick if you want to spend $2.5K to repair the engine with a rebuilt head and gasket or dump it for scrap.

Even if it was the water pump (not likely), the failure caused it to overheat and warp the head = gasket seal failure. The machine shop dohc head rebuild will be about $800+ plus all the extras like gaskets and labor.

I wouldn't advise using a used head without warranty! You then take the risk that a very pricey bill may X2.
Get a new head or the OK from a macine shop that your head is serviceable and worthy of the $ fix effort.

Anyway you can look at it, the car will not be easily fixed if the head gasket is blown from an overheating.
If you replace the head gasket with the original warped head, it will be a total waste of ($) time and effort.
The pressure cap is usually about 15 PSI, when you introduce an external pressure force = instant boiling.
Hot coolant under pressure, then your engine pumps in hot gas pressure over 200 PSI from combustion.
As the gas leak bubbles accumulate, they overtake the 15 psi pressure cap and it all spews in a boil out.

Head gasket or cracked cylinder head.
Time for a new used car.

You need some testing. A head gasket is the worst case scenario (or that is cracked head or block first) But there are less dire possibilities. Is the water pump belt there?
Have someone take off the radiator cap with engine cool, then start it up and let it idle for about five minutes. By now it is warmed up and you should see the coolant rushing by at a rapid rate. Snap the accelerator ( standing away from the radiator. If it jumps up, there's a restriction somewhere. If normal it should flow on through no matter how fast the engine is run. His sight tells you a lot. If it is not rushing by, the water pump is not pumping or there's a major restriction somewhere. Not in the radiator if it is a new one.
Is the exhaust putting out extreme dense fog? If so, head gasket.
Next test, borrow a pressure tester from a FLAPS and follow the instructions to pressure test the engine cooling system, cold. A cylinder leak will show up here, as well as any leak elsewhere in the system. I had the head off an engine which--although not overheating--had a thick wax clogging up the head passages. This, despite efforts to flush it out.
But tests are free and will guide you to the least expensive repair.