2003 Pontiac Grand Prix OVERHEATING

Possibly small crevices in the engine that are supposed to funnel coolant could still be clogged with stop leak. Did you replace the thermostat? I did not see this on your list. Coolant temperature sensor...is this original? Have you pressure tested system to make sure it is holding pressure?

Yes replaced therm

Yes replaced thermistat the only thing I can think of any more is cracked water pump or clog in the system sum where

Pressure test and go from there

Are the fans running?

You may be right in that there might be something wrong with the water pump. Impeller not turning?

Was the thermostat installed correctly? Not only the correct way of flow but upside down will not let the system bleed on some models. Does yours have a small hole on the rim? If so this needs to be installed in the 12 o'clock position.

Fan relay or fuse blown.

Check engine light on for coolant temperature sensor circuit that might uncover wiring issue.

Is your leak a visible leak or using so you don't know where it is going?

Correct I'm not sure if the impellers not turning or if there's a crack in the water pump..no check engine light. Well small puddle but no visible leak while cars sitting idle

Yes fans are kicking on a about 1/2 temp

Yes therm installed correctly

That small leak is enough for it to overheat. It is not holding pressure with a leak.

So what's my next steps?

If you are not exactly sure where the leak is, I would pressure test so you can see the leak. (Might not leak until it is under pressure). Then fix it so it holds the right amount of pounds that your system is designed to hold. (Generally 13 - 18 lbs). Your cap will sometimes have this number. If not test at 13 lbs. if you do not see a leak and you are losing pressure, test the cap and if it passes, you have an internal leak. Generally head gasket.

The car won't overheat because the pressure is low, if it did how would it warm up! These cars have a weep hole in the water pump that's supposed to leak if pump is going out. This sounds like your problem, water pump. Brown stuff was probably dexsludge, flushing the system can make plugged holes reappear. With dexsludge you might do 3-4, more frequent than standard maintenance cycles, thorough flushes to fully purge the system. Bleeding the system is first easiest quickest and often effective(has air been allowed into system) thing to do if temperature isn't acting right. Temp is read at sensor only, rest of system might be different if coolant doesn't flow properly. Any more than that and I'll have to reread the question, so I'm done

I’ve seen the Catalytic converter cause an engine to over heater if they are clogged.

@souldefector Yes, any car with a pressurized cooling system (read: all cars that have an engine) can overheat if that pressurized cooling system lacks pressure. At atmospheric pressure, the water in the coolant boils at 226°F. When that happens, you get steam, which loses the majority of its ability to conduct heat away from engine surfaces. This causes overheating. In a typical functioning engine, the coolant will be pressurized to 15-16 psi, which raises the boiling point of the coolant to 267°F. The coolant stays in liquid form at normal operating temperatures which allows it to conduct heat from engine surfaces.