Cooling System Problem

[GST with PSI]

I have a few issues occurring. First, my car is overheating. Second, my cooling fan switch (sensor) is not activating my cooling fan. Here's the strange part, the entire cooling fan circuit is GOOD, including the sensor itself. I know that the circuit is good for a few reasons. The first being that I bought a brand new sensor. The second is that if I remove the cooling fan sensor switch from the bottom of the radiator and manually heat it to its switching temperature, the fan kicks on and activates as it should. Additionally, if I remove the sensor from the harness, and jump the 2 terminals that plug into the cooling fan switch (thereby bypassing the cooling fan switch all together) the fan also runs as it should. Let me be clear, the fan is not coming on because the coolant temp switch at the bottom of the radiator is not seeing hot coolant. The cooling fan circuit and fan itself are perfectly fine and in proper working order. Even if I manually activate the cooling fan (and A/C fan also simultaneously) the car still overheats at idle.

I believe this is all related to my radiator not properly flowing coolant for some reason. From all this, I can only come to one conclusion; That the cooling fan sensor is not seeing hot coolant because somehow hot coolant is not making it to the bottom of the radiator??? I'm going to flush the radiator tonight and make sure there are no blockages, but aside from that, I don't have a clue what could be causing this. I know the coolant is getting hot up top (at the thermostat housing and top of the radiator), because my temp gauge gets hot enough to go out of the operating temp range, and the top of the radiator is too hot to touch. This being the case, I can't understand how the temperature switch is not seeing this at the bottom of the radiator? Could this be a coolant flow problem? And if so, whats a good way to see if my water pump is generating good flow? I just rebuilt the entire motor (new water pump as well). The cooling passages in the motor and head were clear at that time, and the radiator (aftermarket aluminum) was completely emptied and refilled when I dropped the motor in. Any help or suggestions would be greatly appreciated.

Cooling fan sensor and related components:

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Circuit diagram:

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[GST with PSI]

I have an update and more problems: I flushed the radiator and motor today. I ran water through the radiator forwards and backwards through both the upper and lower hose inlets until the water ran clear. The water ran good and didn't seem to be obstructed in any way. With the radiator hoses removed from the radiator, I did the same thing to the motor, running pressurized water through the thermostat housing and the backwards through the lower water pipe until the water ran clear. I then hooked up both radiator hoses and ran water into the radiator until the system was full. There's no chance there were any air bubbles left in the system.

I closed the system up and started the car. Just idling, the car's temp started to creep past the middle of the temp gauge. As the temp approached the hot mark on the temp gauge, the fan never came on. The car got hot, but fan switch on the bottom of the radiator never triggered the fan relay. I touched the radiator at the top and it was super hot. I then touched the bottom and felt a noticeable temperature difference. For some reason, hot coolant is NOT getting to the bottom of my radiator. At this point, I manually turned on both fans (cooling and A/C fan), in an attempt to get the temp back down using the fans. Even with both fans running, the temp will not come down. The only way I can get the temp back down is to stick the water hose in the top of the radiator where the cap goes and flood the system with cold water. If I have the cold water running into the radiator, the temp drops comfortably past the half way point on the temperature gauge in the car. WTF is wrong here? Is my radiator just not flowing coolant?? The thermostat is completely removed, and nothing but the radiator itself could really be impeding flow.

Additionally, I'm open to opinions on my fan setup. I have a Ford Tarus fan setup in a pusher configuration. I'd like to be able to know if it will cool well, but until I figure out why the hell my radiator won't let hot coolant get to the bottom I'll never know. And just to reiterate, the cooling fan NEVER comes on. The fan switch on the bottom of the radiator never gets hot enough to reach its switching point, and therefore will not trip the fan relay. I know the fan circuit and sensor and both good. I verified this by heating the sensor manually to it's switching point, and it kicked the fan on without issue. I also have a new sensor installed.

I can only come to the conclusion my radiator is somehow impeding flow to the point that the car is unable to cool itself down...Anyone got any other ideas WTF could be going on here?

Pics of my current fan setup:

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[DSMnoobsause]

What's the condition of your water pump and belt?

 

[GST with PSI]

As stated above, the water pump is brand new (gates pump), and was replaced when I rebuilt the motor. The motor has zero miles as of right now, only idle time. The water pump belt is also brand new, tensioned properly, squeal and slip free.

 

[luv2rallye]

All I can think of is something is impeding the water flow
somewhere in the system. Did you perhaps use a gasket somewhere (like wrong head gasket or water pump gasket) that covered up any water ports? Or put thermostat in backwards or it's stuck closed?

 

[mvan0131]

I just had this problem at work. the thermostat was holding open or closed to long causing the car to overheat a little (barley over the half way point on the gauge) and never kick the fans on. Now this was on a toyota but i hope it helps.

 

[thomcasey]

plug the bottom inlet for the radiator, then fill the radiator with water. Once full, pour it into a container and measure how much liquid you have. How does it compare to what the MFG states it should hold? Your radiator may flow fine, but may have area's of blocked passages that will not heat up enough to kick the sensor.

 

[andyb2637]

Have you replaced your HG by any chance? I only ask because Im having the exact problem! The top hose gets really hot and has a lot of pressure but no flow to the bottom of the rad like it should(but there's pressure WTF).
It's like a dam mystery, I have come to the conclusion is probably the head that I didn't get resurfaced when a new hg.. Big Mistake on my part . Could be your answer aswel.