Since my rebuild I've had a baffling cooling issue. I have a 78 engine with electric fan. The fan is controlled with a thermo switch in the coolant coming from block, 205* on, 185* off. This always worked great in the past. Since the rebuild, however, the temp, sitting in the driveway, never drops below 190* on the gauge, so the fan does not shut off. That is with getting major air flow thru radiator. One side of radiator is hot, and the other cool to touch. The temp guage, the type with the probe and metal line, is in the block. I can't understand why the temp stays at 190* when sitting.

 

The block should be whatever temperature the thermostat is. Put your thermo switch elsewhere...I'm guessing you happened to move from a 180* thermostat to a 190* during the rebuild.

From what you describe everything is functioning as expected.

 

If one area of the radiator is cold its likely blocked up with small dirt or rust something and blocking flow.

Maybe try one of those temp guns or an infrared camera to see if the rad is getting hot across the entire surface.

My 300 has a thick aluminum radiator but it only take up about 2/3 of the normal space horizontally. Even with a factory clutch fan sitting 6-8 inches away without a shroud I have no problem keeping it at 198-200* with a 195 thermostat unless it is stupid hot outside. Was the electric fan recently installed? What's the temp of the thermostat? The temp in the thermostat is when it starts to open, if you have a 185 thermostat you won't see lower than 190 I'd bet.

I'm not familiar with electric fans but I have my temp guage running off the plug under the #6 intake (far away from the rad). When the temp first climbs after startup it will reach like 210 then suddenly drop 30* or more in seconds so I have a feeling the temperature reading at the back is much higher than at the rad.

 

That temp drop tells you when the thermostat opens and the block gets cool or cooler fluid from the radiator. It reaches 210* before the first opening because that when the block or thermostat housing has warmed up to the trigger temp. I have my temp gauge sending unit in the same spot as you. My fan activation thermo switch is in the return hose to the radiator, so it is affected by the temp of the coolant in the block. I run a 198* thermostat. My radiator is new, the same aluminum cross flow as I had on previous engine. So I doubt there is a blockage. I can't understand why with the other engine, same fan, same thermostat, same thermo switch to activate the fan, the fan does not cool engine to the temp it used to on the other engine. The only difference is I added Water Wetter to the coolant.

Could there be trapped air in the block, so the sensor is reading the temp of the block itself, and not the coolant???? That has to be the case. If I am right, then I should be able to remove the sending unit and add fluid through a tube. If I'm wrong, I'll find coolant when I remove the sending unit.

The hertz: You are correct: an engine should run the temp of the thermostat. With a traditional pulley driven fan that is true. However, with an e-fan you get a temp swing, depending on your thermo switch activation and shut-off temp. Activation is usually set about 10* higher than the thermostat so you can drive down the road and not have the fan kick in while the wind through the radiator is cooling the engine better than any fan. Then, when you stop, the coolant temp starts to climb and the fan kicks in. Thanks.

 

If the thermostat doesn't start opening until 198* how will the coolant ever get down to 185 so the fan shuts off? With the current settings would it not kick on at 205 then endlessly run since the coolant can't get down below 198? My temp guage while driving usually sits within a 5-10* range and fluctuates constantly

I think if you ran a 180 thermostat or set the fan to cut out at a higher temp you would see the fan cycle. I just don't see how the coolant will get down to 185 with a 198 thermostat.

In terms of an air bubble maybe its possible but removing the sending unit would just drain all the coolant above it in the head.

I would still investigate if part of the rad is running cold. Maybe some debris or rust came loose from the block and is stopping flow somewhere.

 

That would seem logical. However, with an efan you essentially have two systems operating independently. When the thermo. opens hot water hits the fan switch and it starts cooling the water in the radiator. At the same time cooler water enters the block and the thermostat closes. But the fan continues to run and the temp of the coolant in the radiator drops below the temp of the coolant in the block. By the time thermostat opens again, the radiator temp has dropped enough to close the thermostat and shut down the fan. Then the cycle begins again.

 

I think you have it all set up right except for the fan cut out. While not the same setup, my temp changes directions every few seconds and only moves a few degrees. I have to look very closely to see it move, as if the thermostat is somewhat steadily open.

Even with the fans temp reading coming from the rad, the rad shouldn't be able to drop ~20* in mere seconds. If the rad did drop below 185 when the thermostat opens I think the temp gauge would tank as the cold water hit the probe.

Is the fan control adjustable and easy to experiment with?

Hopefully someone else that knows more about electric fans can chime in and help solve it.

 

Thanks for all the help, Jason. I think you may be right about the radiator being partially clogged. That would explain the relatively slow or less cooling ability. It was just having that issue with two radiators made me doubtful.

With efans it is not uncommon to drop the temp 20* in a minute or so. The flow of air would be the equivalent of driving down the street at 40 mph. That much air is flowing across the tubes.

 

I'm not quite following you on this.

You said the sensor for the switch is in the return from the block to the radiator, correct? This is always going to see nearly the hottest temps in the system in this area, and it's not really going to get cooled like the fluid that made it to all the way to the radiator.

Where exactly is your temp sensor for the gauge? From what you've said, unless I missed it, you are never actually taking a temperature of the fluid *leaving* the radiator. Is that correct?

 

Before the thermo shuts, some of the cooler coolant from the radiator escapes past the thermostat. The fan sensor senses it and shuts off the fan. I wonder ... had the bottom hose sitting in garage for four months. I wonder if a critter got up in there.

 

I installed my O2 sensor in the new exhaust finally, and was able to dial in the mix and the timing. I had forgot that the mix changes when I put the air cleaner back on, and again when the air cleaner is on and then I close the hood. With it running very well, idling just over 700 rpm, I began trouble shooting the fan. When I did the rebuild I changed from a 198* thermostat to a 195*. I don't understand how/why a 3* change would cause my fans to go amok, but it did. Rather than pull drain the fluid again and change the thermostat, I simply switched my trigger wire from the low side of the thermal switch to the high side. That makes my fans turn on when the block reaches 207*, and the fan turns off about a minute later when the temp drops to 193*. That is fine with me.