I'm having trouble with the cooling fans on my girlfriend's 1987 IROC w/ 5.7 and dual fans. Everything appears stock configuration.

The engine had boiling over issues from getting so hot. I put in a heavy duty 180* thermostat and discovered the passenger side fan was not kicking on at all. Replaced the temp switch in the head on the passenger side of engine and now the passenger side fan kicks on at about 150* and the other fan kicks on around 160*. They then run constantly because they can't cool the engine back down to 150-160* with a 180* thermostat. At first I thought it was the new switch, but I ordered a second one and even tested them in pot of water on my stove and the switch closes around 195-200 and re-opens around 185 (its a low temp switch).

I've read posts on here saying the switch is the only thing that controls the on/off of the passenger side fan on a 1987, but that doesn't seem right because I know the switch isn't kicking the fan on that early because it grounds / closes around 195*. I'm assuming the ECM is telling both fans to come on too early now for some reason. I've read that the ECM sensor is mounted in the intake manifold on the front passenger side. I see the sensor there but I can't find any sensor at the auto parts store with that connector. It's a rectangle shaped connection just like the fuel injectors. I took some pictures but can't upload them until I get home later.

What part # is for the sensor mounted in the intake manifold?
Does the ECM control both fans, and if not why would my fans be coming on too early now when they never did before?

 

ACDelco 213-4396

 

It's not that ACDelco part # because the connection is not the same. I thought it was and bought that part from O'Reilly's. Got it home and then realized the difference.
I'm attaching pics of the sensor in the intake and the electrical connector. The connector is very rectangular in shape.

 

Cold start injector temp switch

 

^^^EXACTLY. The sensor beside it is the CTS




Blue is CTS Red is Cold Start Injector Switch

 

And being its disconnected in your picture, if YOU didn't disconnect it, chances are you have the 9th injector delete done to it also. Check for the block off plate on drivers side runners toward the back.

 

Thanks! I had no idea there was a sensor for the cold start injector as well. The cold start injector is still present and installed in the runners.

 

Just some thoughts here, no switch experience on these cars, plenty with my career.

Switch designed to come on and cool water in radiator. Radiator water with 195 thermostat will remain in radiator, possibly longer than 180 thermostat, thus giving fans longer time to cool water in radiator, and giving fans a better chance to actually reach set point and turn off. Is it possible your 180 thermostat is the problem? Additionally, it seems you are saying the switch will "open" at 185. Confirming "open" means it should turn off the fan. So if that is what you are saying, its possible the thermostat is just allowing a constant flow of fluid that is higher than 185 temp, thus, switch never closes. The thermostat is an imperfect mechanical device.

Regardless, it would seem the only problem with fans not turning off is they would wear out faster. Its the thermostats job to keep the water in the engine at the proper temp. (more or less)

You should be able to test this theory by putting a 195 thermostat back in the car. Also, make sure your radiator cap is in good working condition, the system needs to remain under pressure, with no leaks, or it will boil over.

 

This right here is your key clue .

Since the engine was repeatedly overheated , you have "thermally damaged" the ECM's coolant temp sensor and in your case the failed sensor is "telling" the ECM that the engine is warmer than it actually is . If you replace the sensor Bob88GTA showed you , I'll bet your problem is solved .

 

Math.

In 87, only the primary fan was ecm switched. The other was dead until the fan switch turned it on. That is how it is wired.