Radiator Fan proper function?; PO 128; 2002 Toyota Tacoma Prerunner

My simple question: Should the radiator fan not be rotating when the engine is cool? What is the proper behavior of the radiator fan for my 2002 truck? Does the radiator fan on my truck run all the time and then shift into faster rate when the engine needs cooling or should it stop about 30 seconds after starting and start up only when the engine begins to get too hot?

The reason I ask:
I have gotten a PO 128 code for My 2002 Toyota Tacoma Prerunner 2.7 liter 4 cylinder truck. The code suggests that the engine coolant temperature is below the thermostat's regulating temperature. The thermostat display in the cab for the engine never gets to the normal running temperature and it shows that the engine remains running cool even on a long drive.

One possible malfunction to rule out is that the fan clutch is not working and cools the engine when it does not need cooling.

I have changed a temperature sensor and the problem continues. There is plenty of coolant in the cooling system; that's not the problem. I may replace the engine coolant thermostat (which may be stuck open cooling the engine); this is often the problem for a PO 128. But before doing that I want to rule out the fan. It is running even a few minutes after the truck has started. It appears possible that this may be why the engine is not properly heating up. If so then I think that I will need to replace the radiator fan clutch. Help!

 

The viscous fan? Yeah, they will spin even when not fully engaged, but it's just drag.

 

If in doubt just remove the viscous fan and take a short test drive. But don't sit in traffic.

 

More likely than anything, you have a thermostat stuck open. If the thermostat is closed and allowing things to heat up, the fan clutch can’t possibly overcool the engine to prevent the thermostat from opening. In other words, a fan clutch can’t cause an overcooling problem because the thermostat sets the minimum operating temp.

and yes the fan is always going to spin. When it’s cold like right at startup, it will somewhat roar until the fluid in the clutch disperses and spreads out and thins out. Then the fan will freewheel as much as it can until warm up, but it will still always spin, just with very little load. Then when the air from the radiator gets warm, the fan clutch will start to engage and it will slip a lot less, moving a good bit of air for cooling.

 

When the engine is cold. Reach down a spin the cooling fan. It should spin fairly easy.
There will some resistance, but not a lot.

Then try it on a war engine. It should be hard to spin.

If yours is always hard to spin, it may be the fan.
But that type of failure is not common.

 

T stat my 1st choice

 

Yes the fan spins all the time, the fan clutch regulates how fast it spins.

In any case it wouldn't matter if the fan clutch was fully engaged or not as far as the engine running too cool, the thermostat regulates the flow of coolant through the radiator and thus engine temperature. The fan is there to force air through the radiator when the vehicle is stationary or moving at low speed to keep it cool, it can't cause overcooling.

I will caution that on some Toyota models the P0128 isn't a thermostat code but instead an O2 sensor code however in this case code set criteria states it sets when coolant temp fails to go above 167F after the engine has been running for a length of time. 95% of the time it's a failed thermostat that is either stuck open or opening too early.

 

Yay!...With these beautiful insights I will go ahead and replace the thermostat. Thank you!

I have never drained a cooling system. I guess just use a clean 5 gallon bucket and pour it back in after the fix...correct? The thermostat easy to get to and I have watched some videos on how to do it. I changed the front brake pads and rotors today!

 

I'd put fresh coolant (Toyota Red) in it if it's never been changed or your unsure of it's history, it only holds about 2 gallons so that's 1 gallon of full strength and 1 gallon of distilled water or 2 gallons of 50/50 pre-diluted.

 

I will take that advice. Thank you.

 

There are 2 coolant temperature sensors on your motor, one is for the PCM and the other is for the temp guage in the combination meter.