My 2008 Nissan Frontier SE 4x4 V6 4.0L automatic does not overheat, no leaks, idles and runs great. The problem is the little Coolant fan comes on when the truck reaches normal operating temperature and doesn’t shut off until I shut the engine off. I changed relays, ETC, full of coolant, no vacuum leaks. I am going to change the thermostat and housing today. No check engine lights come on. Is there some obscure thing I’m missing? Or any thoughts on this would be greatly appreciated.
I'll share some thoughts which may be useful in figuring this out.
It sounds like you're concerned that there may be an obstruction in the cooling circuit as you're thinking about replacing the thermostat and housing ((suspecting that maybe the thermostat isn't opening all the way and coolant flow is being restricted)).
Maybe.
Since this is a 4X4 the 1st thing you may want to check is if the radiator or condenser fins are clogged with dirt and restricting airflow through the radiator fins.
Secondly, are any of the radiator's internal passages clogged up?
These days that's easy to determine using one of those Infrared Laser Thermometers you can find online or at Harbor Freight for under $25.
Engine running at operating temp (thermostat open) with fan ON, take the temperature of the Upper radiator hose. Then take the temperature of the Lower radiator hose.
The Lower hose should be around 25 degrees cooler.
Of course a worn out water pump is also a possibility. I've changed a few which had their impellers worn down to small nubs. (not much coolant circulation being provided).
So to close out the cooling circuit part, here's the diagrams for your 4.0L Frontier:
On the Cooling fan side of the system you can see the criteria for the fan to run at specific engine coolant temperature ranges, vehicle speeds, refrigerant pressure, and AC ON signal.
Maybe grab a cheap scanner and observe the engine coolant temperature at idle and as you drive the speeds in the chart to see what's going on there. For example ECT temps between 208 degrees and 220 degrees, BELOW 25 mph, the cooling fan will run at low speed. But shouldn't run above 25 mph.
Hope this information is helpful in your troubleshooting.
Well, generally that means you have either a bad relay for the fan motors or are the sensor itself won't turn off once it turns on. Pray it's not in the computer system itself, but really if it runs okay that doesn't hurt anything
