Question about engine fans

[TJeeper]

This is about my TJ, although I would think it would apply to any vehicle with a mechanical engine fan.

I have been having a problem getting the TJ up to normal operating temperature, 210 or so. Lately it has been running between 150 and 200, and the engine temperature drops the slower I drive. I don't think it is the thermastat because the heater starts to blow out warm air just a few minutes after I start it, and gets hot within 5 minutes or so.

I was wiping the snow of my Jeep last night, the outside temperature was between 25 and 30 degrees. I noticed the engine fan was on, even though I just started it. Does the fan run 100% of the time? I assumed it would be like an electrical fan, and only turn on when needed. Isn't there some sort of clutch and switch to turn it on when the engine heats up?

Thanks-

Patrick

[Daless2]

Hi Patrick,

You might want to consider replacing the thermostat. I say this becuase on the TJ engines, both the 2.5L and 4.0 L the heater core is fed a steady diet of engine coolant as soon as the engine starts. This is a completely independent of the thermostat or the in-cab temperature setting. This is why you are getting heat rather quickly. It is designed to do that. In the past most engine DID NOT do this. But Jeep does on both TJ engines.


That said, the problem could be a T-stat stuck open, OR it could be the coolant sending unit has gone south on you too.

You might want to check the connector for dirt on the temp sending unit. You may also want to take the sending unit out and clean it with a little rubbing compound as it will get a small scale built up over it over time. This will cause a resistance that can cause false reading as well.

In other words, you engine may be at operating temp and you don't know it becuase the gauge is lying to you.

The fan clutch is a viscous unit, meaning it is filled with an oil. As the oil warms up the fan will stop turning at higher RPM's, but it almost always turns at under 1,200 RPM regardless of engine temperature.

Hope you find some value in this.

Frank

[TJeeper]

Frank-

Thanks for your reply. I have learned a lot about my cooling system in the past 24 hours. It was suggested on another forum that my thermostat could have "lost" its calibration, and is opening up at a lower temperature. I did not realize this could happen as the last vehicle I had that had a defective thermostat failed so it was open 100% of the time, which is why I was confused.

I picked up a thermostat this morning and plan on cleaning the sending unit tonight when I swap the thermostats. I remember (I think it was you) someone suggesting to "burp" the sending unit after changing the antifreeze to make sure there is no air caught there making the sensor less accurate.

[Daless2]

Hi Patrick,

I hope the advice is helpful.

I believe it was Stu who posted the good advice on burping the sending unit.

Have a great day,

Frank

[TJeeper]

Here's a followup. I replaced the thermostat last night with a 180 degree one. I chose 180 because last summer I was having heat problems, on the highway it would run 220-230 degrees, while in town it would run 205 like it is supposed to. I assumed this was because of too many modifications to my Jeep.

This morning the engine warmed up about twice as fast as it was before. In the city (stoplights) the engine temperature is about 195, on the highway, about 205 (which assumes the temp gauge on the TJ is accurate- which I know it isn't). I am happy with that.

The old thermostat was slightly open (you can see light through it). I put it in water and boiled it (205 degrees at our altitude) and it never fully opened. I don't know exactly what it was doing in the engine (opening at a higher temp, getting stuck fully open?), but obviously it was defective.

When it warms up again I'll see if this was the cause of my overheating problems.

[Daless2]

Thanks for the feedback Patrick.

Keep us posted this summer.

Frank