A good one will roar loud enough to drown out the burble of a V8!
So as already said, when stone cold, it should be the most obvious thing you can hear, bonnet up or down.
Then in about 30-60sec, as it starts to decouple itself, you will hear the sound of the motor over the sound of the fan.
Note tho, a few sec after start up, you do need a few revs(1000-1500 ish) to hear the roar of the fan.
The more you rev, the louder the fan will be.
But once it's decoupled itself, you can't hardly hear it over the motor.
So the easiest test is. From stone cold, start motor, wait about 5-10sec for oil to fully pressurise and circulate, then give it some revs.
If you can't hear the roar of the fan over the motor noise, most likely it's not working ideally(or at all).
Could be partially working tho, but a brand new one is very obvious.
Also, in those first 30-60secs, with bonnet up and a few revs, tweaking by hand the throttle cable obviously from the driver side of the front of the car, you will feel massive amounts of air being forced backward of the fan.
Then once it's settled and decouples itself, the difference in air from the fan is very obvious.
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