Why does cooling fan come on after engine shut off

[brianl703]

I've noticed that my 2006 Saab 93 will occasionally turn the cooling fan on after I've parked it and shut the engine off. It only does this on hot days, and the fan only runs for 10-20 seconds. This appears to be normal operation based on what the service manual says. I expect in the case of the Saab this helps the water-cooled turbo cool down.

The only other car I've ever owned that had the cooling fan come on after the engine was shut off is a 1986 Dodge Omni, which isn't the turbo version, and I always figured that the reason why is that the fan temperature switch is wired that way to save a few cents on wire.

But now I'm thinking there may have been a reason for it. Does it help engine longetivity?

Neither the Ford Escort nor the Ford Contour ever turned their cooling fans on after the engine had been turned off..I don't think they are capable of that (they use the engine coolant temperature sensor to control the engine cooling fan, and the engine computer is OFF when the key is off. The Saab 93 is not like that--the engine computer on that goes to "sleep" several minutes after the key is turned off).

 

[badtlc]

When it happens in mazdas, it is due to a faulty fan control module.

 

[eljefino]

I'd be happy if my car did that. Underhood temps peak shortly after shutdown. If the fan ran long enough that the radiator cooled down it would then be a fresh breeze over everything else under the hood.

I also remember 80's dodges doing this.

 

[brianl703]

I read in the service manual that the 2004 Malibu Classic with the Ecotec engine will run the cooling fan:

"After the vehicle is shut off if the engine coolant temperature at key-off is greater than 140°C (284°F) and system voltage is more than 12 volts. The fans will stay on for approximately 3 minutes."

I'm not sure I want to know under what conditions the coolant temperature will ever exceed 284F, or whether the engine will still be worth anything after that. I guess maybe this is an attempt to save the engine in the event of severe overheating.

 

[Merkurwwu]

Does the cooling fan actually come on or does it just stay on when you shut the car off? Just curious. My new S60R leaves the fan on for 20 seconds or so after the car is shut off every time you shut down with the A/C on. It sounds like a jet winding down as you're getting out of the car

Try it with and without the A/C and see if it makes a difference. I'm sure its just normal operation though...

 

[brianl703]

It actually comes on about 30 seconds after I shut the engine off. Both times it happened when I wasn't using the AC. I assume the coolant temperature has increased during those 30 seconds to the point at which the computer is programmed to turn the fans on.

If I'm using AC so far as I've observed the fans (it has two) will always turn off when the engine is shut off.

I think it's probably more likely to run the fans after you turn the engine off if you haven't been using AC, because the normal mode of operation without AC is to run the fans when the engine temp reaches about 210F, wheras when the fans are running continuously the engine temp should stay right around 190F or so.

So shutting the engine off without having run the AC could result in a higher shut-down temp, resulting in a higher likelyhood that the fans will come on after shutdown.

 

[mstrjon32]

Every modern GM car I've ever owned or worked on has had a sticker under the hood that indicated the fans may start with the engine off. If you're noticing the fans starting with the engine off, and the temperature is normal, I wouldn't worry about it too much.

 

[brianl703]

I'm not worried about it at all, I just want to know why the engineers set it up this way. There must be some benefit to operating the cooling fans after the engine has been shut off, I'm wondering what it is. Does it make the engine last longer?

 

[Gary Allan]

I imagine that it's probably due to too many consumers not reading the printed warnings about high speed driving and immediately pulling over (like into a service area) and shutting off the engine. This is rough on some aluminum head engines where, at higher output levels, the cooling system is just keeping ahead of the thermal production. Without a cool down of maybe 30 seconds to 1 minute ..where the lower rate vents the still present "hot spot's heat", you can run into troubles.

Even with the water pump not running, between convection and just "shrinkage" the effects should be greatly reduced.

 

[Volvohead]

Quote:

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Does the cooling fan actually come on or does it just stay on when you shut the car off? Just curious. My new S60R leaves the fan on for 20 seconds or so after the car is shut off every time you shut down with the A/C on. It sounds like a jet winding down as you're getting out of the car

Try it with and without the A/C and see if it makes a difference. I'm sure its just normal operation though...

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I think the S60 also has a feature to engage the interior blower motor so many minutes after an AC running shutdown to dry off the evaporator, but I don't know if it is commonly enabled.

The roar on the R is due to the 12 or 13 fan blades. Not that it makes up for the dreadful intercooler heat soak they suffer from this time of year.

Cooling fans operating after shutdown is not an unusual feature. As long as they turn themselves off, I wouldn't worry about it.

 

[JustinH]

I would be interested to know if the saab is a factory turbocharged car?

This would be a last ditch effort to cool the turbocharger.

The aftermarket sells devices called turbo timers, which will run the engine for an interval that you set, after you remove the key.

This extends turbo life, by letting oil circulate to get the turbocharger temp down before you shut the engine off.

 

[brianl703]

It is a turbocharged car with a water-cooled turbo.

 

[Black Bart]

After you turn off the ignition switch and the engine stops turn the key back on and watch the water temp soar.
When I get home on a hot day I turn the key on because my old car is not programed to run the fan after it is shut off but I turn the key on and run the fan for awhile to help keep it from climbing so high.

 

[Anduril]

When the engine shuts down, a few other things also stop working: water pump, forward movement (which forces air through the radiator), and (generally), the cooling fans. With all of these things no longer working, the engine will "heat soak," and become hotter than it was while running, due to the fact that it hasn't had time to cool down yet, and the things keeping it cool are no longer active. The fans come on in an attempt to combat this, though without a working waterpump (which is driven by the now-shut-off engine), I can't see them being to much benefit.

 

[Gary Allan]

The thermostat is still open. Convection and just plane "shrinkage". The pressure is a result of temp. If you reduce the temp remotely, the pressure will drop and so will the temp locally. It's not immediate ..but

 

[brianl703]

Well it's obvious that, in the case of the Saab, the fan IS having an effect after engine shutdown because it does turn off--and the only thing that will make it turn off is lowered readings from the coolant temp sensor.

I think it's called "thermosiphon".

 

[Volvohead]

Quote:

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The aftermarket sells devices called turbo timers, which will run the engine for an interval that you set, after you remove the key.

This extends turbo life, by letting oil circulate to get the turbocharger temp down before you shut the engine off.

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I just wait for the current song to finish before shutdown.

Turbo cool down has much improved since the widespread use of water cooled designs. Good designs will continue to draw heat off the bearing area after shut down.

 

[Bamaro]

Quote:

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When the engine shuts down, a few other things also stop working: water pump, forward movement (which forces air through the radiator), and (generally), the cooling fans. With all of these things no longer working, the engine will "heat soak," and become hotter than it was while running, due to the fact that it hasn't had time to cool down yet, and the things keeping it cool are no longer active. The fans come on in an attempt to combat this, though without a working waterpump (which is driven by the now-shut-off engine), I can't see them being to much benefit.

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If the engine is off, where does the heat come from?

 

[wavinwayne]

My '00 VW GTI 1.8T does this in the summer after a hard run. The fans will sometimes run for 4-5 minutes after the engine has been shut off.

 

[Drew99GT]

My 94 Corolla does the exact same thing on hot days. If I use the A/C in stop and go traffic, the fans run all the time to cool the condensor, so water temps stay lower, and it usually won't turn on after shut down (they will click on for about 20 seconds if I turn the ignition on but don't start the car).

It's a whole lot better than my Mom's Olds 98; if you turn it off on a hot day, then turn it back on about 2 minutes later, the thing is at like 230. (which is how all GMs seem to be programmed)

Mine never gets over about 205 (factory 180 thermostat).