swporsche

I live in one of the warmer parts of the Country - Phoenix and have a quick question. After driving my car on 100 plus degree days it is not uncommon for the cooling fan to activate when I stop. My question is, is better to allow the fan to do its job or should I open the trunk and expose all of the engine to the cooler air the is usually prevalent outside? Thank you!

kisik

Let the fan do a job. It was designed that way. You can actually idle for a few minutes after you stop to get cooling going. As long as your oil temperature is below 240F then nothing to worry about if you use street oil. Clean your radiators and behind condensers. Add a 3d center radiator ($200 Porsche part) for more cooling capacity and faster cool off.

Devil Boy

I have a box fan that I turn on when I back the car in the garage but all that really does is speed up the heating of my garage.

Mike Murphy

I could be wrong, but on the 996, the engine bay/lid cooling fan pulls air from the outside down and into the bay to cool the engine. I suspect the same thing for your car.

And a lot of folks let thier engine cool down by letting it idle for a long time. I've seen folks do this after pulling off a racetrack or off the highway. However, it is not adviseable for most non-Turbo cars to let it idle for more than a minute or two. An engine will increase in temp for the first few mins after you turn it off, but the hottest parts of the engine will start to cool down immediately. Letting it run for more than a couple of minutes actually doesn't cool the engine down much at all.

adnan76

The convective heat transfer using the fan should be much better than popping open the engine lid.

nzskater

I'd suggest that leaving it idling for any length of time is a bad idea - you've lost air flow over the radiators so you're going to see heat increase (as you do when driving in traffic). As suggested above, the fan pulls cool air through the decklid and onto the engine, exiting below the car.

Even in winter, after a hot run my cars fan kicks in soon after parking up. I've always let it do its thing, even when tracking in summer.

Bruce In Philly

After running a car hard, you should always let it idle a bit to reach thermal stability..... when you track your car, you always have a "cool down" lap for this.... at least that is what the organizers of the events I participated in always did.

If you did run it hard, then a few minutes of idle or driving lightly through your neighborhood is fine. Never pound your car and then just shut it off.

Back when I had a Datsun 280Z turbo, I recieved a letter from Datsun denoting the importance of idle for cool down. This was specific to the turbo cooling as they had a problem with coking (oil turning to sticky sludge on the hot bits) but is still true for any car.

Other than this, worrying about a hot car in your garage is a bit silly to me..... the fans kick on if necessary.... the car is off so there is no added heat.... the engine bay is what is important here, not the engine and these fans will whisk it away.

Peace
Bruce in Philly

jhbrennan

Correct - cooling fan draws air from the outside and pushes it down to escape from the bottom of the car not out through the compartment lid. Since most of the heat is generated from below (headers, cats, etc.) this seems like a pretty efficient way to move the hot air out. Opening the compartment lid, disables the fan and the hot air escapes and rises over the top of the engine. I've done it both ways and haven't concluded one is better than the other...other than concluding the Porsche engineers certainly know more than I do about thermodynamics.

Macster

Keep the engine compartment lid closed. The fan is at blowing cooler air from up top down through the engine compartment and forcing the hot engine compartment air out at the bottom of the engine compartment.

If you open the lid the fan runs but doesn't blow air through the engine compartment. The hotter air from around teh exhaust manifolds then rises and subjects the hoses and wiring to more heat than they would normally be subjected to.

With the lid open the fan runs more and longer since it is controlled by a temperature sensor in the compartment. This will work to run the battery down more.

With my 996 Turbo Porsche says to let the engine idle after a hard or extended run for 2 minutes. Primarly this is to allow the turbos -- the exhaust turbine end -- to shed some of the considerable heat the exhaust turbine can have from this hard or extended running. If the engine is shut down right away this heat flows to the turbine shaft and heats the oil that is in the bearings and this can over time result in bad bearings or damaged oil seals.

During the idling this also gives the engine time to shed its excessive heat load via the coolant.

Some heat goes into the oil as well. After a couple of minutes of idling the oil pressure is down a bit from where it was when the idle time started. This is due to the oil which is contact with the hottest parts of the engine absorbing some of this heat and carrying it away.

With the Boxster which has no turbos I arrange to drive the car easy after an extended run or hard run to give the engine time to shed its heat load.

Addiitonally when I pull into the parking space if the engine compartment fan is running or if the radiator fans are running -- I have shut off the A/C -- turned off the compressor to give the system time to "dry" out -- I let the engine idle until the engine compartment fan or the radiator fans shut off on their own.

The engine fan is running because the engine compartment is hot. If I shut off the engine at this point the fan goes off. The engine compartment is still *hot*. The fan won't come back on if the car is locked and I always lock the car when I park it.

(The Turbo engine compartment fan runs all the time the engine is running and will come on after the engine is shut off --- even if the car is locked and I always lock the car when I park it -- if the compartment temperature rises too high.)

With either of my cars if the radiator fans are running it is because the coolant temperature is 212F (or hotter). If the radiator fans are running I let the engine idle until the fans shut off.

This gives the hotter parts of the engine time to shed some of their considerable heat load and time for the engine temperature to moderate overall.

If you shut off a hot engine the exhaust valves can be close to a red heat in temperature. If the exhaust valve is off its seat -- and some will be -- the only way it can lose this heat is via radiating heat -- not going to work well in a hot combustion chamber -- or by the heat traveling up the exhaust valve stem and flowing to the guide. This path heats the oil between the valve stem and the guide and like in the case of the turbo can result in degradation of this bearing upon the next engine start. Additionally this heat reaches the valve stem seal and can shorten its life as well.

By letting the engine idle the exhaust valves while they still get exposed to hot exhaust gases the exhaust gas temperature at idle is lower than it is when the engine is working harder. Thus the exhaust valves cool down from the cooler (relatively speaking) idle exhaust gas. Also, the exhaust valves spend a good amount of time closed and when closed give up heat to the valve seat (which is cooled by nearby coolant passages).

When the engine is then shut off after some idle time while the exhaust valves are still hot they are not "red" hot and if open while the heat still flows to the exhaust valve stem and to the guide and even the seal the heat is much less and thus these components are not degraded.

To recap and this applies to your car if with my N/A Boxster if I come right off the freeway to say refuel and often the gas station is right close to the exit I let the engine idle a bit to give the engine time to shed the considerable heat load that it develops when pushing the car down the road at speed.

This time also gives me a chance to perform a post flight (so to speak) check out to make sure the car's vital signs are ok. The oil pressure, coolant temperature, battery voltage readings are all ok and the car/engine is not making any abnormal noises.

In some cases -- if it is really hot (and I have been on the freeway in 116F heat) -- when I pull into the gas station the radiator fans can be on even the engine compartment fan. In these cases the engine idles until these shut off. (Actually in the case of the 116F ambient temperature both the engine compartment fan and the radiator fans never shut off but I let the engine idle a couple of minutes then even though the engine compartment and radiator fans were running shut off the engine anyhow. I was monitoring the intake air temperature and coolant temperature. Intake air temperature was 135F which meant the engine compartment was at least least that hot and coolant temperature was 226F which meant the radiator fans were on high.)

If I leave the freeway and drive on surface streets which is the case on my work commute the time on surface streets is as good as idle time to give the engine time to shed its heat load.

However, in doing so this can have the engine compartment fan running even the radiator fans running. In this case, in either case, I let the engine idle until the engine compartment fan or radiator fans shut off.

Kind of long I admit. But I have my reasons. These engines are marvelous inventions and over the years have obtained a very sophisticated level of development. Still one needs to have a bit of mechanical empathy and when possible try to give the engine a bit of a break to help it deliver a long and trouble free service life. This is why I try to give the engine a bit of time to shed its heat load. With over 313K miles on my Boxster's engine and nearly 157K miles on my Turbo engine I think a case can be made a bit of empathy pays off.

ADias

To Macster's excellent advice i would add in the garage (if possible) a fan in the front of the car blowing down under the car towards the rear.

jsalah

I've read others making similar suggestions-- essentially a box fan parked behind the car... tempted to try this. Doesn't seem like it can hurt.

ADias

Behind the car pulling hot air from under the engine, but better yet, a fan in the centerline in the front blowing under the car towards the rear.

david

As expected, the RL wisdom flows! (If I lived in PHX I'd do all of the above-mentioned...except keeping the engine lid open!)

Are you using the 997 to commute or are these late evening / early AM rolls up to Jerome or Holbrook or Sedona or Flagstaff or Page, or other points N and/or E?!

There are so many great roads up there...hmmm...that is what you're using the 997 for, right?! No commuting in PHX in a 911 allowed during the summer or during dust storms!

PS Checking the oil at the end of a run is also a good use of time...

Mike Murphy

The problem with using fans:

1) Cannot hear the engine ticks while cooling (engine ticks on an aircooled car sound the best, but the liquid cooled cars still do it, albeit less)
2) Wastes energy