Do I need to let my turbo "cool down" for awhile ('1990 C-900T)?

When you drive hard (motorway driving, or spirited driving) the exhaust manifold and turbo get really hot. So hot they glow a nice cherry red. This is hot enough to burn pretty much any oil.

The turbo shaft spins on a film of oil - that's all the bearing is. It relies on a continuous flow of oil through the bearing to maintaing the oil temperature below oil burning point. Without oil, the turbo bearing would be metal-metal contact, and would last a matter of minutes.

If you get the turbo nice and hot, then immediately turn off the engine, the oil in the bearing will burn up. Next time you start up, there's no oil in there for the turbo to spin on, so it wears a little. Do this a few times and it wears a lot.

So what do you do? Let the car idle for say a minute, especially after hard driving. This gived the turbo time to cool down.

Later cars (including yours IIRC) have a water jacket around the turbo bearing. This helps a lot with the cooling, so idling the engine is only really important after a hard run.

When I researched this question for my '97 900SE, the clear recommendation was to let it idle a few _seconds_ (like 15 seconds, not a minute or so!) after spirited driving. However, simply logic suggests that one would not want to simply shut down the engine after a spirited drive even on a non-turbocharged cars.

Thanks,

-Dima

Tell you what - next time you're out for a good drive and it's dark, pull over and take a look at the engine. Look at the beautiful glow. Do you really want to cut off oil when it's that hot? Turn the engine off.

Still, it's really not going to cool down with the engine running unless you leave it running for 10 minutes or more.

I suggest you wait 30 seconds after pulling up to let the turbo spin down to idle speed AND using synthetic oil in any turbo motor.

Back to the OP's question: turbo spin-down used to be a more serious issue when turbo units were not water cooled.

MY .02, Bob

The real answer, which I omitted in my previous replies, is this: How many of us come off the motorway (US/Can: Highway) and turn off the engine straight away? We don't - we drive down progressively smaller, slower roads until we reach our destination, by which time the turbo has cooled down plenty. :-)

The original post also mentioned that he had 110,000 miles - and it still ran fine.

However, many Saabs reach 300,000 to 400,000, so 110,000 and running fine is not saying much.

Just my opinion, letting the engine idle for 30 seconds to 1 minute also allows the driver time to close the sunroof and windows. The sound of an idling Classic exhaust echos nicely in a concrete parking garage :-)

On the other end of the scale, it is also important the let the engine warm up gently before using the mighty turbo power, otherwise the head gasket will be in trouble.

True, however, when you are cruising on the highway in 5th gear, the engine speed is low and the turbo is not doing a whole lot. OTOH as you blast down the 2 lane side roads, stopping or slowing at corners and accelerating away with a crazy turbo induced grin on your face (as in the recent series of SAAB TV ads) your engine speed and turbo will be spinning up nicely and heating the housing to the familiar cherry red. The first time I saw this on my old '92 9000T I was astounded that the housing could become so hot and yet the turbine bearings continue to work at all!!

I am of the opinion that a minute of idling will actually allow things to cool down, even more so with the modern water cooled turbos. But, of course, YMMV...

-Fred W

That can be said for any engine - turbo, supercharged or NA. This includes

the driver as well. More heart attacks occur in the morning when people try to exercise as soon as they wake up.