I have been told that a mercedes diesel does not need to be "spooled down" before shutoff. Is this true, and if so why is it that the mecedes diesel does not need this procedure? With my Volovo 240T I always let it spoll down before shut off, per Volvo recomendations.

I don't know. But I always drive lightly towards the end of a run and let the engine idle for about 30 seconds to a minute before shutting it down.

__________________
2001 Dodge Ram 2500 Cummins Turbo Diesel - 4x4, auto, 3.54 gears, long bed
-------------------------------------
'92 300D 2.5 Turbodiesel - sold
'83 300D Turbodiesel - 4 speed manual/2.88 diff - sold
'87 300D Turbodiesel - sold
'82 300D Turbodiesel - sold

It is done not so much to ‘spool down’ the turbo as it is to cool down the turbo. By allowing oil to continue flowing through the bearing area, it cools it down and reduces the possibility of coking.

RTH

It's because the exhaust temperature of a diesel is not nearly as hot as a gasoline car, so coking of the oil in the turbo bearings is not a problem.

- Jim

It's because mercedes turbos (on the older cars) have such incredible lag and produce relatively small amounts of boost (~10 psi) that under normal driving conditions, the turbo doesn't get hot enough to require a cool-down period.

Don't get me wrong, if you drive the crap out of your car, it's probably a good idea to let it idle for a minute or so just to let the turbo cool down a little and also let the engine coolant cool down.

The owner's manual of my 87 300D recommends "several minutes of idle" before shutting down from highway speeds.

If you've been booting down the freeway, the turbine wheel will be nearly white hot, reguardless of the power output. Takes 5 minutes or so to cool down -- still got hot exhaust on it! -- so that the oil won't burn onto the bearings.

In real life, I use synthetic oil and don't worry about it as nearly everywhere I go I have several minutes of slow speed driving before I shut off. On the highway, especially rest stops on the Interstate, I let it idle for a couple minutes.

Much cheaper than a turbo rebuild or ($$$) replacement! -- I've done the rebuild thing on the Volvo, but that wasn't my fault -- it didn't work properly when I got it, and 40,000 miles later stopped altogether.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

psfred, good point about the highway driving... rest stops are murder on a turbo!

yes it's true... if you have a boost gauge on a MB, you'll notice that it's running full boost all the time at highway speeds. Turning off the engine at a rest stop to take a pee break will surely result in coked oil on the turbo bearings (not good). I usually just leave my car running at rest stops or I let it idle for 2 - 3 minutes before shutting it off.