I accidentially knocked my car into neutral and the engine briefly hit 5000+ rpms. Very quick and I let off the fuel immediately. This is an 83 617 with the turbo. It still seems to run OK. Did I overspeed the engine or is 5K not past red-line.
Thanks Much.

Yes, that probably was an overspeed. But from the sounds of it, must not have been bad enough to do any damage. Redline can be hit or even pushed when under load but freereving past red isn't good.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

The car won't over-revv.. the injection pump prevents that.. your engine is fine.

I'd agree the IP would stop it from staying over redline but wouldn't the inertia of the motor accelerating there rapidly cause it to go over?

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

At lunch time I'll check the oil and take it for a ride and report back. I'm hoping nothing is wrong. For all I know, the inertia of the tach may have also been indicating a higher speed than the engine actually hit. It's not knocking (other than diesel knock) any different than before. Lunch-time run should tell.

Yes the inertia may let it spin another ~100rpm but it will keep cutting the fuel when it senses the car is near redline. Don't worry about a thing.

How does the IP do that?What gets damaged first?

On robust engines that don't break easily, throw rods and the like (I think the 617 qualifies) the next potential damage from over-reving can come from valve float where the valve springs don't close the valves fast enough before the piston comes up from below and hits them. Given the considerable stroke and compression of these engines it's probably pretty hard to do that unless you get the RPMs really up there. In any event, you'd know it if you've hit a valve. You should be fine.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold

I did the exact same thing, it was rather frightening having it hit 5K RPM all of a sudden when i tried to shift it back to "D." Engine still runs fine, and that was like 5-6 months ago.

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1985 CA 300D Turbo , 213K mi