My father drove a concrete mixer for 30yrs and told me that the engines sometimes started to run in reverse, to stop this event they simply turned off the engine and then restarted it. Anyone ever heard of this? Why do they do that? Jerry.

Absolutely. Some boat diesels run in reverse in order to back up the vessel. Certainly can happen in some trucks, usually 2 cycle diesels I think. You then get multiple reverse gears and only one forward gear.
It can happen if a person attempts to pop start the engine with the clutch and it turns the engine in reverse.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

It happens because the injection is near TDC. If the first combustion forces the crankshaft in reverse, it can keep running that way. The injection will then be after TDC, but close enough to still run.

I don't know if it's a common problem for modern MB engines as I have not heard of an occurance, but the old engines had a flap on the intake which closed if it did start in reverse. The danger of running backwards is that your oil pump is pumping the wrong way.

An MB would not run in reverse. It would cause the chain to go slack before the injection pump and skip teeth or jam.

That is because the timing is very close to TDC and of course they only have exhaust valves, intake air comes in through ports, and of course it fires every time it come to TDC. On the 4 cycle engine the way the valves operate, (valve timing) would preclude a backwards running engine.

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Junqueyardjim
Christianity, if false, is of no importance, and if true, of infinite importance. The only thing it cannot be is moderately important. C.S. Lewis



1983 Mercedes W123 240D 4 Speed 285,000 on the road with a 617 turbo, beautiful butter yellow, license plate # 83 240D INDIANA

2003 Jaguar Type X, AWD. beautiful, good mileage,
Mom's car, but I won't let her drive it!

Many 4 stroke diesels will run backwards - albeit a bit roughly. I've seen it happen on older British trucks, where the timing is all gear driven, and there were no anti-backfire devices fitted.

This can happen on 4 strokes because the valve timing and injection timing is close enough to symmetic.

Running forwards;

The piston goes down on the intake stoke, pulling in fresh air from the inlet manifold
The piston goes up on compression, squeezing that fresh air (all valves closed)
Injection happens sometime shortly before TDC
The piston goes down on the power stroke (all valves closed)
The piston goes up on the exhaust stroke, pushing products of combustion out of the exhaust

Running backwards,

The piston goes down, pulling fresh air in from the exhaust system
The piston goes up, compressing this air
The injection happens sometime shortly after TDC, but the gases in there are still hot, so the fuel burns (but not quite so well)
The piston goes down
The piston comes up, pushing the products of combustion out of the inlet valve

There are a few ways to prevent this - as mentioned, one way throttle valves, and also making the cam in the injector pump asymmetric so that in the reverse direction, injection happens at a useless point in the cycle (a so-called anti-backfire cam)


The exhaust gas passing through the inlet ruins the air filter in short order!

My grandfather's Oldsmobile (gasoline engine) used to do this. If the RPM was a little high when you turned off the very warm engine, it would start "dieseling" - my grandfather's word - chugging backwards. To fix it, we had to turn the ignition back on, thus somehow allowing the thing to start going around forward again. Then we could kill it with it in gear (to lower the RPM) and it would shut down.

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~Michael S.~
Past cars:

1986 300SDL
1987 300SDL
1982 240D
1982 300SD


Current:

1987 300SDL

It wasn't going backwards during those after-runs. The engine is still turning forward. The engine is very rough because ignition is probably occurring on only a few cylinders. Fuel is still supplied by the carburetor whenever the engine is turning, and hot carbon deposits can ignite the mixture.

my yamaha golf car went fwd/bkwd depending on which way you wanted to go.

it would really move in reverse.

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1984 123.193 372,xxx miles, room for Seven.

1999 Dodge Durango Cummins 4BTAA 47RE 5k lb 4x4 getting 25+mpgs, room for Seven.

Not sure about running in reverse, but my Ford IDI will run with the pump a full 180 degrees out of time. Not very well, and they will smoke like a house afire, but they will run. The first one I bought had the pump installed 180 degrees out, but I drove it home.

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1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown

As far as accidently running backwards heard of 2 stroke diesels taking a gasp and chugging backwards..

I had a Harley XR750 flat tracker that on several occasions at start up took a deep breath, stopped, and chugged backwards until you killed it.