Numpty Question i knw but,

kash · Post by **kash** » Thu Nov 29, 2007 11:52 pm

Would someone be kind enough to explian/define to me what is meant by the term over-run

Heard it mentioned plenty of times, was to embarrased to ask, tried googling it but didnt get anything LOL LOL

Bikernut · Post by **Bikernut** » Fri Nov 30, 2007 12:08 am

It could mean a couple of things actually.

To be Over-run can mean inundated with something.
Or in the context that i assume you mean...when a engine is coasting in gear with no throttle applied.Like going down a hill.

BlueRover · Post by **BlueRover** » Fri Nov 30, 2007 12:51 am

Over run is where you get the engine continuing to run even when the ignition is switched off. You are very unlikely to get this with modern fuel injected engines as the injectors would not fire when no power is supplied but I suppose in very unusual circumstances it can still happen.
It's more likely on carburettor engines when the fuel mixture is rich and the hot plugs could ignite the fuel in the cylinders when supplied by a mechanical fuel pump as the pump is driven from the engine. While the engine turns, the fuel pump pumps.

I think I'm right but welcome any corrections where necessary.

98Ti · Post by **98Ti** » Fri Nov 30, 2007 12:54 am

I'd say that sounds abobut right. me and my mates used to have an old mini to drive round the farm and you had to stall that old heep to stop the engine.

E_T_V · Post by **E_T_V** » Fri Nov 30, 2007 1:04 am

Over-run is the same as engine braking, i.e. the wheels are turning the engine rather than the other way round.

Running on is where the engine continues to run or misfires after you have switched the igntion off.

BlueRover · Post by **BlueRover** » Fri Nov 30, 2007 1:08 am

What he said ^^^^^^^^^ ( or is it ?)

Null_Byte · Post by **Null_Byte** » Fri Nov 30, 2007 11:24 am

I must say now i have heard both terms "running on" and "over run" used to describe an engine continuing to run once ignition is off.

I still get this occasionaly on my landrover - where the engine will run on for a few seconds, usually backwards.

Never seen it on a modern car, although I have seen it happen with a few diesels.

Diesels can be even worse as they can spiral out of control and start to burn their oil.

Scott · Post by **Scott** » Fri Nov 30, 2007 11:44 am

fuel injection cars shut off the fueling at like 1500rpm if the throttle is closed. Im sure we have all noticed when letting the car roll / slow in gear with the throttle closed, when you hit 1500rpm (just above as the rpm meter is always over) it gives a little judder... Mems does this to save petrol (but only when the engine is warm) it will still fuel if the car is cold and throttle is closed...

our A series maestro used to over run, turn it off, used to run for upto 20 seconds. Carbs cant cut the fuel after switch off, and petrol was ignited by hot carbon inside the combustion chamber...

Post by **Punx0r** » Fri Nov 30, 2007 1:56 pm

"Running on" is the pre-ignition scenario

t16omcat · Post by **t16omcat** » Fri Nov 30, 2007 9:17 pm

E_T_V wrote:Over-run is the same as engine braking, i.e. the wheels are turning the engine rather than the other way round.

Running on is where the engine continues to run or misfires after you have switched the ignition off.

^ all true ^

running-on is also known as 'dieseling'