Q: Engine revving on closed throttle

When the throttle is closed from high revs, the engine is in what is called over-run.

Running on, occurs when the engine fails to stop. This should only happen on Carbed engines that have no fuel cut-off. It is caused by a hot spot in the cylinder (typically excess carbon build up) that causes the still entering fuel mixture to still ignite despite having no spark (hence it shouldn't occur with any fuel system equipped with a fuel cut-off).

As for what happens during over-run, on a carbed engine (and some early FI systems) fuel is still entering the engine, albeit in greatly reduced quantities. And the engine is technically running very lean, but there is not enough heat created during this process to cause any engine damage usually related to lean running. On a modern fuel injection system, fuel is cut-off on engine overrun, to improve economy and reduce emmisions.

Momentum.

A minute amount. Certainly not enough to keep the revs up.

Fuel injection actually stops supplying completely above tickover speed.

No, really?

The message from "Krycek" contains these words:

That's correct. Certainly modern engines with fuel injection and even some late carb engines shut the petrol off completely until the revs drop to idle - or wherever you've left the throttle.

Well, almost. The fuel generally comes in a little earlier to make a sort of soft landing at idle. I forget what the cut-off point was on the

1.6 Maestro, but that had a little vacuum powered external fuel cutoff on the SU carb which cut the fuel completely on over-run. So, for example, if you were runing down a long hill using engine braking no fuel was passed at all.

The mixture isn't so much lean as non-existant, so there's nothing to overheat a cat with.

You may be confusing running on - when the engine fails to switch off when you tell it to, with over-run - when the engine is running faster than the throttle setting would suggest. Either 'cos you've just shut the throttle and it hasn't slowed down yet or 'cos you're engine braking.

Probably.

Many thanks to all :)

Krycek formulated on Tuesday :

On a carbed engine the carb will shut down to the air and fuel mixture needed for tick over, except there will be a high vacuum in the inlet manifold.

As above, however... Running on, is what you used to sometimes get with a lean burn carb engined car after the ignition was turned off. The engine continued to turn, drawing fuel in and it was self igniting in the cylinder with no spark from the ignition, a little like a diesel. One fix was I think a relief valve in the inlet manifold to kill the induction.

A fuel injected system only injects fuel when it is actually needed. On the overrun it is completely cut off until the engine speed falls to a tick over, then just enough fuel and air is allowed in to maintain a steady tick over speed. You can drive a fuel injected car a little like a diesel, in that you can release the clutch and the injection ECU will try to maintain the engine speed and actually drive the car at a constant low speed - useful in very slow moving traffic.

[...]

Only on very ancient carbs. Most of the last carbed engines had a shut-off valve that stopped fuelling on overrun.

Chris