Engine Braking?

[aaronng]

aaronng,
The throttle plate and IACV don't do anything out of the usual when the engine is on the overun. Overun fuel cut off isn't cotrolled using aiflow, though the IACV does remain momentarily open immediately after the throttle plate has been closed to (I think) assist in burning still unburnt fuel as it flows through the exhaust system (an emissions control measure).

My understanding of what happens is that the ECU detects that the throttle is at the closed position (using the TPS input) but also detects that engine rpm is above X, and using these inputs the ECU 'decides' that an overun condition must exist and as programmed cuts the injectors until either the throttle is opened again or the rpm reaches a set point somewhat above idle speed.

This feature lessens fuel consumptioon and emissions, and helps to prevent backfiring (which can damage the cat apart from anything else...).

The braking effect comes from internal engine friction (and to a much lesser degree from internal gearbox friction). Some of the braking effect comes from the compression of gas in the cylinders, but since the throttle plate is shut very little air should be able to flow into the cylinders from the intake side, so perhaps there is some reverse flowing gas coming in via the exhaust valve...?

Thanks for explaining. If there is still air going in and the injectors are cut off, wouldn't there be a split second of lean combustion when the left over injected fuel in the manifold enters the combustion chambers after the injectors are closed ?