Backfiring
 

I have a dump valve on my 93wrx and I have been told by some people that it is supposed to backfire, but others have said it isnt. Who is right?? It used to backfire before I had the cambelt changed but it doesn't as much now. Please help me!!!!

You usually get a backfire when there is a leak in the exhaust system or, you are running very a rich mixture and it gets burnt in the exhaust. hense the 'pop'. Dumpvalve shouldnt affect this in any way.

if you have a VTA dump valve and you change gear, it can dump a bit of fuel down your exhaust and cause the popping and back fireing, more so with a full decat

chris purvis doesnt seam to know about this

I stand corrected.All Im saying is that when I changed my exhaust to a blitz system, popping sound went away ( lots of rusty holes in old exhaust! ) Also fitted a turboxs dump valve and still no popping

The principle is that if you have a VTA dump valve then this releases metered air from the induction system when lifting off the throttle. The ECU sees this and interprets it as the engine requiring more air (and hence fuel) and injects fuel to get the mixture correct. However the air is not in the inlet manifold, it has been vented by the dump valve, so you end up with an overly rich mixture. This can get burnt off in the hot exhaust system causing pop, bangs and sometimes flames.

The downside is that it can cause bore wash, the excess fuel gets past the piston rings and contaminates the oil. Not good :(

Of course none of this happens with the standard recirculating dump valve.

But surely the ecu wouldnt pump more air+fuel in would it? The throttle is closed, ie, lifting off?

Airflow into the engine isn't goverened by the ECU, it's governed by the mechanical movement of the pistons and of course the turbo.

The ECU controls the fuelling.

Graz is right, the airflow meter measures the air enetering the engine and tells the ECU. The ECU then applies the appropriate amount of fuel, but if the dumpvalve vents some of that air to atmosphere, the ECU doesn't know about this and so fuels the car as if that air were still there, hence temporary over rich mixture.

Thanks a lot lads, it seems a common thing in scooby's with dump valves!