But doesn't the IAC valve open whenever the throttle plate is shut?[/quote

 

When the engine is doing 5000+ RPM due to engine braking and the TPS reads that the throttle is fully closed, I believe that a common engine management strategy is that the ECM cuts the fuel to the injectors and the IAC would be closed. The fuel is restored once the engine speed drops below the 1500 rpm mark and the IAC will begin to open once the engine speed nears normal idle speed. (Please note: I don't know for a fact that Subaru's engine management strategy works this way, but I do know that many cars use a similar strategy.)

 

Nathan

Do what you want, but you may need to add oil and change plugs more often because of this. Using engine braking is making the engine suck as hard as it can, pulling the most vaccum in the intake manifold. This also pulls the most vacuum in the cyl's, which tends to pull oil past the rings. This also pulls the oil past the intake valve stem seals. With the injectors shut off by the ECM, you're not going to have any firing going on in the cyl's, so fouling the plugs with the oil sucked in is more likely.

 

Big trucks= diesels. These use engine compression to slow the truck. No throttle plate means no vacuum.

 

Car's use a throttle plate, so they are using the engines ability to pull vacuum to slow it down.

 

The way Jake brakes work is by popping off the exhaust valves right at TDC on the comp stroke, which dumps all the stored energy in the compressed air.