Quote:
Originally Posted by Beadybc
It idles ok when stationary but I've found that when on the move, if I put in the clutch and coast from 3-4k rpm, it will idle at around 500rpm and if I brake the car will cut out completely, first thought was that dump valve but not sure how to reintroduce it as I'm not 100% as to where it should be dumping to. Looking at photos I'm pretty sure it should be connected to the lower connection on the inlet manifold. This connection has been repurposed for a boost gauge by the previous owner so i'll just introduce a t joint for that instead if it is the correct place?
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Yeah if it doesn't dump it will not idle after acceleration (when pressure is built up). It dumps to the atmostphere (out the air filter) by allowing the air to bypass the spinning turbo. It take a good few seconds to clear the pressure from the intercooler and pipework if there is no dump and the throttle is closed. You can usually keep it going with a little gentle reving for 5s or so.
It is called a recirculating dump valve because it is open always during idle because its all after the MAF. Other non-recirculating types open momentarily until the pressure equalises, they dump after the MAF so have to close otherwise air is sucked in via the valve and not measured.
Some people think the recirculation means the pressure is recirculated and used by the engine and maintains the mixture from measured air, however not really what is happening, the ECU does not keep a log of past airflow, it only knows air flow now. When you close the throttle it immediately starts trying to do closed loop operation with the O2 sensor using a weakish mixture, it cannot respond quickly enough to enrichen the muxture so it just dies pretty much immediately. A MAP type system might work better in theory, but the 940 doesn't measure MAP(manifold absolute pressure).
If there is pressure in the system and no demand for power, it just uses the air in the inlet after the turbo and there is no measured air. Also its not really using the MAF, its mostly using the O2 sensor which is not a dynamic measurement. I think the MAF is involved somehow but its different to open loop mode during non-idle.
You just need the diaphragm, spring and the proper cover with the vaccum line nipple. Its connected to any of post throttle manifold nipples I think, some are larger than others, mine is the lower one of the 3.
It might be they have disabled or adjusted the throttle closed sensor too (this can keep the mixture rich), so you need to find out what they have done to achieve this stupid modification.