Humming from engine area on start up + wastegate valve quest

On first time starting in the mornings, I instantly get a loud humming noise from the back, the only way to explain it is it sounds like the noise a petrol pump makes when its running out. Only happens on startup. Also, since giving the car some serious 32 lap rapeage at the 'ring I've noticed that under load when I change gear, I can hear the dump valve hiss a little (like the chavs have it). Could the wastegate valve have burnet out a little? Thanks in advance.

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Re: Humming from engine area on start up + wastegate valve q Exhaust Gas Re-circulation (EGR) pump operating perhaps??

Re: Humming from engine area on start up + wastegate valve q Is that a normal thing then? :?

I must firstly confess to being no expert in this matter. :judge: However, I understand that on initial engine start-up, in order to get the cats up to temperature in order for the necessary chemical reaction to take place (so that the engine emissions are within spec or reduced) an amount of Exhaust Gas Re-circulation takes place assisted by an electric pump (or fan). Please don't take my word for it - I could be well off course on this one (but I would like to learn more about it one day!) :success: The wastegate valve is usually constructed of a butterfly or flap valve designed to withstand high exhaust temperatures and, when in operation, diverts exhaust energy away from the turbine chamber so the turbo effectively does less work and therefore produces less boost. This valve is often integral to the turbo. The wastgate actuator is often connected to the wastegate valve by means of a rod (which can be adjustable). The actuator is often vacuum operated (needs vac to operate) which then opens the wastegate valve, diverts exhaust energy away from the turbine and less boost is produced and so on..... The dump valve simply "dumps" boost pressure from the inlet system to atmosphere (if it's an external one) following a condition of high-boost and then the throttle valve is shut off quickly (engine load then to no load) in an attempt to allow the impellor of the turbo (inlet side) to run freely rather than be impeded by the pressure build up and reduce turbo spindle speed. So when power resumes, the turbo hasn't as much catching up to do. What I've wrote is not engine specific to the 996TT and I apollogise for that. I hope someone will be kind enough to expand on the above points and relate it to the TT as they are very lean at the moment! Hopefully I'm having a TT next and then I will be able to answer this fully!

"I can hear the dump valve hiss a little (like the chavs have it). Could the wastegate valve have burnet out a little? " The GT2/TT's are closed loop DV's - you should not hear any hissing as the open loop units are the ones which dump into the atmosphere and hiss/honk( re boy racer Scobbies and EVO's) The std plastic DV units are garbage(leaky? - so get them off) and fit either EVOMS or Forge units(check the little boost hose that connects to the DV's for any leaks also). I'd also fit silicone boost hoses, seals and clips if/when replacing the DV's. Never heard of a wastegate flap buring out - so I think you are onto plums. What you do need to check is that the actuator(s) is not seized though and moving freely(clearly only do this when the engine is cool LOL) What level of boost pressure (bar) are you seeing on the cars on-board computer when you open the taps (0.9 or 1.0) is the norm for an X50? If the wastegate flap has lost it's pre-load via the actuators( I think they are factory set at 0.6 bar on std X50's) you will lose boost. I cannot stress enough the importance of having both actuactors functioning/set perfectly. Many >5 year-old cars are running around with worn actuators and/or seized arms. Easy to have checked-out/replaced. A good pair of actuators + upgraded DV's and a well sealed set of boost hoses makes a 996T go like stink again. Small faults creep-up over time and sometimes the driver does not notice the tail-off in power. Reports of X50's running only 350BHP due to leaks and/or knackerd actuator operations are not that uncommon! If you are seeing 0.9/1.0 bar then the chances are that you have a well sealed boost system.....but a well set-up pair of actuators/wastegates help keep the boost going just that little bit longer while a good pair of DV's help the turbo's to spool well :thumb:

Mat I think you are refering to the throttle body hum, they do buzz quite loud and it is perfectly normal, I will listen when I eventually get five mins to pop over... make me a list! 1. Door rattle 2. Screen squeeking 3. Humming noises