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Thread: N75 vacuum control - varying pressure?

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  1. N75 vacuum control - varying pressure? 
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    Hi,

    My T5 1.9 BRS is low on power when accelerating. Turbo kicks in for a period of time but slowly phases out, especially when running up hill. Pushing the accelerator harder forces the N75 to open the turbo again but it slowly backs off.

    Heres a video of the issue - T5 BRS N75 vacuum issue - YouTube

    Monitoring the vacuum pressure inline with the N75 to VNT hose shows that the pressure quickly jumps to normal levels but then eases back before jumping up again. I previously found the hose connecting to the vacuum pump was leaking a little so replaced it, but I can’t find any more leaks in the system and so I’m now at a loss! What the fudge is happening!?

    Thanks in advance for any advice.
     
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  2. Re: N75 vacuum control - varying pressure? 
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    Do you get an odd quickly fading humming noise when you turn the engine off!
    Anthropogenic climate change, the biggest con inflicted on mankind since religion...

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  3. Re: N75 vacuum control - varying pressure? 
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    No, but the occasional “moo” on depressing the clutch. I’ve previously replaced the N75 but suspect a dirty filter on the N75 might be causing that “moo” when the vacuum to the VNT is bled off.

    On the varying vacuum issue, I found that unplugging the EGR removes the problem.

    Turns out that a varying vacuum to the VNT has an effect on exhaust back pressure when the EGR is open, so that explains why I don’t get the issue when the EGR is unplugged and thus closed. (https://people.bath.ac.uk/enscjb/FuzzyLogicControl.pdf)

    Problem is, I don’t know why the vacuum to the VNT isn’t stable - I can’t find a leak and the tandem pump pulls a good vacuum.
     
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  4. Re: N75 vacuum control - varying pressure? 
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    Have you applied a vacuum to the 3mm source hose and check the servo or hose is not loosing vacuum integrity?
    Anthropogenic climate change, the biggest con inflicted on mankind since religion...

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  5. Re: N75 vacuum control - varying pressure? 
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    Yes, all hoses hold vacuum, as does the reservoir on the rocker cover. One way valve operates fine, N75 is new.
     
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  6. Re: N75 vacuum control - varying pressure? 
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    Does it hold vacuum when plugged into the turbo actuator? In other words, actuator itself not point of leak?
     
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  7. Re: N75 vacuum control - varying pressure? 
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    I’ll check again but last time I tested that a few weeks back it was fine. I’m going to replace all the hoses for new too just to be sure. For a few quid it’s worth ruling out the possibility that I’ve missed a leak on one of my tests. I can’t think that there would be anything else to cause such an issue?

    Someone mentioned it could be a leaking rocker cover, though the vacuum chamber which is built into the cover is it’s own sealed unit. So unless that chap meant there could be an issue with the CCV (which I haven’t checked) then I’d have thought that possibility is a dead end.
     
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  8. Re: N75 vacuum control - varying pressure? 
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    Do you have a pump to pull the vacuum and observe for slow leak? Sealey hand-pump helped me, but frustratingly lost its ability to hold a vacuum itself a few months after purchase .
     
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  9. Re: N75 vacuum control - varying pressure? 
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    I do - just a cheap one but does the job. It’s 5mmHg out of calibration but can still observe changes. Very handy bit of kit.
     
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  10. Re: N75 vacuum control - varying pressure? 
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    Do yourself a favour get yourself a mity-vac pump & a small vac gauge













































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































































    you would need a mityvac pump & a small vac gauge take a vac reading at turbo actuator at start up the readings should be approx 26/27 inhg.This must be your first port of call. Your n75 is easily enough checked out either on or 0ff the vehicle when vehicle is first started turbo rod should be fully retracted at 18 inhg
     
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