Reply With QuoteThey dont hold 15PSI to the redline Kris.Originally Posted by CupraGTi
Volkswagen Fan
SOOOOO...
you fix one thing and find another problem (or perhaps I was the problem all along....)
New spark plugs, back to having n75 in total control of boost, and in high load (4th,5th gear) full boost of 10psi is achieved and held for a short period before boost increases to 15psi where I let off throttle so as not to fuel dump.
Is it my wastegate flapper?
I think I might take some valium and lie down.
Dub Crazy
VWwA Super Moderator
They dont hold 15PSI to the redline Kris.
Volkswagen Fan
Hey gav,
Nah the opposite was happening with JUST the n75 controlling things... really weird actually. It was hitting the requested boost of 10psi, holding it for a few seconds, then as exhaust flow was increasing the boost increased from max requested.
I jumped the gun.. its not the wastegate or wastegate flapper.. I think I've really really upset the n75 (Gav I know what you're gonna say... I told ya so)
To make sure I didn't have a problem with the wastegate/actuator/flapper, I plumbed the MBC only in to take control of the boost and just bypassed the n75 as we have discussed before. I took it for a drive, and it held at 5psi, so I increased it to 10psi which is the standard level and took it for a drive out on the open road. It held the 10psi through all gears, with a bit of trail off of course. There was no sign at all of hitting full boost, then the boost ramping upwards of 15psi. I then wound it up to 13psi and had the same result of keeping the boost in control throughout the rev range..
I think I've screwed that n75 valve....????
Volkswagen Fan
And this is where I end my foray into screwing with things for a while I think...
Dub Crazy
VWwA Super Moderator
Make sure you have the N75 plumbed in correctly. The longest of the 3 goes in the TIP. If it's not plumed right it doesn't work.
Google a diagram if you need to.
Gavin
Volkswagen Fan
Gav,
the n75j I have looks a bit weird, but it is kinda around the opposite of stock. its plumbed up correctly. This is a pic of what it looked like with the n75 in series. Its no longer like this, the MBC is in control with the n75 out of the system. Still hooked up, but looped of at the intake and outtake nipples.
its running fine. with good air/fuel ratios at higher RPM.
Dub Crazy
VWwA Super Moderator
I wouldn't know if you plumbed the MBC correctly, the last one I touched was on Fiat Uno turbo in 1985.
Gav,
the n75j I have looks a bit weird, but it is kinda around the opposite of stock. its plumbed up correctly. This is a pic of what it looked like with the n75 in series. Its no longer like this, the MBC is in control with the n75 out of the system. Still hooked up, but looped of at the intake and outtake nipples.
its running fine. with good air/fuel ratios at higher RPM.Is that N75J a stock one? again I don't play with those as all our tunes are written for the factory hardware.
I understand you're keen to play around, I used to be the same. I predict you'll be going back to the OE set up soon.
Volkswagenist
the only way to reliably test the wastegate is to get the MBC removed completely. With it in there even if it is fully open it could be bleeding to atmosphere out of its ports. And if you suspect the N75 of being cactus then eliminate it too for now just for testing purposes. Just get the line that goes down to the wastegate actuator and connect it directly with the line coming from inlet with a joiner. Your boost should go to 6-7psi or whatever is stock for a mk4. If not then you have an air leak in the line or a shagged diaphragm internal to the actuator.
If that's ok though, then plumb your N75 back in how it was on day dot when everything was fine, but don't plug it in electrically. No MBC though. Leaving it unplugged leaves it in a failsafe open position that directs all the air to the wastegate actuator. When you drive it like this, your boost should be the same as step 1 above. If that's ok, then the N75 isn't leaking any air down the other port (atleast at 6-7psi) so you can go ahead with the next bit.
Now plug in the N75, clear the code you'll have, and drive. If its back to stock then smiles all round - you got back to square one and it was the influence of your MBC that was causing the boost surge (even when fully open). If not then you could still have a leak that only starts to loose air once you are sitting at peak boost. ie it won't leak meaningfully below 10psi, but above it it will.
let us know how you go.
sam
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