Intermittent boost...what to check

[willg54]

Just got upbto derby and I'm getting some intermittent boost.

One pull will be 10psi, next normal at 20ish. What should I check? Unfortunately I can't pull codes atm so doesn't make life easy.

I'm thinking...

Maf
N75
MFTS
Boost leak?
Waste gate
Knock sensors

Any other thoughts
Cheers

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[UrS4boy]

No use thinking until you can pull the codes. Otherwise, it is a wild goose chase. Starting with the codes (even if there aren't any) helps.

[willg54]

Need to get my ecu comms working then...not great timing!

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[Error404]

Will I am about 40 mins from derby I have a stock ECU here

[willg54]

Where abouts? I'm hopefully picking one up from Keith soon to try

Also meeting tonks tomorrow to pick up some carpets!

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[Error404]

Stone, Staffs

[willg54]

Not too far but won't have time tobdo that tomorrow, I'm on meet the girlfriends family weekend!

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[newsh]

Is the diag 12v fuse ok? You could try pulling blinks.

[willg54]

Yea I checked the fuse and it's fine. Can connect to abs unit fine.

Getting a stock ecu off Keith tomorrow so will give that a try.

I remember there being a boost hose finder the back seat, am I right? What does that do?

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[martinws]

I have a similar problem and pulled blink codes and got nothing. Im very interested in your outcome.

[newsh]

Boost hose under back seat is vac line to reservoir for diff lock actuator. There's a one way valve there as well.

You could try giving your maf connector a clean.

Could be a loose knock sensor. Intermittent MFTS

My ABY did this until I put new rings in it though This I think was because it would suck oil up past the rings on overun causing knock.

[willg54]

Not that then

I've tried cleaning the connectors so I'll see what happens in the morning

I'll give it all a good check over and try a diff ecu!

not using oil so not rings!

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[newsh]

Make sure you haven't got any air in the cooling system too.

[UrS4boy]

I remember there being a boost hose finder the back seat, am I right? What does that do?

As newsh suggested that "boost" hose under the back seat is the vacuum line for the rear diff lock. There is a check valve, that when it fails, you get diff lock on boost, (not a good idea in the middle of a corner). More info in the link:

http://forums.quattroworld.com/s4s6/msgs/170940.phtml

[martinws]

Could be a loose knock sensor. Intermittent MFTS

My ABY did this until I put new rings in it though This I think was because it would suck oil up past the rings on overun causing knock.

Would unplugging the MFTS throw up a code and cause low boost?
I have a PRJed ECU that should run 23psi but it seems to only boost to around 15 or lower sometimes and only ever goes up to 20ish when starting at about 20mph in 5th with foot down and even then it makes a weird noise.

I thought my rings could be away as well as it uses a bit of oil. What boost were you running when you changed the rings?