Boost stops at 1.4bar - N75?

[hunker7]

When I start it, it is .6 bar on the dash. In the link you gave me, it says if sensor is dead, it will show 968mbar, mine is at 1000 mbar.

[UrS4boy]

...when i use measuring blocks in vagcom, i get 1000mbar reading for map at idle.

1000mbar = 1 Bar = 14.5 psig = slightly less than nominal atmospheric pressure.

On idle, I get about 12 inches of mercury (Hg) vacuum. 12 inch of mercury [0 °C] = 0.406 366 64 bar so I would think that a digital absolute pressure gauge would read something like 0.6 Bar at idle, not 1 Bar. You might have a faulty MAP sensor if you don't have a line leak.

[hunker7]

Sorry, i will try to be consistent from now on.

Got a day off work tomorrow, so will try to find the source of the problem.when i use measuring blocks in vagcom, i get 1000mbar reading for map at idle.

I will check the rest of the piping tomorrow and report back.

thanks

[UrS4boy]

G71 Manifold Absolute Pressure (MAP) sensor info

Thanks guys. Checked the moisture trap hose, and i just pulled it off, changed the fastener and now it's all tight. Should I be seeing more than .4/.5 bar in 2nd gear@6000rpm?

If you want our help, please be consistent with what you are reporting as "boost". If you are using the OE trip computer boost gauge that digitally reads out boost, that is absolute pressure, i.e. the summation of atmospheric and boost *above* atmospheric.

On that basis, your question above suggests vacuum, not boost.

Sounds like you still don't have tight tubing/hose connections between the intake manifold and the Manifold Absolute Pressure (MAP) sensor in the ECU (or the N75 really is disconnected/dead).

G71 MAP sensor info:

http://forums.quattroworld.com/s4s6/msgs/20417.phtml

[hunker7]

Thanks guys. Checked the moisture trap hose, and i just pulled it off, changed the fastener and now it's all tight.

Should I be seeing more than .4/.5 bar in 2nd gear@6000rpm?

[UrS4boy]

C4 UrS Moisture Trap Info

Hi. I have a problem, my AAN won't boost past 1.4 bar (gauge), sometimes will tick to 1.5 and that's it. As before, it juddered when reaching 1.4/1.5, after I changed fuel lines, it just stops boosting, but runs without any problems otherwise. Before I had vacuum in the tank.

Ha. You've just be caught by the simplest easiest fix for lack of boost. Or at least that is what I would put my money on: The boost/vacuum hose has come off the fuel vapour ("moisture") trap on the UrS4 firewall. Re-attach small gear clamp.

http://forums.quattroworld.com/s4s6/msgs/20401.phtml

[UrS4boy]

...and how would I do that?

All you need is paper clip:

http://www.sjmautotechnik.com/trouble_shooting/ecuf20v.html#outs4

[hunker7]

Try the WGFV in output tests on VCDS it should audibly click on and off

...and how would I do that?

[Monkeymagic]

Try the WGFV in output tests on VCDS it should audibly click on and off

[hunker7]

I would say so, I'm not sure if there is another condition that causes no active boost control on the urS4/6, Dave will be along in a bit to tell us. Maybe the MFTS (Multi Function Switch) under the water manifold. You could try unplugging that and see if your boost comes back.

BTW.. Did you try blipping the throttle and seeing if the WGFV ticks?

Haven't tried anything, will do tomorrow and see what it does. As I said before, at first it had a power loss sort of thing when it hit .5 bar, but now just won't go further.

[newsh]

I would say so, I'm not sure if there is another condition that causes no active boost control on the urS4/6, Dave will be along in a bit to tell us. Maybe the MFTS (Multi Function Switch) under the water manifold. You could try unplugging that and see if your boost comes back.

BTW.. Did you try blipping the throttle and seeing if the WGFV ticks?

[hunker7]

With the WG connected to the face of the turbo then the spring will compress and open the gate at 0.4Bar of boost (1.4Bar absolute on your dash). The exhaust gases pass though the WG and into the exhaust without passing through the hotside so that boost is limited.

To get more than 0.4Bar of boost the ECU comes into play, at full throttle the ECU starts to control the WGFV.

What the frequency valve does is to modulate the pressure from the face of the turbo to the WG so the WG "thinks" it is getting less than 0.4Bar and so closes allowing more exhaust gases to pass through the hotside therefore creating more boost.

If you disconnect the WG from the turbo by pulling off the pipe at the banjo on the WG and plugging the end, it will keep on boosting for as long as you hold the throttle open till it blows up. This doesn't really prove anything though.
You either have no active boost control because the WGFV is broken or because the ECU is not giving it for some reason (which would normally show a code)

Thanks for the info mate. So my best bet is faulty WGFV, as it only gets spring boost?

[newsh]

With the WG connected to the face of the turbo then the spring will compress and open the gate at 0.4Bar of boost (1.4Bar absolute on your dash). The exhaust gases pass though the WG and into the exhaust without passing through the hotside so that boost is limited.

To get more than 0.4Bar of boost the ECU comes into play, at full throttle the ECU starts to control the WGFV.

What the frequency valve does is to modulate the pressure from the face of the turbo to the WG so the WG "thinks" it is getting less than 0.4Bar and so closes allowing more exhaust gases to pass through the hotside therefore creating more boost.

If you disconnect the WG from the turbo by pulling off the pipe at the banjo on the WG and plugging the end, it will keep on boosting for as long as you hold the throttle open till it blows up. This doesn't really prove anything though.
You either have no active boost control because the WGFV is broken or because the ECU is not giving it for some reason (which would normally show a code)

[hunker7]

Except that would show a code so maybe stuck.

Would removing middle vacuum line would allow it to boost over 1.4/5?

WGFV?

[newsh]

Except that would show a code so maybe stuck. Can you hear (or feel) the WGFV ticking when you blip the throttle?