S4 overboost problem

[Delroyb]

Hi Guys.

I am having some considerable fun and games trying to eliminate a max boost pressure exceeded code on my B5 S4. The car is running a MRC Stage 1 map, apart from that it is bone stock. When i bought it, it had a couple of codes I knew needed dealing with, egt's were shot etc. With everything replaced, including plugs which eliminated the under load misfire, I thought all would be well. Alas not!

If I floor it hard in second gear, the boost spikes to about 2200mb absolute, so 1.2bar over atmospheric. This kicks in a overboost code as the ECU is only requesting about 2050mb.

Now I have done a pressure test, all of the big hoses etc are fine. Initially I thought it would be a leak on the wastgate hoses, however, there is no noise/hiss coming from lower down. I am getting a hissing noise from the area of the N75 valve once the pressure goes over about 12psi. It does not however seem to be a leaking to the atmosphere. I coated everything in soapy liquid and there were no bubbles anywhere to be seen!

Could the N75 be leaking internally? I am abit stumped by it at the moment, and if I cannot find the source I will proably just end up making the schlep to MRC to get them to have a look.

Any suggestions greatly appreciated.

Ta

[Delroyb]

Another point, checked resistance accross the pins on the N75 and coming in at c.60ohms. I read somewhere that it should be 25-35 but that was for the N75 on a 1.8T, are they the same spec?

[Madmonkee]

MAP sensor iffy? I know on tuned VXR's you had to think about changing the MAP sensor to a 3bar sensor as the mapped car used to boost to pressures very very close to the limits of the OEM Map sensor, used to cause issues similar to this.

[Delroyb]

A quick update on this problem. Got around to pressure testing the feed lines from the N75 to waste gates. O/S wastegate fine, pops open no porblem, n/s had a loud hiss. Checked the hose over, no holes. Then felt that there was air leaking from the acctuator body itself. Arse. If I blow down the line it won't happen, but a 15psi blast with the airline and it is clear as day.

This would explain the over boosting, one wastegate is not keeping up with the ECU, and is slow to open hence the spikes.

Now the question is, has anyone changed an actuator in situe, after a preliminary investigation it looks like a right ball ache! Someone must have had a similiar problem