[RS4-Limp mode] Still problems

[Enne&Effe]

Hello,
I've tought to have solved my car problems: there was a broken Dump Valve and an air leak.
The car worked fine for 2-3 weeks.
Now the car sometimes goes in limp mode with a great loss of power...so I've to turn off the engine then turn on and the cars works fine.
The VAG-COM says:
Charge pressure control poritive deviation intermittent

Bank 1 fuel trim (Add) too lean: intermittent

Bank 2 fuel trim (Add) too lean: intermittent.

Any ideas? The MAF was changed the last year.
What about the N-75? Can I chek it or is better tho change it?
Thanks in advance

[Jani]

Best replace the N75. You can also clean it if you are handy. Spray with a brake cleaner (inside), and simultaniously apply 12V voltage to it, on and off, to make it move. Keep doing it until it clicks loudly, and brake cleaner doesn't get dirty anymore.

It tends to get clogged with breather oil, making it move too slowly -this would cause too high boost.

Could be something else too.

[simple1]

Jani
Best use a 9v battery, the current for N-75 is actually 9.2v so be careful. The system for cleaning it remains the same.

[Enne&Effe]

Yesterday evening I changed the N75 boost control valve. I tested the old one with 9V battery: sometimes it remains closed. The second aspect is this: if I pump air from the bottom hose it escapes also from the hose that has to be closed if no voltage is applied. On the new one the air escapes only if I apply 9v on the terminals. So the old valve seems to be sticky.
Then I drove the car for about 70 km (most of them at WOT - wide open throttle) and I haven't found any problems. I've only a question: the boost gauge on the VAG-COM sometimes reaches 2.0 bar (including atmospheric pressure) while I'm in third gear (for example) at WOT at engine rev limiter (7.250 rpm)...then - obviously - it goes to zero. The most of time the upper limit is about 1850 mbar. By the way the VAG-COM gives me always "0 errors found" and the car works great! Also the N75 duty cycle is normal and at WOT it doesn't reach the 100%: that means no boost leaks.
Any comments?
Thanks in advance.
Nicola - Italy

[Jani]

Do you run stock by pass valves? Maybe they are shot -seem to be sticky or slow in operation. That is why the boost goes up when you lift the throttle or hit the limiter. If you have Forge, then I'd say the valve is in need of cleaning and lubrication. Or is this normal? I think a correctly operating BPV should not let the boost spike.

Nice to hear your problem seems solved.

70 km WOT, well you won't be in the furthest by a tankful -thread then

[Enne&Effe]

Well...I'wasn't always at WOT...by the way the trip computer shows 14.9 ltr/100 Km.
My car has now 2 new Bosh 110 Dump Valves that were changed 3-4 weeks ago. The "limp mode" was really sporadic. Infact 5 days before changing the N75 I reached (on the highway) the rev. limiter in 6th gear reaching 280Km/h on the dash with no porblem at all. So I supposed I've a "gremlin" inside my engine. Can the old N75 be the source of that sporadic "limp mode"? Now with the new N75 on the VAG-COM I don't see spikes: the boost pressure rises as exponential curve without "spikes". If the correct boost pressure has to be equal to 1850 mbar +- 200 mbar my graph should be correct in some circumstances: for example at WOT at rev limiter. Any confirmations?