B5 S4 intermitant overboost (with logs)???

[Biggied]

Hi all, I'm usually over in the 8L section but have been looking @ a mates B5 S4 for him today & done a couple of logs with another mate whos a member here (Cheers Ross).

The car is having intermitant overboost problems so me & Ross did a couple of logs today, the car intermitantly overboosts then logs a fault code for it & holds back quite a bit until the fault code is cleared. The car has an unknown remap, uprated recurculating valves & a full de-cat exhaust system. The logs were done in 3rd gear, full throttle straight after clearing the code, after both logs it stored the overboost fault code & held back boost giving a lack of power.

I've a fair bit of Audi know how but I'm quite new to logging, as far as I can make out everything alse in the logs looks ok but I think it looks a bit slow to come on boost & then the actual exceeds the specified a fair bit which is why it logs the fault code.

With the fault being intermitant I'm thinking its either a problem with the N75 being a bit sleepy or mabey the wastgate rod/rods sticking or a faulty wastegate actuator, although mabey not the actuator as a fault in the diaphram mabey more consistant?

Can any of you guys look @ the logs bellow or suggest any further logging/testing we could do or any other possable causes you could think of? Any help would be much appreciated.

Cheers Ewan.

 

[Blue_Thunder]

Yeah, would be looking at the N75 as the culprit.

 

[aragorn]

You want to do a full boost leak test before changing anything.

 

[rossbrownlee]

You want to do a full boost leak test before changing anything.

When we were logging it was consistently making good boost pressure on every run apart from when it kicked in the max boost press exceeded and went to limp home mode, what are you suspecting regarding doing the pressure test?

 

[Biggied]

Thanks for the replys guys, as Ross said boost leaks isn't somthing I've even considered as the car will boost quite strong when it does but then intermitantly overboosts & hits limp, this symtom is somthing I've never associated with a leak, what makes you suggest a leak test aragon?

The more I think about it the more I'm leaning towards a sleepy N75, or are you suggesting a leak in the boost pipe going to the N75, the leak acting like a bleed valve as such & letting the car overboost?

Cheers Ewan.

 

[andybnwc]

Any kind of overboost and limp mode is usually due to boost leaks in these cars unfortunatly . Cou;d also be a bad map requesting too much and causing a throttle cut . First things first do a leak test it will eliminate a lot of problem areas.

 

[Biggied]

Ok quick update on this one, I've had this car back to me now for further investigation into the over boost issue, right enough as you guys said I've found a leak in the pipe going to the actuator on the bank 1 turbo (driver side), I've followed the pipe down as far as I can and it apears to be split right @ the end next to the actuator, theres a metal pipe & then a short rubber pipe before it connects to the actuator, I can't see much from the top even with the airbox out but it would apear to be the short pipe anyway. My question is have any of you guys replaced one before? obviously its real tight for space down there & I've yet to get it jacked up to look from the underside but if anyone has any tips on getting into the little swine it would be much appreciated.

Cheers Ewan.

 

[aragorn]

Nothings easy to access on these engines, but hopefully you can get it from underneath.

Once fixed do another test, as there are quite a few common points that leak including the F hose and Throttle Boot.

 

[28cmod]

Nothings easy to access on these engines, but hopefully you can get it from underneath.

Once fixed do another test, as there are quite a few common points that leak including the F hose and Throttle Boot.

Sorry, but that is not accurate. A leak in the throttle body boot will in no way cause overboost.

 

[andybnwc]

Split throttle body boot is one of the most common causes of overboost and limp mode usually found undrneath when removed , obviously the engine never actually gets the boost and will over compensate and the turbos work overtime goes into limp mode and trowing the code max boost limit exceeded .
Also it can be intermitant and leaks can sometimes not show until certain pressures in the full system are met.