s4 not running boost....

[s4 osh]

hi guys, i have been on the s4 forum for a while and cannot seem to get any closer to solving my problem, so now i am looking for a new angle on things
these are things that i have changed or inspected...
02 sensor bank 1 no codes coming back now
boost controll valve 'n75' code still there
maf sensor, never been a code but the are known to be hidden faults
checked all boost pipes including tbb and f hose

there is no restriction on revving it drives so smooth, and every now andagain she will kick in full boost will be ok for a gear change, then its gone back........

any input welcome, thank you in advance
osian griffith

 

[aragorn]

If its making no boost the N75 valve code is the one to be looking at.

What exactly is the code?

We were getting an intermittant open circuit code on the N75 on our 1.8T and it turned out that the wiring was broken. I replaced the last 12" of wiring with that from a loom from a breaker and the fault vanished and my boost returned.

It could be the valve itself though, have you tried changing it?

If the valve or wiring is broken it will run at actuator pressure, which is about 4psi.

An intermittant wiring fault like ours would likely cause the symptoms you describe, ie sometimes it will boost perfectly and othertimes theres nothing.

 

[Lee Goodall]

Yeah I'd def say either faulty N75 wiring (if the code is open circuit) or faulty N75.

I had the same issue as Aragorn but the wire had snapped right near the N75 plug/connection so I just trimmed it down and rewired back into the plug.. job done!

 

[Blue_Thunder]

You haven't mentioned if you've run a pressure test. I'd look there first.

Have you checked/swapped any of the common failure points, ie: DV's, TBB, F-hose, etc?

You should also run some Vagcom logs if you are able to.

Which S4 forum have you been looking at?

 

[Lee Goodall]

If there is a fault showing with the N75 then that will be causing boost control issues.

A boost system pressure test would obviously rule a boost leak out too though.

 

[Blue_Thunder]

Sorry, just re-read the original post and saw that your N75 code is still there.

Get it sorted!

 

[s4 osh]

no i havent done a pressure check, i really want to do it just for peace of mind, can any one put me through the steps of doing it, and an easy way of making a tester rather paying £50 for one haha.....
the n75is a new one genuine audi part,,, didnt get the codes cleared but the car has been driven roughly 500 miles, same as always.
i will check the wiring on the weekend.
cheers for the input guys, my car is running 4 psi by the way haha

 

[Broken Byzan]

4 PSI is Limp mode, you deffo need to get that N75 checked out

 

[andybnwc]

physically check intercooler pipes throttle body and F hose ie take it off and check the underside .

 

[Blue_Thunder]

You didn't get the codes cleared??


1) Get on Vagcom, go into Engine module, then 'measure blocks' then choose block 32 and post up the values.
2) Clear the codes.
3) Go for a decent drive.
4) Check for fault codes again.
5) Let us know how you get on.

 

[s4 osh]

the engine management should re evaluate all reading after 300 miles so it should have picked the new one up by now, i have checked all pipes, removed and checked all over,,,,, twice.... ill check the wiring on saturday ill let you know how i get on

 

[aragorn]

The codes will remain, even if the fault has been gone for several thousand miles...

The only way to know if they are current is to reset them, and then see which return.

 

[Lee Goodall]

I do seem to recall reading that the ECU will only keep codes stored for 300 or so miles.. So in theory if you've corrected the issue the codes will be erased.

 

[Blue_Thunder]

the engine management should re evaluate all reading after 300 miles so it should have picked the new one up by now, i have checked all pipes, removed and checked all over,,,,, twice.... ill check the wiring on saturday ill let you know how i get on

I'd not heard that.

300 miles is a lot of laps round Anglesey! lol

I picked up my S4 from Anglesey. Spent it's first 8 years there.

 

[s4 osh]

wel hopefully i will have vag com tomorrow to see what the hell is going on lol, was it from holyhead you got the s4 from, negaro blue???? if so i believe thats a good buy

 

[andybnwc]

nogaro

 

[Blue_Thunder]

No, mine's a Minger. Bought it from a guy called John in LLanfechell just over 2 years ago.

 

[s4 osh]

coolio i know someone had a nogaro in holyhead, but ihad no idea bout the one in llanfechell, my missus's step dad lives there... small world