Boost issue / Car feels flat UPDATED: pressure test result

[marty.h]

I have a 1999 S4 and the vehicle will boost and perform well then it goes flat with no boost and when i plug vag com in i get the code

17965 charge pressure control. Positive deviation
P1557 35-10 intermittent

So far i have checked all the VAC lines and have replace the rubber Y pipe to the throttle body as that was split and used the revised clips that was 3 days ago and i was hoping that was going to be the fault but clearly there is another problem somewhere.

The car its self drives faultlessly it does not smoke its runs very smooth and has good power delivery until i get that fault it does not put up a MIL light it just drives flat. It has a full Audi history and has been maintained

My next step today is to pressure test the boost circuit to see if i can find any other leaks.

The car also ocationally stumbles as you back of throttle i was thinking this could be down to warn re-circ valves.


So im thinking of replacing

Both re-circ Valves
N75 valve


Im fairly new to audi ownership so im still learning about them i am a qualified mechanic and am finding this quite frustrating not being able to pinpoint it

If you have any suggestions i would apreciate it. i will also report back later if i have any findings from the pressure test

Thanks for you time in advance
Marty

 

[aragorn]

Boost leak is defo the most common cause of this issue.

 

[marty.h]

I have done the pressure test and have found a leak its on this pipe

That pipe lives as you look at the engine down to the left of the throttle body and the hard pipe then runs to a valve at the back of the block not sure what that is for ill have to look into it the part number on the pipe is 0781333 784 AJ ill have to order one monday now and ill pressure test the system again after just hope this is the last leak i find in the system and hopefully it will cure the P1557 code i have been getting


cheers
Marty

 

[Blue_Thunder]

You can delete the F-hose completely and it's a fairly common mod to get rid of this failure prone part. I had mine done about 5 years ago. Do a search for 'F-hose delete S4' for details.

 

[marty.h]

Thank you very much i will go have a poke about

 

[marty.h]

I have been having a poke about and came across the N249 delete as well is it worth me doing this at the same time

thanks for your time
Marty

 

[marty.h]

I have done the F hose delete but im unsure where to put the pipe that comes of spider pipe and runs under inlet do i vent it to atmosphere thats how it presently is and it goes spot on or do i T it into another pipe any info would be apreciated

Thanks
Marty

 

[Blue_Thunder]

No, it doesn't vent to atmosphere. Can't remember off the top of my head, but can check on Monday.

Think you need a t-piece at the back of the inlet manifold and route it to one of the pipes which used to run to the F-hose.

 

[Blue_Thunder]

Also worth doing N249 delete too. One less component to worry about failing, you can get rid of a load of pipework and the part throttle control is improved (as DV's don't dump all of the boost when coming off WOT to part-throttle).

 

[marty.h]

thanks if you could let me know where to T that pipe it would apreciate it

Thanks for your time
Marty

 

[Blue_Thunder]

Hi Marty, got some good pics today.

First is the replacement hose which replaces the F-pipe itself (the pipe used here is an OEM RS4 one, but you could make do with any silicon hose):

Second off is the T-pipe. You can see where it t's into the line which used to go to the N249 (which you will also see is deleted). This T's into the spider hose pipe which used to run under the inlet manifold to the pesky F-hose.

 

[marty.h]

Thanks for them pics mate much apreciated. The pipe that use to run under the inlet i have cut it back so its now at the rear of the engine and have re-attached the 1 way valve to it at least now i can see where it goes once again thanks for taking the time to get me a picture. so i take it on the aother valve delete its a strait forward join up

Cheers
Marty

 

[Blue_Thunder]

Yep, the N249 delete is simply the other two pipes coming out of the T. You are effectively allowing the DV operation (see blue hose clip on bottom right of tyre photo) to be controlled directly from the manifold pressure/vacuum (rightmost hose going into the back of the inlet manifold).

 

[marty.h]

Thanks for your advice and help much apreciated

 

[marty.h]

This is how i have done it i have also replaced any other warn vac pipes i could see have road tested it and going well. Done a vag-com log peak boost and holding 1860 3rd gear WOT no fault codes prestent

Let me know if i have done anything wrong thanks for your time

Cheers
Marty