Boost Problem

[Alex123]

Hi Guys,

Have recently purchased a '99 facelift S4 in black. Awesome piece of kit, totally standard apart from duel Milltek (sounds fantastic) and APR boost gauge.

From looking at the boost gauge, most of the time it's been pulling about 7-8 psi. Every now and again, it feels noticeable quicker when putting your foot down. When I look at the boost gauge, its pulling 11psi!
There doesn’t appear to be any pattern as to when it does this. It's just every now and again. Totally random! I assume that 11psi is what is should be doing all the time rather than the 7 or 8.

I was told that both DV's and N75's where done not long before I bought the car. Could it still be the N75's causing the problem or something else?

[Stigter]

n75 all the way - and get someone to pressure test it and check it with vagcom (VCDS)

where are you in the world?

[Alex123]

Thanks for the quick reply Stigter. I assumed as the N75's had been done that it couldn’t be that but guess it wouldn’t hurt to swap them anyway. TBB had also been done. I will def take it someone to pressure test. Maybe an MRC health check as they are not too far from me.

It's strange because I've kinda got used to it boosting at 8psi and thought that’s pretty quick (used to have a 1.8t!) then every now and again it hits 11psi and it's like wooooooohhh!

I'm in Bristol by the way.

[Stigter]

I think mine shows about 16-15psi but thats running a poo superchip. From where you are located go and see MRC and get a real remap and healthcheck.

[AdamS4]

11 psi is standard and thats what mine hits and then drops to about 9 psi at top RPM.

Mine was recently only boosting 7-8psi and i changed the N75 valve and that got my boost back.

Defo do a pressure check and change the N75 valve.

[Alex123]

Finally got my VAG-COM working and it looks like i've found the problem ...

1 Fault Found:
17861 - Exhaust Gas Temp Sensor 1 (G235): Open or Short to Plus
P1453 - 35-00 - -

I'm guessing its putting the car in limp mode. I believe these cost about £150 each which sucks! Do you think I can get away with replacing just 1 of them?

[grizz]

Just fit the one. Not hard to do .

[yorkie]

That will definitely lower your boost and up the fuelling too.

That is the easy one to change though.

Before you replace it just check the connection to it is ok.

[Turbo Joe]

At least it is not the passenger side one you have to deal with, not an easy job, though saying that it is easier if you have a ramp to work with. Driver's side a doddle, 15 to 20 minutes at most.

[am8631]

I must be a <beep> then as I couldnt get the drivers side one out! Its easy enough to find i just couldnt get any movement with my ratchet. Is there a trick i missed?

[S4]

???????????? how did you get a ratchet on it..... you would have had to cut the cable?

jack it up and do it from under the car.... if you really get in crap, undo the driveshaft..... you can see much more.


Darren

[am8631]

I cut a slot in a suitable socket so that i could slip it over the cable - but i found i didnt really have the room to then undo it. In the end i got my local garage to do it while it was in for the mot.

At least i know its easier from underneath now.

[Turbo Joe]

Driver side should be straight forward, hmmmmmmmmmmmmmm..........

[s4woody]

At least i know its easier from underneath now

underneath...are you kidding..15-20 mins top from above...no need to take off a drive shaft even to do the pasenger side...awkward yes impossible..no

[andybnwc]

open spanner and some strength i did my last one outside in the rain in about 20 mins but engine was still hot and got a few burns , not a good idea . on the other hand having them hotter may be easier to undo them?