AAN Boost Drop Off ..grrrrrrr

[yodaboy]

Hi

I'm slowly working through some problems with my UrS4, thought I'd mention some issues here on the off chance that another S car nut has experienced the same or similar issues but worked through it...

When I floor the S4 in any gear, once I hit 5,200 revs the boost drops right off, then at around 6,200 revs it picks back up again

If I accelerate progrssively and dont floor it (again in any gear), it passes the magic 5,200 point with no problem, but of course I dont get to experience maximum power

Every now and again, it will fly through the rev range and when it does It completely catches me off guard, its flippin rapid trouble is this has only happened half a dozen times in 12 months

Its a bit of a pain to be honest, overtaking in third (especially on our local A roads) is a bit of a pain as you have tio change quickly to keep boost and then rebuild power in 4th. To date I have been changing gears before hitting 5,200 revs to keep boost on the go, but there's no way I'm going to annoy M3s that block my mission on the outer lane of the ***.....lol

The car has full audi or specialist service history, standard set up and has 187k on the clock....to date I have with help from friends replaced the following:

all coils with wiring harness with OEM replacements
oxygen and knock sensors (blue/green)
ignition / speed sensor
cam shaft sensor
fuel filter
Vag Com shows no issues
No obvious boost pipe leaks
No leaks in the scorpion exhaust system

The only other thing I can think of is that the MAF may be playing up...but I have no idea re how to test this.

Any suggestions welcome

It's spoilling what is otherwise one of the best cars I have ever owned

Cheers

Dylan

[Ruffiano]

Knock sensors?

[yodaboy]

Knock sensors?

Hi, yep have changed knock sensors

Have also replaced the blow off valve with a new one.

I think the next place to pull apart is the MAF

[CREASE]

Have you done a boost leak test, they're not always obvious! Check all the vacuum lines also-mission but a leak in one the these critters can be just as bad as leaks in the pre IM boost hoses

Try swapping out the n75 to eliminate that aswell

[yodaboy]

Have you done a boost leak test, they're not always obvious! Check all the vacuum lines also-mission but a leak in one the these critters can be just as bad as leaks in the pre IM boost hoses

Try swapping out the n75 to eliminate that aswell

Thanks for the welcome CREASE

I will take another look at the vac' lines and boost hoses

The N75 was replaced by qst last year as the previous owner asked them to diagnose the fault (they also swapped the ecu over on the off chance) at that time the fault was intermittent hence the problem wasnt diagnosed

Its going to get a good service this weekend so I will have a good look around

Thanks for the suggestions- much appreciated

Dylan

[Audi-Porsche Avant RS2]

Hello, your problem is when overboost turn on I think?

Alessio

[yodaboy]

Hello Alessio

Yes, overboost stops at 5,200 revs when throttle is wide open. It comes back after 6,200 revs and then carries on until the red line.

If I open the throttle very slowly overboost is OK across the rev range..

Its all a bit weird

[yodaboy]

Thanks for all pointers to date

Today I pressure tested the system for leaks..... and no leaks found

I have cleaned each electrical switch I can find (and access) and twisted each pin to give better connection.

But the problem persists

Anyway, after 9 months of trying to sort it I've given in and booked the car in with a local specialist next week, if they get to the root of it I'll post the outcomes as it may help someone else in the future

[S2central.net]

throttle position sensor maybe - or bad connections there - could be something flaky in the loom if its intermittent like that. Those are often difficult to find.

[istoo]

Hello, your problem is when overboost turn on I think?

Alessio

I like this train of thought, seems to make a lot of sense. LEt us know how you get on Dyl, I have a sort off similar issue with mine.

[yodaboy]

Car was given a good going over yesterday by local garage (Prestige on Anglesey)

Ive had lots of help in trying to get the problem sorted and as you can see from my initial post we had changed quite a few parts over for known to be working or new items...but to no avail.

Anyway it turned out to be quite a minor thing in the end. VAG COM kept coming up with cam shaft sensor codes, so I agreed to replace the sensor with a new OEM one.

While they were doing this they checked the timing and found that it was out. So they fitted the new G40 and corrected the timing and hey presto....boost works A1 and car runs like new....oe better

I cant tell you how relieved I am to have this problem sorted, I can now experience this iconic engines power across the entire rev range and it feels so good. After 10 months of sometimes frustrating ownership, I think I've turned the corner.

Tomorrow I'm going M3 baiting lol

Big thanks to Ryan

Dylan

Next on the list are:

Sump Gasket
Rocker cover gasket
Rear handbrake cables
Rear top outer joints

[S2central.net]

Cool... Its wise not to ignore fault codes then !

I'd wager a G40 was unnecessary if the mechanical timing was out, but its a good result !

[yodaboy]

Yep the original sensor was fine, but I'm guessing the timing wouldn't have been spotted if the cambelt hadn't come off for the new sensor !-)

Anyway old (18months old to be exact) sensor will be on ebay tonight lol

I'm over the moon though B-)

[HMS-S6]

I have a very similar problem, if it was the timing, how did it start as intermittant?

[yodaboy]

The drop in boost would only happen under WOT between 5 and 6 thsnd revs, when I first got the car it would drop boost in all gears, then after changing the original cam shaft sensor it would work in second gear but none of the others. Xomparing it now to how it was, its aa lot samoother, starts better, doesn't miss and goes like stonk....B-)