S3 8V backing off a bit on full boost

[Damo S]

Hi All,

I posted about this a month or so ago with my car being a bit intermittent regarding backing off a bit when on full power. It was doing it quite a bit again this morning, and I noticed for the first time that when it does this the boost gauge drops down to 7 lights on rather than 8. Ive never noticed that before as Ive usually got my eyes on the road on full chat, but Im wondering if this sheds any more light on what the problem could be.

So the symptoms are the on full throttle the car will come on boost fine, and then after a second or so on full boost it backs the power off a bit, and the boost gauge drops from 8 to 7 lights on. Come off the throttle and repeat and it will do it again.

TIA

 

[AlS3BE]

What gear are you in?
I think the s3 reduces boost in 1st and 2nd.

 

[Damo S]

6th shows the problem best, doing about 70mph in 6th and then floor it

Yes it definitely runs lower boost in 1st and 2nd

 

[AlS3BE]

A few reasons the car might back off boost.
Misfire/pinking caused by bad plugs, coils etc
Over-boost, sticky actuator, had this problem on a few TDi’s not sure if this can happen on petrol turbos.
Traction control kicking in to slow you down as it detects slip.
These are a few thing off the top of my head.

 

[Damo S]

"Misfire/pinking caused by bad plugs, coils etc"

This was my first thought. I'll get the garage to do a full diagnosis on Thursday to see if there are any fault codes logged. If not it might be a case of replacing bits until it goes away, but that could be a lot of things (also injectors, fuel pump, duff sensor)

 

[AlS3BE]

"Misfire/pinking caused by bad plugs, coils etc"

This was my first thought. I'll get the garage to do a full diagnosis on Thursday to see if there are any fault codes logged. If not it might be a case of replacing bits until it goes away, but that could be a lot of things (also injectors, fuel pump, duff sensor)

Starts getting expensive replacing bits. Plugs and coils are perishable type items so depending on mileage could be worth replacing but swap them between cylinders first and see if it moves to another cylinder. If it does it confirms bad plugs or coils.
I would go with the cheap service items like fuel filter, air filter and while you at it give it a good oil change. A good service never hurts.

 

[Damo S]

Car is 25k miles

Yep, you are probably right. I've got a pipercross panel filter to go in. I'll fix that at some point. Had no end of trouble with coilpacks on the old 3.2 so I might just bite the bullet and replace all of them. There is no evidence of misfires in any cylinders yet although it may well be running lean which would indicate fueling issue or possibly a failing PCV valve (which I read about earlier, and have several of the symptoms including lumpy idle)

 

[AlS3BE]

Is the pipercross an oiled filter?
If it is it can give all sorts of problems if it’s been over oiled.
Other thing my mate mentioned, not sure if it applies to s3, if it has a common fuel rail even though it’s only misfiring on cylinder 4 it could be something to do with fuelling across any of the injectors. If one is weeping/leaking or the injectors seals have worn/broken then the fuel pressure is not high enough so the one on the end of the rail normally loses out.

 

[Damo S]

yes, but I havent put it in yet, plus I wasnt planning on ever oiling it (just buy a new one when it gets dirty)

 

[Damo S]

Ive now got a OBD dongle and Torque Pro app. I was using it for my commute home and according to that my car is running around 14psi on WOT (well below the 17, 18 Ive read that it should be). Then when the car replicates the intermittent boost problem as per thread it instantly drops to 10.5 psi. No wonder it feels pants but also confirms what I thought and that I'm not going mad.

Coolant temp does seem to be consistent with what the chap said in my other thread. So, some problem but I dont know what, but it seems that the turbo is not being allowed to boost as it should ALL of the time and sometimes its worse than others.

 

[AlS3BE]

What fuel are you using? Could be poor fuel or low octane fuel and the ecu is winding back the timing to compensate.
Real way to see is run it on a dyno to see if it’s making the correct power and at the same time check the afr see if it’s a fuelling issue.

 

[Damo S]

Ive been using tesco momentum for a few months now, which actually feels better than the tank of v power I ran it on.

A bit more testing this morning, where it replicated one instance of it hitting only 10psi in 6th (after a 5 mile stint of crawling at 40mph in 6th). Looking at the data there were a bucket load of misfires (195) on cylinder 1, and none on the others.