Surge? Creep? Advice needed - now with logs!

[Dave B]

[Dave B]

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Hi All,

In the past few months a strange problem keeps popping up with the S3. If I pull away in 1st with progressive use of the throttle (i.e. not flooring it) sometimes the car will accerlerate more quickly than pedal depression would suggest until 4k rpm, then suddenly lose all power for second (jerking the car forward slightly), and then regain power and starting accelerating again? Other points to note:

- It only seems to happen in first gear
- It doesn't exhibit the same behaviour if you nail it in first
- It hasn't thrown any ECLs yet and I haven't seen any fault codes on previous scans (but I'm scanning it again tonight to confirm)
- I have also noticed that at full throttle in 5th/6th gear you can hear a slight fluttering which almost sounds like air escpaing somewhere
- On idle the revs can be very slightly lumpy (varying by 50-100rpm when warm)

I originally thought it might be DV related so the OE DV has been replaced with a Forge 008p but the issue remains the same. Mods wise, its a BAM engine in standard form apart from a B5 TIP, JR filter and Customer code stage 1 map +3 degrees timing (IIRC).

I'm not really clued up on things like compressor surge/stall or boost creep, but does this sound like it could be one those? If there continues to be no fault codes then my only other thoughts were maybe a split vac or boost hose?

Comments appreciated!
 
Last edited:
N75 valve can cause low/high boost issues. When i first bought my car my N75 valve was knackered. It would overboost in 1st gear then sometimes whilst im doing about 50mph and you floor it, there was no power until 4-5 seconds later. This fault didn't bring up any lights on the dash but luckily i had a good idea of what it was!
 
In regards to putting your foot down and the car jerks and then kicks in again, i had that issue 3 months ago which turned out to be 1 of my coilpacks on the way out! Then the weekend came along and i couldn't even drive the car as the coilpack failed completely so i got 4 new ones and it was all good to go!!:)

I have a strong feeling it's a coilpack on the way out, mine was exactly the same way you're describing it.
 
There will be another meet soon where we'll be doing vcds logging. You are welcome to come along bud. I'll have a few spare bits off my S3 to try on yours if we need to, saves you buying a MAF, N75 etc :)
 
do you have a boost gauge?

surge would only really happen at full throttle but unlikely on your state of tune.

coilpacks would explain missfire/jerk
air leak/vacuum leak would explain dodgy idle.

get it scanned
 
Have a quick look to see if this hose is split. You won't actually see the split because it splits on the inside but if it has split you'll have some oil misting on the intake manifold
P3070251-2.jpg
 
I got the same problem.I changed my coil packs less than a year ago,dunno what it could be.also no boost leaks
 
Thanks for the replies guys :)

- N75: Changed for a new one about 10 months ago, only done about 4k on it.
- Coilpacks: Changed for new ones as part of the recall about 6 months ago.
- No boost gauge, but logs below!
- (Sandip) thanks for the offer of logging but i'm sorted. Not had a look at the hose you mention, would this cause these symptoms?

Ok, so i've done quite a bit of logging. First logs below are a 3rd gear run, flat throttle from ~1500rpm:

3rdgearrun.jpg


To me (novice!) the boost looks good and so does the g/s! Not sure on the other elements? Any other blocks I should be logging?

I also thought it would be good to do some logging on part throttle in first and second gears, logs below:

1stgearrun2.jpg


1stgearrun1.jpg


The logs seem to reflect what i'm feeling when i'm driving it i.e. the boost pressure being higher than what it should be? Could this be a sticking wastegate/dodgy actuator meaning that it's not opening on mild boost but opening under higher boost meaning its fine when booted but not on part throttle? Or could it be map related?

Cheers gents,
Dave.
 
That hose was split on mine and it used to make my boost feel quite harsh on part throttle, kinda used to lurch forward. If i planted the pedal it didn't happen, literally only happened on the odd occasion when i tried to set off lightly in 1st
 
Right, that's something to check at the weekend then! Thanks Sandip.

Mods wise, there's just a B5 v1 TIP, Jetex filter and CC stage one map with some additional timing added by Bill. It does feel pretty quick on maximum attack, it just struggles with taking it easy, wierd eh!?
 
audi upgrades
Definitely looks quite strong, i reckon your boosting quite high, 24psi+

I've got a spare boost guage so if you can come to the meet we'll plumb it and have a look at your boost. Have you ever logged AFR/lamda, block 031?
 
When's the next meet?

I've converted the pressure @ intercooler to psi and graphed it against the g/s divided by 0.8. I'm not to bothered about the numbers but at least it looks to be in the right ball park.

psibhp.jpg
 
I was getting similar boost pressure like you when i had the bad map, 2550 is the max the MAP can read i think??

Mine was hitting 26/27psi at this pressure (2550)...scary

Yours is much more controlled though!
 
I'm quite experienced with bad maps actually! Originally I had a Jabba 'custom' map on which ran 26psi boost (i.e. off the clock for the cars sensor) and AFR's of mid 14's! I took a trip to Bills last Feb and he applied the custom code map with some tweaking so it peaked at 22psi and ran AFR's in the 12's, hence i'm more confident AFR's should be in check. Although I could log it if needs be.

In fact it's the original bad map with huge boost that makes me think the wastegate or actuator may be a bit goosed?
 
sandip said:
Have a quick look to see if this hose is split. You won't actually see the split because it splits on the inside but if it has split you'll have some oil misting on the intake manifold
P3070251-2.jpg
Click to expand...

So what is this hose for? What does it connect to/from?
 
sandip said:
There will be another meet soon where we'll be doing vcds logging. You are welcome to come along bud. I'll have a few spare bits off my S3 to try on yours if we need to, saves you buying a MAF, N75 etc :)
Click to expand...

When's the meet? This time I'll definately come!
 
Ok, i've had a look around that hose and can't see any splits or any misting on the inlet manifold or other surrounding areas. Any other thoughts?
 
I've been having a browse on the net and wondered if this issue could be caused by a small air leak in the hose that runs from the N75 to the wastgate actuator? Would this cause these symtpoms i.e. the actuator not seeing enough pressure to open the wastegate under part throttle and therefore not bleeding boost appropriately?

[Please feel free to correct any noob statements/assumptions].
 
Funky_Junky said:
I've been having a browse on the net and wondered if this issue could be caused by a small air leak in the hose that runs from the N75 to the wastgate actuator? Would this cause these symtpoms i.e. the actuator not seeing enough pressure to open the wastegate under part throttle and therefore not bleeding boost appropriately?

[Please feel free to correct any noob statements/assumptions].
Click to expand...

Why dont you buy a few meters of silicone and replace the little bits and see if it helps at all?
 
That's the next step but I want to see if the theory's correct before I start potenitally changing parts that won't help.
 
True. I'm starting to think the issue is better described as overboosting, pointing to n249, n75, hoses or wastgate...
 
Ok, after more investigation I found this thread on Tyresmoke: S3 (8L) BAM Over-boost problems

The problems sound similar to mine yet mine isn't throwing any fault codes. One part of the thread that interested me was:

"The N249 valve can fail or be sluggish without recording a mechanical fault but this is not too likely as the N75 is the primary control for over boost control. The N249's main uses are to regulate part throttle boost and to recirculate boost on gear change/throttle release."

I have noticed recently that the car can seem to bog down slightly on gear change, so this does point to N249 or related pipework issues. I had a look under the bonnet at the n249 and n75 pipework but there doesn't look to be any splits or misting anywhere and all the connections are tight. The only thing I did notice is some misting around a connection to the hockey puck. It looks like the clip had come loose so I've replaced it with a decent jubilee so its now nice and tight. Could this have been causing the overboost issue? I would of thought it was definately contibuting to the mildly bumpy idle? I've not been out for a proper run yet so can't report if the car feels better or not.

I've also logged fuel trims (block 032) and lambda, but only whilst reving the engine out of gear, see below:

032006logs.jpg


If i'm reading this correctly it looks like the car is running a little rich on part throttle but still within tolerance +/-10%?

I'm thinking of ordering some vac hose and doing the n249 bypass as this might help if the issue is n249 related.

Thoughts?
 
Hi funky, have you tried to do a TB alignment or ECU reset by pulling the related fuse?
I asked this because i've been suffered the opposite problem of yours wich was under boosting under 4500 rpm.
Today took the ECU completely out to clean onderneath it, connnected it and now car boost as it should 24 psi spike, 22 till' redline.
 
Ok, for those interested here are the AFR logs:

afr.jpg


Didn't manage to adapt the trottle body as I had the wrong instructions so it threw a code and errored, but I've got the right ones now so should sort that this week. I didn't notice any overboosting today but I'll see how it goes this week.
 
Funky_Junky said:
Hi All,

In the past few months a strange problem keeps popping up with the S3. If I pull away in 1st with progressive use of the throttle (i.e. not flooring it) sometimes the car will accerlerate more quickly than pedal depression would suggest until 4k rpm, then suddenly lose all power for second (jerking the car forward slightly), and then regain power and starting accelerating again?
Comments appreciated!
Click to expand...

Is there any sound associated with this lurching loss of power? If this is at part throttle, and under very low boost I think your N249 is simply not pulling the DV open smoothly. So instead of feathering boost away, it opens like a cork in a bottle. Boost is suddenly gone, manifold pressure suddenly drops, and you're thrown forward in your seat. When ask you about an associated sound, can you hear the DV fluttering open and closed? it sounds a bit like this:


 
Last edited by a moderator:
Westy said:
What blocks in VCDS give you the AFR numbers?

You'll soon be a VCDS logging guru Dave ;)
Click to expand...

I logged block 031 and then multiplied the values by 14.7 as per the info in the TTweakers guide. I've been going a little bit mental with the logging recently, I also logged knock, EGT's and intake temps just to see what's going on. I can't go out in the car without the laptop Lol..

slappy_dunbar said:
Is there any sound associated with this lurching loss of power? If this is at part throttle, and under very low boost I think your N249 is simply not pulling the DV open smoothly. So instead of feathering boost away, it opens like a cork in a bottle. Boost is suddenly gone, manifold pressure suddenly drops, and you're thrown forward in your seat. When ask you about an associated sound, can you hear the DV fluttering open and closed?
Click to expand...

Loving the mutley comparison! I can't hear that sound but that's prob because the filter is making a louder intake 'woosh' at the same time. The symptoms are very similar to your description though. I have some silicon on order to do the bypass so hopefully that'll solve the problems. It's annoying really as the car is running great other than this.
 
Could this be caused by too strong a spring in the 008p, I'm currently running the yellow spring, which according to the Forge specs should be ok....

Green - 5-15 PSI
Yellow - 15-23 PSI
Blue - 23-30 PSI
Red - 30 + PSI
 
Funky_Junky said:
Could this be caused by too strong a spring in the 008p, I'm currently running the yellow spring, which according to the Forge specs should be ok....

Green - 5-15 PSI
Yellow - 15-23 PSI
Blue - 23-30 PSI
Red - 30 + PSI
Click to expand...

Green or Yellow should be fine mate. The only time I've seen people experience issue with their DV springs is when they have big boost. Saying that though my yellow spring seemed fine on 25psi!!!

Might be worth giving the DV a service if you've never done it. I did mine last year for the first time in 3 years.
 
as I'm having the exact same problem on my AUM ill try the n249 bypass to see if that help. I know exactly what you mean when you say the cars great apart from this problem,just so annoying! I also noticed that I have got a jubilee clamp on the hockey puck valve so ill sort that out tomorrow and report back. I'm running stage 2 buy I keep forgetting to get a yellow spring.I just don't think that my car is making over 15psi of most below 3k. someone correct me if I'm wrong
 
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