3rd Gear Stutter

ninjag

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A bit of a strange one this and I was wondering if anyone can point me in the right direction for what to check first so I can start a process of elimination.

When accelerating with just a moderate amount of power sometimes I get a weird stutter, only for a split second and often only just once - but sometimes it can be like a double jerk. It's enough to be noticeable. I'm pretty sure now that it's only when in 3rd gear, possibly in 4th gear once or twice. It's not when it's changing between gears, only when actually in gear.

So far I've got this:
  • It doesn't happen under gentle or hard acceleration, only when doing a specific amount of moderate acceleration.
  • It happens whether the engine/transmission is hot or cold (but I notice it more when cold probably because I'm accelerating mildly)
  • It happens in Drive, Sport or Manual whilst staying in 3rd to replicate it.
  • I think it is always below 2000rpm when it happens but I'll need to check this more.
  • Oil & filter, air filter, spark plugs, transmission fluid and filter etc all recently done
  • I initially used BP 97 but changed to Esso 99 but now just filled up with Shell 99 and will stick with that. It's always had this issue though.

I think I can rule out coil packs as it's fine under hard acceleration? Could this be the supercharger clutch starting to engage at some awkward/hesitant point and causing this? Please don't say it's the DSG!!! Or is this just one of those weird S4 quirks like the 1st gear clunk which the billet thankfully sorted?

I've got icarsoft and Autel scanners which can read Audi codes etc but I don't have that special VAG one everyone seems to have. Could anyone advise on the best modules/areas to look at first? (s/c clutch current?, fuel trims? etc)

Any advice would be greatly received and I'm a little stumped at the moment!


Many thanks
 
Could this be the supercharger clutch starting to engage at some awkward/hesitant point and causing this?
Possibly. Do you have ADS? I know that the clutch is always closed in dynamic but I can't remember if just Sport mode keeps it closed - but I don't think so. Personally I have never "felt" my clutch operate - and I can even see it happening on my FIS-Control MMI where I monitor supercharger revs. However I believe I remember having a discussion with at least one forum member (maybe not this forum) who claimed to be able to "feel" the clutch operate.

If you want to monitor your car's health and do some of your own diagnostics instead of relying on a service centre then there is no substitute for VCDS.
 
Possibly. Do you have ADS? I know that the clutch is always closed in dynamic but I can't remember if just Sport mode keeps it closed - but I don't think so. Personally I have never "felt" my clutch operate - and I can even see it happening on my FIS-Control MMI where I monitor supercharger revs. However I believe I remember having a discussion with at least one forum member (maybe not this forum) who claimed to be able to "feel" the clutch operate.

If you want to monitor your car's health and do some of your own diagnostics instead of relying on a service centre then there is no substitute for VCDS.

Thanks for the reply and I think I'm with you on it not being the s/c clutch. I was watching things more closely today and I think the reason I see it more in Drive rather than Sport is because the revs are usually lower in Drive (unless I'm deliberately trying to make it happen in Sport at low revs).

I also felt it in 4th and 5th today but it was less of a stutter and more of an 'about to' stutter, as in feeling a little slower to pick up power - almost like turbo lag. It didn't happen when working through the gears during acceleration, but when letting the revs drop and it stays in 4th or 5th and then I accelerate normally. Sorry if I'm not explaining this very well.

I'm pretty convinced now that it only happens under 2000rpm, I'm not sure if that helps? It definitely doesn't do it when accelerating very gently, as in just touching the throttle. It's not overly concerning me, but I do like to catch thing early if possible.

I'm trying to think what could cause this at low rpms, maybe something sticking? Another thought is the Manifold Port Injectors, I'm sure I read that they are the only ones used under normal load and the Direct Injectors only come into play when needing more power? It's had the injector recall done, I'm not sure if that's MPI or DI?

Looks like I'll have to get VCDS. I think someone posted a link to it here before, I'll have a look.


Thanks

EDIT: Oh I just remember something. I wonder if this could be early signs of the PCV valve? I do remember seeing a sporadic code come up for it last time I did a code check.
 
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Possibly. Do you have ADS? I know that the clutch is always closed in dynamic but I can't remember if just Sport mode keeps it closed - but I don't think so. Personally I have never "felt" my clutch operate - and I can even see it happening on my FIS-Control MMI where I monitor supercharger revs. However I believe I remember having a discussion with at least one forum member (maybe not this forum) who claimed to be able to "feel" the clutch operate.

If you want to monitor your car's health and do some of your own diagnostics instead of relying on a service centre then there is no substitute for VCDS.

Apologies, I missed this. I don't have ADS, just the standard Drive and Sport on the transmission (plus manual by popping it over to the left).
 
Just thought I would update this, thanks to advice from Deutschtek in Glasgow who said it could be some water in the fuel, I ran through two bottles in two tank fulls of that Wynns Dry fuel and it's solved the stutter.

I now need to find out the source of this water and if it's a petrol station I go to or if it was the previous owner somehow.
 
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Just thought I would update this, thanks to advice from Deutschtek in Glasgow who said it could be some water in the fuel, I ran through two bottles in two tank fulls of that Wynns Dry fuel and it's solved the stutter.

I now need to find out the source of this water and if it's a petrol station I go to or if it was the previous owner somehow.
A strange one. Wouldn't of thought the water would only affect one gear.
 
A strange one. Wouldn't of thought the water would only affect one gear.

It ended up being other gears as well, I think 3rd was just most noticeable as that was often the gear I would be in when accelerating in such a manner. I'm happy this has been such an easy fix.
 
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It ended up being other gears as well, I think 3rd was just most noticeable as that was often the gear I would be in when accelerating in such a manner. I'm happy this has been such an easy fix.
Nice easy fix.
Would be interesting to know how the water is getting in.
Filler cap & fuel station seem to be main suspects.
 
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I’ve had a dodgy filler cap in the past, albeit in a different car but that allowed water into my diesel at the time, discovered it in the filter.
Rightly or wrongly I was also told that condensation can build up in the tank so an additive should be used periodically.
 
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Interesting you both mention the fuel cap because that little drain hole seems to be blocked in mine. The outer flap isn't water tight either, well from a hose anyway lol I will be keeping my eye on it.

So far it's continued to be fine and I'm using my same local petrol station, therefore, at the moment I'm going to assume it was the previous owner but I think I will use a bottle of this Wynns Dry Fuel every few months as it seems to be one of the few additives which I've seen obvious results from. Cheers.
 
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I just remembered something! When I first got the S4 I used my usual BP and their BP Ultimate 97 for quite a few tanks but the car didn't like it. I figured the octane rating wasn't high enough so I switched to ESSO 99. But my previous car was a Jag XF I only ever used that same BP garage for their ultra diesel and the XF had a water in fuel sensor which pinged a few times after filling up there.

I remember about five years ago their pumps were very old and knackered, so they sourced some second hand ones to replace them with. I'm now wondering if their tanks underground aren't too healthy? I'm certainly going to avoid that garage though.
 
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