HCGxKaLiBeR

If you had the engine auto stop on. I think the car may got confused in stop and go traffic when you had both feet on the paddles.
Me personally I always disable engine auto stop. I only use both feet when doing launch control, not anything else.

MGBerridge

The trail of responses seem to be about both turbo lag and poor throttle response. I think they are two separate issues. I'd like to add my experience with a 2018 S4 in the US. I have occasional poor throttle response and it is nothing to do with turbo lag. Turbo lag means that the response to opening the throttle starts a bit slowly and builds as the turbo spins up. I'm used to turbo lag in other cars and the S4 doesn't seem unusually bad. My problem is that intermittently, when I open the throttle, it will initially respond and then the engine cuts to idle and stays there for anything from 1/2 sec to 4 seconds. Yes, 4 seconds (counting 1 mississippi 2 mississippi etc). During that period, there is NO response from the engine. In the worst instance recently, I actually started to steer to the kerb as I thought the throttle had failed completely (but note that there was no suggestion of it stalling). That happened once. Usually it is only 1 or 2 seconds, which is enough to be a concern when the initial response was just enough to get you moving into a lane of traffic. I have done 12,000 miles in 10 months and it was like this from the start. It mostly happens just after a warm start. The dealer has looked at it twice and can't fix it. I have experimented with D, S and M modes and there is no difference. I have tried either easing the throttle gently, normally, or mashing it to the floor. No difference except that in the latter case, when it finally responds, it gets a bit exciting. Sadly, I experienced the same thing in a loaner Q5 they gave me while looking at my S4 which makes me think it is a general Audi characteristic. I wondered if I was doing something unconsciously but I frequently rent cars, turbo and non turbo, and I never have the problem with those cars.
It seems to me that something between the throttle and the engine is sometimes responding too slowly to the throttle input. Like a committee that takes some time to decide how to respond. Most of the time it still feels a little bit laggy but nothing serious. Then about once every two or three days, I get a serious delay between throttle and response.
Does this sound like something anyone else is experiencing or is it just my car?

jondin2000

Yep driving me crazy...I usually downshift when I need a fast respond but sometimes I forgot to and I find my self on the left lane not moving which it seems like forever and hopping guy behind would slow down or crash on the back of my car

Eli McCraig

If you are experiencing a 4 second delay between input and response, even occasionally, there is a major problem that needs to be addressed by the dealer.
This is not normal.

Glisse

Completely agree, adding that rpms dropping to idle speed during the process indicates a hardware issue.

But then the poster went on to say that the Q5 showed the same behaviour as their S4. A car with a different engine, a different transmission, and a different drivetrain. So then I lost interest.

MGBerridge

The Q5 didn't give a 4 second delay, only 1 - 1/2 sec, and the S4 has only done it once. More typical is 1 - 2 second delay, and then this only happens every 3 or 4 days, but when it happens, it is a bit scary when you have pulled into a lane into a gap in traffic and then you are dead in the water. The dealer can't duplicate the issue and I can't isolate the combination of circumstances that leads to the delay. It is mostly (but not always) after a warm start but is independent on things like ambient temperature, gearbox mode, amount of throttle opening, how long it has been idling before opening the throttle, whether auto start/stop is engaged, whether it is in dynamic or comfort mode. I can't duplicate it on call and sometimes after a few days I dare to hope that it has sorted itself out and then it happens again.
The Q5 is also a turbo auto. I'm thinking it is an intermittent Audi feature arising from their approach to meeting emission regulations. I note others have experienced a similar lag but mine appears to be a bit more extreme.

Glisse

That your S4 has done this only once wasn't obvious in your first post. Even so, there shouldn't be a lack of throttle response in S mode, so something is wrong with your car. Or with how you are driving it.

I do not understand why your dealership can't duplicate a lack of throttle response in S mode, which would be very obvious. If it is a reasonable sized dealership, they should have an S4 or S5 on hand, drive one of those with a senior technician (not a service advisor) and then compare it to yours.

The Q5 uses a dual sequential transmission, an Otto cycle in-line 4 cylinder engine, and a quattro Ultra drivetrain. The S4 uses a torque converter transmission, a B-cycle V6 engine with a dual scroll turbo inside the V (hot side), and a permanent self-locking centre differential. They are very different. I haven't driven a Q5, but have the A5 and A4. They have a more linear throttle response in D mode, so I am very surprised you found the Q5 feels the same as your S4. In D mode or S mode presumably.

jc123's earlier post contains useful information, which should be highlighted amongst the noise from people who forgot to test drive the car they bought. Audi, as do a number of other manufacturers, cut the throttle if there is any (no matter how slight) brake application whilst the car is moving. This is to remove the potential for unintended acceleration, as research conducted a decade or so ago concluded that the majority of unintended acceleration cases were due to drivers applying equal pressure to their left and right feet in an emergence situation. People who left foot brake, and have a habit of resting their left foot on the brake pedal, will have the throttle cut issue just as jc123 explained.

The only other issue I can think of is fuel quality. Some US states, those with high elevation, may not have 93AKI fuel. Whilst 91AKI may not normally cause any issue, if a 91AKI winter grade fuel has come from a refinery which is using a lot of naptha, then I can imagine the car not being happy.

MGBerridge

Thank you very much for the detailed response. I apologise for not stating my situation clearly. My bad.

What I'm trying to find out is if my problem is unique to my car (and therefore fixable) or common to either S4s or Audis generally. The hesitation I experienced with the Q5 made me think it is an Audi trait and may be a result of their particular approach to achieving emission limits and target fuel consumption. A long time ago I was a vehicle emission development engineer for an auto manufacturer back in the days when the only tools available were leaning out the fuel mixture, retarding the ignition and maybe throwing in Exhaust Gas Recirculation. This was before catalysts and fuel injection etc. We would lean out the mixture until it affected driveability and then go a bit richer. The driveability issue was off idle with the transition from the idle jet to the main jet if the throttle movement wasn't enough to engage the accelerator pump. It went flat as it went off idle and the Audi behaviour reminded me a bit of that which is why I was wondering if it was designed in as some sort of compromise which of course they would never admit to. I left the Industry 40 years ago and have not kept up with technologies and have no idea any more how engine management systems work so I'm feeling around in the dark.

The reason the dealer can't replicate the problem is that I can't either. It happens so infrequently. I can't determine the combination of events that leads to the hesitation. It only happens periodically, maybe only once every 3 or 4 days and the only consistency I have determined is that mostly it happens after a warm start and mostly it happens only within a minute or two of that warm start. After that it mostly behaves faultlessly. 9 times out of 10, the hesitation lasts less than 2 seconds. I could live with it except it frequently gets me into a very dangerous situation when it happens, as the car will accelerate just enough to stick my nose into the lane and then the engine cuts to idle for usually 1 or 2 seconds (but on the most egregious occasion 4 seconds which really got my attention). The reason I drew attention to the 4 second incident was to try to dispel discussions about turbo lag, which this definitely isn't. I'm sorry it led to people just thinking I was crazy.

I have experimented with everything I can think of - driving mode (D/S/M) Auto Stop Start on or off, easing the throttle open. I have even wondered if it is the proximity collision avoidance which might be cutting the power because I am too close to something but that doesn't seem to fit. I wondered if it was me, but over the recent winter I had occasion to rent 5 different cars and none of them gave me this problem although they were very utilitarian cars. My previous car was a turbo also (not Audi), and I never had this problem with it.

However, your comment about left foot braking interests me as I always use my left foot for braking, a legacy of a job many years ago as a forklift driver. It is quite possible that I have both feet on the pedals lightly but enough to cause the problem you describe. I was not aware of this possibility. I'm very grateful you suggested this and I will certainly experiment. Thanks for your comment on fuel also, but I'm at 900 ft (and ALWAYS use the highest octane fuel) so I don't think that is the issue.

Thanks again for your detailed response. It has given me food for thought. I apologise again for my lack of precision in my description. I've been getting so frustrated by the problem.

Glisse

Thanks for clarifying. It would frustrate me too.

It was jc123 who raised the left foot braking issue in post 220, it has been mentioned before in the forums (possible in this long thread!), but I I think gets overlooked. Kudos to jc123.

If you can adjust, try forcing yourself to a week of right foot only braking and keep your left foot firmly on the rest. That you had this issue in S mode, that you had the same issue in the Q5, and that you have seen your revs drop back down to idle suggests your issue is different from the lazy throttle response in D mode which is what most others are complaining about in this thread.

You are quite right on turbocharger lag. In S mode, the car will idle at just under 1000rpm. It is on boost and making maximum torque at 1370rpm. In suitable road conditions, see how long it takes to go from 1000 to 1370rpm. Hard to time, but it will be in the millisecond range, perhaps 200 or so. Everything about this engine design is minimising lag, and maximising low rpm torque. Twin scroll turbocharger into a V configuration, turbocharger inside the vee for the shortest possible path length, etc. Throttle mapping and B-cycle application in D mode is intended to calm the car down to it doesn't use the fuel required to generate 500Nm of torque 200 milliseconds after applying the throttle. The primary benefit of B-cycle is to lift torque below boost, so in D mode the programming wants you to be driving the car between 650 and 1300rpm as much as possible. That needs a very gentle throttle map, I think Audi overdid it myself, but then I bought the car to drive it primarily in S mode. And the cars I bought behave in exactly the same manner as the several versions I test drove before deciding on the car.

MGBerridge

Update. For me, the problem was left foot braking. I have forced myself to use my right foot to brake for the last couple of weeks and haven't had a problem. I forgot a couple of times and felt the hesitation. I still prefer to use my left foot for the quicker braking response time in stop start traffic, but also since the force required for accelerator and brake are quite different, using different feet for each function seems more efficient. Still, I can live with it as I have driven a stick shift for most of my life.
Now that I have that resolved, I'm experimenting more with D and S mode and finding myself using S around town most of the time and this gives an acceptable throttle response for my needs.
It is a major relief to get to the bottom of this and I thank jc123 and Glisse in particular for their assistance. It has significantly improved my driving enjoyment with the S4.