Stephen_S5

Hello all. I've had my 2018 S5 coupe for almost a year now. I've driven over 12000 miles (currently at 43000 total) and it ran beautifully up until the last 2 weeks. I took it to the Audi dealership for an oil change, but they told me it was time for the 40K service so I let them take care of all of that (Fluids, spark plugs and a few other items basic checks.)

Two days after the service, I started the car and it ran really rough before turning off after about 10 seconds. It flashed a message on the dash saying "Engine RPM reduced to 3500" and threw a check engine light. It did this twice before I left it alone. I tried a third time and it didn't start at all. As I was about to arrange a tow truck, I decided to try it again about 10 minutes later and the car fired up and worked as through nothing had ever happened. No rough idle, and started very quickly.

I was able to get an appointment at Audi the next morning and took the car in for them to take a look and at least determine what caused the check engine light. At this point the car was still running perfectly. They told me the code was a "sporadic error" but they couldn't find anything. They cleared the code and gave it back. The car ran perfectly fine for about 2 weeks before doing the exact same thing again yesterday. I started the car, it ran really rough for a few seconds and cut off. This time I waited an hour before trying again. Once again it fired right up as though nothing happened. I had an old OBDII bluetooth reader and was able to determine that this time the car threw a fault code of "P2635 - Fuel Pump A Low Flow / Performance". (Not sure if it was the same code last time.)

The error suggests a fuel pump issue, but I'd expect to also see some issues while driving. So far this is an intermittent issue that happens only at startup. I'm just wondering if anyone else has seen anything similar or know of a potential cause of this behavior? It could be a coincidence that this all started after the last service, but I never had a single hiccup with the car before this so I do wonder if something they did triggered this.

I'll add an update on what Audi says, and anything new I find.

ShiftyWolf

I had a similar issue in my 2018 RS, although engine speed was never reduced. Also I could hear the fan noise didn't sound right. In my case the initial problem was with the thermostat and the SA said rough starts could absolutely be related.

They couldn't find a fuel issue and cleared the fuel pump code but it returned a month later. Right now the car is in the shop and it looks like they have ordered parts for the fuel system. Still waiting on them to arrive and then get my update.

I'm lucky that mine is still under the CPO warranty and techs saw both errors. I'll try to keep you informed if they solve the fuel pump error.

Nikon1

There is a fuel pump issue that Audi has acknowledged — but IIRC, it was for the A5 series, not the S5 / RS5. If you search this sub-forum (which is a PITA) you’ll find a thread or two with the information. I also wonder if it could be a clogged fuel filter somewhere in the fuel line system?

Good luck and Welcome to AudiWorld.

— John

synergize

Afternoon John:

Is this what your are referring to, an Extended Fuel Pump Warranty for 15years or 150k miles, not sure if it is for all B9s or just the 2019. I got this about 1.5 months ago. Did you get it? If I'm not mistaken, I believe this extended warranty is for all B9 cars?

Nikon1

@synergize — That’s the one I got as well: Thank you!

— John

SloopJohnB@mac.com

find the fuel pump relay and get the dealer to replace it. On BMW E46 it's (salmon relay) notorious for SMG power problems.
It's also possible the fuel filter or fuel pressure regulator is acting up, at 40K it probably should have been replaced but I guess Audi doesn't specify it. I went through a fuel pump, fuel filter/regulator around 60K on a port injected M3 motor...your direct injection is likely more sensitive to fuel. Additionally, consider scoping the intake ports...DI can often end up with carbon deposits all over the intake valves/ports. Deposits can cause hard starting and rough idle/running.

Stephen_S5

Thanks for sharing. The first time this happened, the car was cool and parked in the shade, but the second time it was in 100+ degree temps in direct sunlight. I suspected heat could be at play as well since this is the first time it's really getting the full Texas heat since I've had it. I hope they get your car taken care of. Intermittent issues are the most annoying.

Thanks for the welcome and for turning me on the the fuel pump issue. If that turns out to be part of the problem, this is good info for me to have when talking to the dealership. So far they've been helpful, but you never know.


I didn't even think about the relay. Based on the responses here and a lot of searching I'm leaning towards the relay or the pump having an issue. I'm saving all this to continue troubleshooting on my own as the dealership may take awhile based on the scheduling call I got today. This is really helpful.

Stephen_S5

This thread has already reminded me why I appreciate forums so much!

My current theory is that either the relay or the electric fuel pump is having an issue. Whenever the car is running, it never has any issues or hesitation while accelerating. I'm thinking that an intermittent issue with the electric pump or relay prevents fuel from reaching the engine so it can start and allow the mechanical engine driven pump to take over. Since the engine driven pump takes over, that would explain why I haven't had any issues while driving.

I'm guessing that the initial rough engine start happens because of fuel already in the system, but once that runs out the car turns off. This is my current working theory. I'm taking the car in tomorrow and will see what they say although its possible that they may not be able to see it due to being backed up. I got a call today that they may have to keep my car for up to a week depending on workload so we'll see how that goes. Either way I'll post an update here.

SloopJohnB@mac.com

a fuel injector(s) leaking would also fly...no pressure in the fuel line until the fuel pump(s) bring it up. DI problems....

ShiftyWolf

I just got my car back yesterday following repair for this and a separate unrelated concern. In my case, the P2635 error popped up twice as a soft error code so the CEL never came on. The first time, Audi couldn't see an issue and cleared the code, but when it popped up again a month later there was clearly something going on. In my case, they replaced the in-tank electric fuel pump assembly and tank seals. For my RS, this was P/N 4N0919088A and 1J0919133B but it looks like the 2018 S5 part is 8W0919087K. I've only had my car back for a day but so far the code did not reappear even though the tank briefly dropped to 50 remaining miles of fuel remaining.

Hopefully Audi has or will solve your issue as well.