RS4 intermittent flat spot/judder

[Majik001]

Hi,

B7 RS4 has an intermittent fault, odd occasion it will judder when at 4-5k Rpm. Car feels like its completely lost power for a second and then it powers down. I can drive the car hard for a long time and won't do it again.

Car has 90k miles its had a recent Carbon Clean, Throttle Body cleaned and tested, new accelerator pedal, injectors reconditioned, fuel pumps tested, New Maf, recent service and spark plugs changed.

There's days where it won't do it and other days where it can do it a few times. Worried I might be causing more damage of I continue driving it.

Cars had manifold flap delete, and cats removed.

I've had it smoke tested and couldn't find any leaks.

Current codes:

P0174- system too lean bank 2
08569- fuel metering system too lean at idle

Above 2 codes have come up twice together

And I've also had 08584- fuel sizing system too rich at idle with the other codes.

Any ideas as I'm thinking to replace the exhaust back to standard with Cats and replace the o2 sensors.

[JRC1]

Has it been mapped? Assume it has with the work you've listed as being done.
Because if not, with cats removed and flap delete it won't run right / will throw codes if it's not been mapped properly.

If it has been mapped, then I'd take it back to the mapper as they should be best placed to assist with those type of codes.

[r8wut]

'System too lean' codes is basically stating too much air into the combustion chamber and not enough fuel - and yes they can be a nightmare to chase, as you've clearly tried a few avenues

Try the following whilst "..at idle" and engine hot

Carefully. spray short sharp bursts of Carb Cleaner in and around the Intake Manifold and Throttle Body area. Listen carefully to hear if the engine note changes ...

I suspect you may have a vacuum leak, which a smoke test (unless when an engine is hot) - will sometimes not find.

I would also try the following:

- Check all bolts are nice and tight and torqued to the correct spec on the Throttle Body and Intake Manifold etc.
- Check all PCV vacuum lines
- Check PCV valve at rear of engine, if original - then replace. When they go bad, they can cause similar issues too
- Replace Fuel Filter - with either Audi OE or UFI (original manufacturer) worth changing anyway if original, and I think it contains a 6-bar fuel pressure regulator? which could be partially clogged

When all Injectors were reconditioned - were they all tested as showing similar flow rates?
Try some injector cleaner additive anyway, Liqui Moly 1803 Injection Cleaner is apparently quite good

To be honest, ideally - you really need to see or log real-time Fuel Trim Values with VCDS and check if they are within spec. to rule out any type of fuel delivery issue.

At the same time you can check the MVB's for the MAF and O2 sensors etc..

I've done this before on my R32, can't remember which MVB's (Measuring Block Values) and specs - it's midnight, and I'm half-asleep
maybe someone on here might know or done this already ..

I have only owned the RS for a few months now.. with all of it being off the road

[JamesBaby]

The fact that it happens at 4-5k rpm points to the air flap in the air filter housing. That flap opens at certain conditions at those revs.

It is operated by a vacuum solenoid. If that set-up is faulty and has a leak, you will get a rush of unmetered air and thus a lean code. Followed by the ECU\knock sensors having a hissy fit.

Before you start changing expensive items check your vacuum system.

Fuel Trims in block 032 at Idle and Partial throttle values will tell you. Vacuum is highest at idle, so if you have a vac leak your Idle reading will be off. I don't have a vac leak and my idle reads -0.2 in the trims.