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Weird thing happened last night. In 6th gear, my car would not accelerate past 5000rpm, but only when I was in 6th gear. 4th and 5th went through 5k easily, but in the highest gear, as the engine approached 5000rpm, I could feel the engine "pull back" - felt like an accentuated flat spot. Any ideas?

Maybe the MAF.

It probably realised youwere breaking the law [img]images/graemlins/nodder.gif[/img]
Where did you find a stretch of road to try this trick ? I mean, 5000RPM in 6th must be getting on for at least 125mph plus... I have never tried it, so I don't know the exact speeds involved but presumably you weren't up at the limiter at 155mph ?

Sounds like your ECU has the engine in limp mode as a result of any number of reasons ranging from a dodgy MAF as Phil suggests through to sticking dump valves, a ruptured throttle body boot or some other air leak. All of these are common faults on the S4.

ECU's sometimes reset when the ignition is turned off but if the fault is there the situation will occur again. Get the car to an Audi dealer soon as some of the fauls, like a faulty MAF can cause the engine to run lean with serious consequences for components like the turbos.

I can't see how any engine fault like MAF or valves would be gear-related.

I can't remember what the road speed vs rev in which gear formula is, but as has been pointed out, 5000rpm in 6th must be very fast. I certainly wouldn't know how my car responds at that speed.

AFAIK the stock S4 has a 155 mph speed limiter. This would operate by retarding the engine, giving a 'flat-spot' feel.

Pete, without admitting your driving habits, does this make sense to you?

...and maybe if I had read the full thread properly I'd have realised that Carps has already written the same response - doh!

It probably realised youwere breaking the law

Ooops silly me; I forget to mention I popped over to Germany for a bit of late night shopping [img]images/graemlins/033102bigblink_1_prv.gif[/img]

but presumably you weren't up at the limiter at 155mph ?

No, I've been up in that range plenty of times before (on the continent [img]images/graemlins/gotmesome.gif[/img]). Guess I'll have to get it checked out - you guys definitely think my local dealer will be best? (they've been great up 'til now with regular servicing).

You could try running VAG-COM on it before it goes to a dealer, just to see if it gives any clues.
If you don't have it, you are welcome to have a go on mine. Whereabouts in Surrey are you ?

I don't think it is gear related but I have experienced exactly the same thing at Elvington. My standing quarter runs were fine but on the timed quarter mile the car failed to accellerate past 140 or so, with a noticable flat spot. I thought at the time it was a bad dump valve but now, looking back I thing it may have been the MAF - perhaps dealing with a higher airflow, not sure.

But certainly worth getting these components checked asap

Cheers for the help lads. But although an experienced (but pretty average driver), I'm unfortunately a little technically unitiated! Could someone be so kind as to quickly explain what MAF and VAG-COM are?

Also Joshie: even though you experienced the same problem, did you then continue to run the car without getting it checked immediately without any further problems?

While I'm on here, I think it was Dippy who recommended upgrading to K04's. I'm presuming these are the turbos fitted to the RS4. Does the RS4 simply have the same 2.7 V6 as the S4, just with the bigger turbos? What I'm really asking is, if I go down the upgrade route, would I have to mod anything else on the car apart from the turbos? (reading other posts on the forum it seems a complete exhaust system is also in order)

All questions, me!

MAF = Mass Air Flow meter. This is used for sorting out the air/fuel mix for the engine. Hence it can cause a lean run, and potentially massive damage to the engine

VAG-COM = Diagnostics hardware/software to plug into the car and read off any fault codes that have occured. Can also monitor various things 'real-time'. Fault codes are logged for numerous components, one of them being the MAF.

The S4 engine is not the same as the RS4. The RS4 lump was tinkered with by Cosworth, and I believe just about every component is different. The K04s are the turbos from the RS4. From my reading, ideally you would change other bits and pieces too, to help out with the extra power. I believe a proper K04 installation costs in the order of £6K.

At a minimum, I would guess you should get your car checked on a diagnostics rig (dealer or VAG-COM) just to ssee if it is reporting any problems.

Good luck

Thanks mate. The replies make complete sense now!

BTW I'm based in the Guildford area. You?

Also Joshie: even though you experienced the same problem, did you then continue to run the car without getting it checked immediately without any further problems?

I continued to have the symptoms until it was dignosed as faulty dump valves (which were causing the turbo's to stall) however, several months later I discovered that in fact the MAF was faulty. This meant that the engine was running lean which ultimately caused the turbos to overheat and necessitated their replacement [img]images/graemlins/blush.gif[/img]. The point is that you sould get this checked by someone who has the skill to correctly diagnose the fault (not necessarily an Audi garage). I would recommend one of the Audi specialist tuners such as AmD or QST. It is very important that you don't ignore it.

Joshie is right. Get the car checked out ASAP you definitely have a potentially serious prob.

I too have experienced the same problem as you. The fact that you are changing down and getting the revs in a lower gear is telling me that you simply don’t have the power to pull 6th through 5000rpm as the power required is proportional to velocity squared. As the rest have guessed that's more than 140mph.
(For the record I've had 168mph on the flat with of my chipped S4. Well that's what the speedo indicated)

Tech. stuff aside your car is being seriously robbed of power and more than likely boost pressure for any number of reasons noted above. Mine was due a ruptured TBB in combination with a faulty exhaust temperature probe (not the (lambda probe).
However, I still have the occasional limp mode occur which I'm convinced is related to the DVs as I have read "intermittent - boost pressure exceeded" from VAGCOM which send the car into limp mode resulting in what your talking about.

However, by simply flicking the ignition on and off I can reset the ECU and off it goes for another 3 months or so. Joshie made the very easy and fatal mistake of resetting the ECU upon limp mode events and suffered seriously expensive turbo failure.

I had my car checked out several times by my local dealer. They said the car was fine. Convinced it wasn’t and 14+ visits to my dealer I eventual made the 8hour drive to AMD. They rolling road tested the car and found it was 40bhp down and then discovered the burst TBB and eventually noted the intermittently faulty exhaust temp probe while chipping the car. There are two of these like so many things on the engine and each bank of three is monitored separately by the ECU. Any problem occurring in any one of the numerous sensors will induce a limp mode or reduce power.

I’d recommend AMD or Kim Collins over an Audi dealership anyday. But if your stuck at least a dealer can check for error codes to give some clue.

Warning if your going to try switching the ignition on and off while still moving, be aware that you'll lose power steering and NEVER take the key out, as the steering lock could come on!

The K04s are the turbos from the RS4. From my reading, ideally you would change other bits and pieces too, to help out with the extra power. I believe a proper K04 installation costs in the order of £6K.

You can have just the turbos and a remap for less than £4K (I think). But to really make them work you need to uprate the fuel pump & injectors and maybe the MAF too. Also it would be worth considering uprating the intercoolers, the flywheel and the clutch at the same time.

As you can see, modding can be an endless addiction. Joshie's going through 'cold turkey' himself and as a result of his delusions he's gone to the dark side...