S8 - Revs Wobble at Idle

ahd91 · #1 29th April 2015, 10:31 AM

This morning on my way to work, I noticed that every now and again my revs at idle were a little bit bouncy. The car actually cut out at some lights at one point, but started up again without issue. No lights came on.

Where should I look first - throttle body perhaps?

moltuae · #2 29th April 2015, 10:42 AM

I'd check for fault codes first.

Lumpy idle can be caused by many things: Bad plugs, dribbling injectors, faulty cam/crank sensor, manifold air leak, sticking valves ... to name but a few.

Also, what petrol are you using?

My S8 sulks and runs a bit sluggish and lumpy if I feed it on the cheap supermarket stuff. I always stick to the recommended high octane petrol, preferably Shell V-Power Nitro+.

ahd91 · #3 29th April 2015, 10:59 AM

I'll find someone to read the codes then, thanks.

Now that you mention it, I did have to grab some lower grade fuel from a station whose brand name I can't even remember.

Cornershop · #4 29th April 2015, 11:13 AM

I tend to carry a couple of bottles of Millers VSPe with me for the rare time I can't find super unleaded (usually Vpower). I don't know if ethanol content is a concern on our cars but the octane boost is welcome (works especially well on my high-comp 911 to avoid pinking)

Is there an idle control valve on these cars?

Hope you get it sorted easily

ahd91 · #5 29th April 2015, 05:55 PM

Needle didn't give me so much as a murmur on my 90 min commute home. Typical. I'm going to put it down as some dodgy fuel.

ahd91 · #6 27th May 2015, 07:42 AM

So this appears to have taken a turn for the worse, and it seems to be an electrical issue?

In the past few days, the car has died on me three times. The revs suddenly start to fluctuate and drop completely, along with flickering EPC, battery and engine lights. If I'm lucky, the car will simply start up again without any issue, lights or anything.

If I'm unlucky, the car will turn over a few times without catching, and then eventually nothing will happen at all. And I mean nothing - no power whatsoever, no lights on the dash, no locking, as though the battery is *completely* dead. If however, I get out, unhook the battery and leave it for a couple of minutes, power returns and the car starts without issue or warning lights.

I'm perplexed, and when I took the car to be read a couple of weeks ago there were no codes that pointed to anything like this.

ainarssems · #7 27th May 2015, 08:26 AM

Dodgy earth connection or positive connection at the junction box, quite common problems on D2 and can give all kinds of odd issues. Do a search on forum you should find what to check and where.

ahd91 · #8 27th May 2015, 11:20 AM

Ah excellent.

Found an image of the location - not sure if it's o/s/f or n/s/f though - any help? And is it readily accessible?

David's8 · #9 27th May 2015, 12:38 PM

Quote:

Originally Posted by ahd91

Ah excellent.

Found an image of the location - not sure if it's o/s/f or n/s/f though - any help? And is it readily accessible?

Its o/s/f and is behind the wheeel arch plastic liner and then carefully concealed behind another plastic piece (forget what thats for).

I had similar issues and it turned out to be fuel pump related and occurs when 1/3 - 1/4 full as the pump cannot draw the fuel out of the lower parts of the tank via the eductor pumps (which require the main pump to provide a good flow to "lift" the fuel out of the lowest parts of the tank using a venturi effect). The issue hasnt re-occured since the pump was removed, given a good optical bollocking, and re-seated. I thought at first that it was a battery/electrical problem due to the symptoms you describe AND the fact that I had left the car for a month and battery was v weak. Has the car been sitting for a while?

ahd91 · #10 27th May 2015, 12:52 PM

Hmm - I'll look at that too - it is about 1/4 full right now.

Hasn't stood for more than about 24 hours in the two months I've had it.