Slight rough running at idle, what could it be?

[GeoffDunk]

Hi all,

The S4 has gained a slight rough idle which is unpleasent, all ok any other time.

Anyone got any good ideas on what could casue this? This is prior ot its full service and 100K refresh as posted here

Cheers

Geoff

 

[TheDriver]

Sticky injector(s)?

 

[Greg190t]

Slight air leak? Vacuum pipes will be getting old and cracked by now!

 

[GeoffDunk]

Thanks Greg, I had read about that before, I'll add vacuum pipes to my 100k list.

 

[Greg190t]

Easy way to test, grab a tin of brake cleaner from gsf etc and while the car is idling, spray it over vacuum pipes etc see if revs change (go up) etc or buy some good silicone and change the lot!

 

[GeoffDunk]

sounds like its wise to change th elot, are there suppliers out there doing kits at all?

when you say "buy some good silicone and change the lot!" can sets be purchased or do you mean by a role of pipe and make to measure replacements?

 

[Greg190t]

Buy a roll from someone like RAMAIR so you know it's good, remove them and replace one at a time! You may need different internal diameters for different pipes but 3mm id, 4mm id and 5mm I imagine would to the trick!

390757967085

On eBay for example

 

[Mike.M]

Could be plugs, coil packs, fuel filter, bad grounding.........process of elimination and can be a pain in the @rse but usually the simplest or last thing to try.

 

[GeoffDunk]

Could be plugs, coil packs, fuel filter, bad grounding.........process of elimination and can be a pain in the @rse but usually the simplest or last thing to try.

Cheers Mike

Plugs will be swapped out with service along with fule filter, coil packs a 1 yr old and where swapped out FOC by Audi.


These are all good pointers and give me things to investigate.

Cheers all

 

[Statler]

Does it have an airflow meter?
A lot of idle issues are sourced to a faulty MAF especially if you run an aftermarket foam or oiled type filter.
Several forums I've read on suggest cleaning the MAF internals with some carb cleaner and allowing it to dry. I'd be cautious but it might help.

 

[GeoffDunk]

Is it worth replacing the MAF sensor?

I am running the OEM filter so no change there.

 

[dualmono21]

could well be plugs
coil packs will usually start to show a little further up the rev range with a misfire
could be an old fuel filter which is starting to restrict flow but this would be worse when you boot it
maf sensor is a very real possibility .... it is possible to see what its doing and if its working correctly with vcds
you can clean it by removing and spraying the thermistor inside with electrical contact cleaner to remove any dirt or grime
then allowing it to dry in the sun
vac lines are susceptible on many vag cars so its allways a possibility
short or long term fuel trim from the oxygen sensors again can be looked at via vcds to ensure all are working correctly

intake manifold has a device inside which opens at 5000rpm to allow more air in if this has broken then it will be allowing too much air in at low revs
you ideally need two people to see if this is working correctly , one to rev the engine one to look down in between the radiator and the
front of the engine block ..... there is a tiny actuator which should alter 90 degrees when the engine reaches 5000rpm
if this isnt working correctly it can cause rough idling problems

 

[Greg190t]

Weak chain tensioner at idle? Allowing cams etc to slightly move? Higher rpm more oil pressure, these do suffer from chain issues!

 

[dualmono21]

camshaft adjuster unit is mechanical and not subject to chain tensioner

 

[Greg190t]

The chains going to the cams look like they have a tensioner

 

[dualmono21]

there is no separate tension-er on the camshaft units on both sides
there are two tension-ers on the bbk engine ....
one on the main chain which connects the crank and camshafts together (but this doesn't alter timing the mechanical units with gold bolts do this... one adjuster per camshaft )
the second tension-er is on the auxiliary chain which drives the air con , steering pump etc etc

 

[Greg190t]

Yeah so the tensioner that tensions the chain that joins the crank/cams etc may be on its way out! A weak tensioner/stretched chain could allow everything to move out of sync slightly at idle/first start?

 

[dualmono21]

Yeah so the tensioner that tensions the chain that joins the crank/cams etc may be on its way out! A weak tensioner/stretched chain could allow everything to move out of sync slightly at idle/first start?

tensioner works on hydraulic pressure of about 10psi - 12 psi the oil pump on tickover is around 25psi rising to in excess of 50 psi when doing around 5000rpm
if the tension-er was loose or slack or not receiving oil pressure at all it would immediately put up a warning for the crank sensor out of position and go into limp mode limiting power this isn't happening .... the tension-er works in tandem with the crank not the camshafts as they have there own mechanical units which alter the timing not the tension-er
the easiest way i can explain it is when changing a cam belt you have a side which is fixed (between the two sprockets) and a side which is slack and has a tension-er to take up the slack ( the tensioner between the crank and camshafts is on the slack side not on the fixed side the mechanical units alter the camshaft on the fixed side )
if what you are thinking were happening with geoff,s car then it would be throwing warning lights and be very very unhappy
this is a slight misfire not a serious malfunction