Engine sound

[V6reu]

Has anyone ever noticed or had this problem !

Stationary which the  engine running on idle and when you press the clutch down the exhaust sound changes and it’s very noticeable, it has a euro cup gt exhaust fitted and it goes from a nice burble to sounding like it’s blowing it’s t*ts if !!

[s3swiss]

weird - pressing clutch just disconnects the crankshaft output from the gearbox idler shaft so shouldn't affect engine running. Clutch is hydraulic so not even going to be the cable pulling engine on worn engine mounts. Seen similar on old mini engines (wrong or worn crankshaft endfloat and when clutch pressed, crankshaft moved causing extra friction as pistons off line - caused engine to run rough and nearly stall).

Don't know if the clutch pedal has input to the ECU? If so, that would be my first port of call - possibly bad pedal position sensor?

Let us know what you find...

[½cwt]

Disengaging the clutch often drops revs a bit as transmission drag is removed and throttle body/fuelling can adjust down a little to sustain normal idle.  Have you got a lightened clutch or flywheel or and underdrive pulley kit fitted?  These would possibly mess with idle but in a closed loop ECU system it would be unlikely unless something it out of calibration if it is all at minimum before the transmission drag is removed.

[the baron]

2 minutes ago, ½cwt said:

Disengaging the clutch often drops revs a bit as transmission drag is removed and throttle body/fuelling can adjust down a little to sustain normal idle.  Have you got a lightened clutch or flywheel or and underdrive pulley kit fitted?  These would possibly mess with idle but in a closed loop ECU system it would be unlikely unless something it out of calibration if it is all at minimum before the transmission drag is removed.

This^^^

It might also be a sign your clutch is on its way out if its that noticeable 😱

[½cwt]

1 minute ago, the baron said:

This^^^

It might also be a sign your clutch is on its way out if its that noticeable 😱

Oh, and I missed the obvious...

[edc]

A 4" or 4.25" underdrive pulley doesn't affect the idle. 

[V6reu]

Clutch and pulley all genuine 

[V6reu]

Just tried disconnecting the clutch pedal switch and the car sounds rough on idle permanently  , so the ECU is altering something when it see’s the pedal down 

[Cheddar Bob]

3 hours ago, V6reu said:

Just tried disconnecting the clutch pedal switch and the car sounds rough on idle permanently  , so the ECU is altering something when it see’s the pedal down 

Does it make you go faster..... if it does I'd leave it as is 

[V6reu]

No it dont

[Paul P]

A lot of cars ( and some bike by the looks of google) have different idle based on clutch engaged or disengaged - fuel saving or some such. My focus would vary the idle if i was in gear clutch down rolling to a stop on the driveway - as soon as the car stopped moving the idle dropped 100rpm - still in gear, still clutch down - so there's some "jiggery pokery" there in some cases. Given disconnecting the switch changes the behaviour there's something there, wonder what the logic is - if it was fuel save then I would guess neutral and clutch up would result in lower idle, clutch down maybe up the revs slightly incase you are going to crawl in 1st.

Maybe you notice it more with the exhaust you have, interesting to hear what others find.

[edc]

I think the later 986 may have some sort of anti stall system. Could it be linked to the clutch switch? 

[½cwt]

15 minutes ago, edc said:

I think the later 986 may have some sort of anti stall system. Could it be linked to the clutch switch? 

Not from the number of times I stalled mine when I first got it before I got used to the heavier clutch and the slightly baulky Getrag 'box from more recent fly weight ones in other cars.

[Mr96er]

Any reason to suspect a lightened flywheel?

might this result is more chatter if the ecu hadn’t been programmed to account for it?🤷🏾‍♂️

[map]

One reason the clutch switch is there - as I understand it - is to facilitate Cruise Control. When you depress the clutch in a car with CC it disengages the CC - @V6reu is your car fitted with Cruise Control?

A depressed clutch could alter the fuelling parameters for the engine - have read somewhere that a stalling engine was fixed with a replacement clutch pedal switch  

On my admittedly tweaked 986S I haven’t noticed any change in idle between clutch pedal up or down. 

If there were a lightweight flywheel installed the only time you would hear the characteristic chatter is in neutral, foot off the clutch pedal with the engine idling.

The OP describes the change in sound as happening when the clutch pedal is depressed so I’d be very surprised if it is a lightweight flywheel. 

For what it’s likely to cost I’d be tempted to swap in a new clutch pedal switch. 

[½cwt]

18 minutes ago, map said:

One reason the clutch switch is there - as I understand it - is to facilitate Cruise Control. When you depress the clutch in a car with CC it disengages the CC - @V6reu is your car fitted with Cruise Control?

A depressed clutch could alter the fuelling parameters for the engine - have read somewhere that a stalling engine was fixed with a replacement clutch pedal switch  

On my admittedly tweaked 986S I haven’t noticed any change in idle between clutch pedal up or down. 

If there were a lightweight flywheel installed the only time you would hear the characteristic chatter is in neutral, foot off the clutch pedal with the engine idling.

The OP describes the change in sound as happening when the clutch pedal is depressed so I’d be very surprised if it is a lightweight flywheel. 

For what it’s likely to cost I’d be tempted to swap in a new clutch pedal switch. 

This would be the cheapest first diagnostic move given what has been written above.  If it solve the problem it would be one of those rare cheap fixes to an curious problem.

 

Re lightened flywheel/clutch and under drive pulleys, I was just thinking of mods that could have resulted in changes to the 'load' at idle to help diagnose, it was not to say that these things will cause this problem.  A correctly set up management system with all the correct sensor inputs would be able to cope with these unless they were extreme. The clutch switch would appear to be one of those inputs.

[map]

2 hours ago, ½cwt said:

Re lightened flywheel/clutch and under drive pulleys, I was just thinking of mods that could have resulted in changes to the 'load' at idle to help diagnose, it was not to say that these things will cause this problem.  A correctly set up management system with all the correct sensor inputs would be able to cope with these unless they were extreme. The clutch switch would appear to be one of those inputs.

👍 Understood completely where you and @Mr96er are coming from - just wanted those who weren't familiar with Lightweight Flywheels (LWF) to understand the conditions needed for the tell tale chatter to be apparent.

Have run LWFs on my 986 for most of the 100k I've driven mine - the current LWF along with upgraded con rods and pistons have taken 8.6kg off the crank.......  I am a big fan of them.  

More real world performance gain - albeit without dyno bragging rights - than any other bolt on engine/drivetrain mod I can think of.  Sure it makes almost zero difference to a dyno curve, nor does it lift power or torque but the difference to engine responsiveness is phenomenal especially at higher revs.  Add in some smart intake/exhaust upgrades and a proper live dyno tune and there are some startling gains to be found both.

[V6reu]

Update 

if the clutch pedal switch is disconnected it runs rough so this will eliminate Mechanical problems

[½cwt]

1 hour ago, map said:

👍 Understood completely where you and @Mr96er are coming from - just wanted those who weren't familiar with Lightweight Flywheels (LWF) to understand the conditions needed for the tell tale chatter to be apparent.

Have run LWFs on my 986 for most of the 100k I've driven mine - the current LWF along with upgraded con rods and pistons have taken 8.6kg off the crank.......  I am a big fan of them.  

More real world performance gain - albeit without dyno bragging rights - than any other bolt on engine/drivetrain mod I can think of.  Sure it makes almost zero difference to a dyno curve, nor does it lift power or torque but the difference to engine responsiveness is phenomenal especially at higher revs.  Add in some smart intake/exhaust upgrades and a proper live dyno tune and there are some startling gains to be found both.

No increase in power output but a significant reduction in inertia to less internal losses in overcoming that inertia so more responsive.  I'd consider a LWF if I need to do the clutch but as it was only done around 10k miles ago along with the RMS by the PO, I'm hopefully not be having to get the 'box out any time soon to give the opportunity.

[½cwt]

25 minutes ago, V6reu said:

Update 

if the clutch pedal switch is disconnected it runs rough so this will eliminate Mechanical problems

Pedal down and disconnected would both show the ECU open circuit on the clutch switch so same effect which you have confirmed.  Time for a specialist, @Richard Hamilton do you have any ideas?

Have you done a Durametric or PIWIS scan for codes that don't give an EML?

[edc]

@map I can't remember if you've listed this before but which lightweight flywheel have you used or have you had the DMF lightened, is that even possible? 

[map]

2 hours ago, edc said:

@map I can't remember if you've listed this before but which lightweight flywheel have you used or have you had the DMF lightened, is that even possible? 

Have had two installed the first was from an EU supplier - can't remember who as it was part of a package of early stage tweaks to my 986 back in 2008 - a web search will produce a number of vendors but I suspect there are fewer actual producers of these.

Current single piece light weight flywheel took some finding - looked to EU vendors, USA and RoW but drew a blank in terms of what we were looking for.  Even my usual source out east wasn't able to help at all.  In the end we were up against it in terms of time and needed something effectively overnight - the one fitted to my car is bespoke, machined from billet in the UK to match the crank assembly given the reduced mass of upgraded con-rods and pistons.  We wanted it as light as possible without making it too difficult for the ECU to manage or impacting drivability in traffic. It is noticeably lighter than the normal LWFs that are available but stops short of being a full house race LWF.

Not sure how you'd go about getting the Dual Mass Flywheel lightened - like you I don't know if it's possible, imagine you'd have to be careful about matching the weight loss from machining operations across the two components to avoid creating a dynamic problem.

[V6reu]

8 hours ago, ½cwt said:

Pedal down and disconnected would both show the ECU open circuit on the clutch switch so same effect which you have confirmed.  Time for a specialist, @Richard Hamilton do you have any ideas?

Have you done a Durametric or PIWIS scan for codes that don't give an EML?

I’ve only scanned with a Launch kit , not very helpful 

[Richard Hamilton]

9 hours ago, ½cwt said:

,...., @Richard Hamilton do you have any ideas?

Not a clue.  Never heard anything like the clutch pedal switch affecting the idle and causing a rough idle.

[½cwt]

19 minutes ago, Richard Hamilton said:

Not a clue.  Never heard anything like the clutch pedal switch affecting the idle and causing a rough idle.

That's a turn up!

@V6reu looks like you've got something rare here.