996 4S sounds ill when cold

[Paul-M]

Hi everyone. My 996 C4S is sounding ill when starting up. It's fitted with the factory sports exhaust but when starting from cold is much louder than usual and unhealthy sounding. This lasts for the first 5 minutes of driving or so. Despite this it's performance seems the same as ever. Has anyone ever had this before? Or any suggestions as to what is causing it? Thanks in advance.

[Acf69]

I guess a sound clip would help people assess if it's normal or not otherwise its speculative. If it didn't used to make these noises it would be a good idea to have it checked in case there is anything amiss

[sportsandclassic]

Hi,

Could be the manifold to cat joint securing fittings corroded and allowing the two sealing flanges to become insecure and when it warms up it seals the joint.

Seen this loads on 996/997 cars.

Easy to check.

Mike

[Lewy]

Mine always sounds louder on cold start up. I put this down to cold cats, when they warm up after a couple of minutes its back to normal.
No unhealthy sounds though, just very raspy.

[Callughan]

sportsandclassic said:

Hi,

Could be the manifold to cat joint securing fittings corroded and allowing the two sealing flanges to become insecure and when it warms up it seals the joint.

Seen this loads on 996/997 cars.

Easy to check.

Mike

+1
Happened on my old 996

Edited by Callughan on Tuesday 19th February 15:11

[Shurv]

Sounds like a leak to me too that seals itself up when all warmed up.

[kayc]

Paul-M said:

Hi everyone. My 996 C4S is sounding ill when starting up. It's fitted with the factory sports exhaust but when starting from cold is much louder than usual and unhealthy sounding. This lasts for the first 5 minutes of driving or so. Despite this it's performance seems the same as ever. Has anyone ever had this before? Or any suggestions as to what is causing it? Thanks in advance.

I think that is normal..had the same on mine..actually i thinks it just an exaggeration of the rather strange noise the Porsche sports exhausts make imo..always sounded artificially 'hollow' to me but much worse when cold.

[Ikebaddog]

When cold, injection systems run a different mix than when warm, usually much richer, this causes more noise and can cause pops and bangs, my Aston sounds superb when first starting up, even at tickover.

[Ian_UK1]

Ikebaddog said:

When cold, injection systems run a different mix than when warm, usually much richer, this causes more noise and can cause pops and bangs, my Aston sounds superb when first starting up, even at tickover.

The cold start mapping is designed to get the catalysers as warm as possible as soon as possible for emissions purposes. To achieve this the mixture needs to be lean - it burns hotter - and ignited as late as possible so the fire hasn't gone out when the exhaust valves open - that way burning mixture travels down the pipes to heat-up the catalysers.

Obviously, this is at odds with the need to run a richer mixture when the engine is cold. The richer mixture is required when cold as some fuel will drop-out of suspension in the very cold incoming air charge. To reconcile the requirements of a richer mixture in the cylinders - and the need to heat the catalysers more quickly - what Porsche did on the 996 was to use a secondary air injection system (utilising the pump you see in the n/s corner of the engine bay). Essentially, the engine is given a very rich mixture that is ignited very late. As the exhaust valves open, the secondary air system injects air into the exhaust ports. The oxygen in the new charge of air causes the excess, un-burnt fuel in the (still burning) rich mixture to ignite. This causes an 'afterburner' effect down the manifolds towards the catalysers (in a similar way to what a very lean mixture would do) heating them to operating temperature as quickly as possible . Clever stuff.

The odd sound produced by the engines when on cold programming is due to the combination of very late ignition timing and secondary air injection causing this afterburner effect in the exhaust manifolds.

Ian

Edited by Ian_UK1 on Tuesday 19th February 16:47

[Paul-M]

Thanks for all the answers so far. I have had the car 3 years and I know it usually sounds a bit different when cold. What started 2 days ago though is very different to the "usual" cold noise!

[Rockster]

Paul-M said:

Thanks for all the answers so far. I have had the car 3 years and I know it usually sounds a bit different when cold. What started 2 days ago though is very different to the "usual" cold noise!

Do not run the engine any more. Get the car to a qualified Porsche tech and have him give a listen if he decides this is what he needs to diagnose what's going on.

[996c2]

Paul-M said:

Hi everyone. My 996 C4S is sounding ill when starting up. It's fitted with the factory sports exhaust but when starting from cold is much louder than usual and unhealthy sounding. This lasts for the first 5 minutes of driving or so. Despite this it's performance seems the same as ever. Has anyone ever had this before? Or any suggestions as to what is causing it? Thanks in advance.

Unfortunately, I am having the same problem with my 996C2 today. Did you ever find out what the problem was?

sportsandclassic said:

Hi,

Could be the manifold to cat joint securing fittings corroded and allowing the two sealing flanges to become insecure and when it warms up it seals the joint.

Seen this loads on 996/997 cars.

Easy to check.

Mike

Did Mike guess right? How do I check to see if the "manifold to cat joint securing fittings" have corroded?

Thanks!

[996TT02]

smiyff said:

It's losing water, not loads, but have replaced the cap and it's still losing water so assuming it's either the expansion tank or rad(s) leaking as I can smell coolant after a fast run.

I'm at the point now where I try to just drive the thing

I would urge you to make an exception in this case, the coolant leak will never sort itself out but only get worse, and for the cost of a straightforward repair which HAS to be done at some point in time anyway, you are risking far more.

Since you are smelling coolant my guess is radiators, if so replace all three to save on labour costs, only exception perhaps the middle one if it really looks good, easiest to remove anyway. And budget for condensers, (and replace o-rings) too, for an extra £150 OEM plus aircon recharge cost, almost a no-brainer. Gives you 4-5 years hassle free front end if you don't bother cleaning out the rads (bumper off jobbie) or fitting mesh, longer if you do.

Oh and inspect the rad hoses, the metal collars corrode.

[chris_w]

996c2 said:

How do I check to see if the "manifold to cat joint securing fittings" have corroded?

Thanks!

Hi - mine went last week with the symptoms described above (loud on start up, normal once warmed up).

I put mine up on ramps at the weekend and visually inspected, it was very obvious as all three nuts had corroded away leaving only the studs/bolts. I could put a (gloved!) hand next to the join and feel the gases coming out.

One thing that slightly threw me was that the noise was coming from the left hand side of the car (when standing at rear) so I assumed it was related to the right hand bank and started by checking the pipework on that side. As it was the joint that had failed was on the left bank but directly behind the left hand exhaust.

I've got a pic of the joint with the nuts missing, please PM me if you'd like a copy for reference.

A quick question for Mike at S&C - you say you've seen a fair few of these now, what's the usual remedy - just new nuts or will I have to have the studs renewed too (looks like they'll be a pig to get out...)?

Chris

[PorkaFly]

OP, I had two issues just like this with my C4S back in 2004 and then 06...rusted nuts and heat shield rattle were one cause the other was the baffles going in the off-side back box requiring a new one. They would rattle like a bd when cold and then calm down when warm, but it was an embarrassing racket and sounded ill! Anyway...another potential cause for you.

PF

[thegoose]

chris_w said:

A quick question for Mike at S&C - you say you've seen a fair few of these now, what's the usual remedy - just new nuts or will I have to have the studs renewed too (looks like they'll be a pig to get out...)?

Chris

I happen to have seen Hartech doing this job - it required a gas-axe to heat up the studs and an air-chisel type thing to bash each one out once hot enough. Not the end of the world for skilled technicians using a ramp but definitely not something an amateur home mechanic should be attempting.

[chris_w]

thegoose said:

I happen to have seen Hartech doing this job - it required a gas-axe to heat up the studs and an air-chisel type thing to bash each one out once hot enough. Not the end of the world for skilled technicians using a ramp but definitely not something an amateur home mechanic should be attempting.

It's ok, certainly had no intention of trying myself, just trying to gauge who to take it to. My biggest concern is how fragile the manifolds look at the engine end; worried that messing around too much at the other end could finish off what's left of the manifolds.

[996c2]

I finally had a look under the car today and found what looks like a broken muffler (picture below).



Could this be the cause of my ill sounding car? Strangely, the noise only last for about 5 mins after starting up and then settle down to normal.

Finally and, most importantly, how much will it cost to replace?

Thanks.

[PorkaFly]

996c2 said:

I finally had a look under the car today and found what looks like a broken muffler (picture below).



Could this be the cause of my ill sounding car? Strangely, the noise only last for about 5 mins after starting up and then settle down to normal.

Finally and, most importantly, how much will it cost to replace?

Thanks.

Err yes...read my previous post to you OP, 4posts up.

PF

[996c2]

PorkaFly said:

Err yes...read my previous post to you OP, 4posts up.

PF

Hi PF,

Thanks for the heads up. For some reason I missed the second half of your post.

It does seem I have the same problem that you had. How much did it cost you to replace the mufflers?

Thanks.