991.1S Very loud back fire at high revs.

[richieruss1]

Sorry about the long post but I've been anxious about my pride and joy for a while now and I don't know where to turn...

I have owned this 991S for about six years bought it from the dealer it's done about 45K miles and its always been serviced at the opc and at a reputable indy for the last two years. I didn't renew the warranty as I wasn't expecting any issues as I assumed that if it is serviced properly and it has always ran extremely well with no issues. It usually does about 4K miles per year.

The summer before last the rear exhaust tip bottomed on a low bit of road at a very slow speed junction and I noticed that the exhaust sounded a little bit louder. I don't drive it often so it is in my garage for long periods on a charger. A few months later on a drive during the first lockdown I noticed silver smoke spewing out the back when going up a hill. It was locked away and I took it at very low revs no smoke to my indy which is a short distance away when they reopened who replaced the AOS. The silver smoke was gone it seemed fine until at high revs occasionally there was a very loud backfire! I very rarely drive the car then a few months later an amber check engine light came on but suggested that it was okay to drive on. I went back to my indy who checked the car out and couldn't find anything wrong. They cleared the fault and all seemed well apart from the louder exhaust but they seemed to think it was okay.

In the meantime the car went back to Porsche Reading for a recall and they noticed the cleared message

Primary Cat convertor bank 1 inadequate effect limit exceeded.

Then you guessed it the light came back on and it was booked into the indy to have two new O2 sensors at a cost of about £800ish
So the backfires seemed to have gone then until this weekend when I had it out for a spin and it back fired again.

There is obviously some issue I don't want to wait again until another amber light comes on. I'm lost about what to do? Should I go back to Porsche? My indy, another expert? I don't want to sell or trade it as it was always my dream car.

I'm pretty clueless when it comes to modern vehicles so any help from you guys would be massively appreciated.

 

[Zingari]

DeMort is probably best to answer this but could it be a failing coil pack(s)?

 

[lk993]

That or a plug failing?

 

[Dizzy991]

Sounds like an exhaust problem, did you crack anything when touching the ground?

 

[deMort]

I have an idea whats wrong but would have thought either of the garages would have looked for this ..

Need a build up first .. can't just have the last page of the book with the ending can we now ....

This all started when the exhaust ground out or at least that's what it appears to be the case .. coincidence does happen i'm afraid and it could be something totally unrelated .

So .. exhaust sounds louder .. question is why ?

You have had the car for 6 years .. you know full well what it sounds like and every quirk of the car .. and its louder now .

Smoking out the back under a load .. its interesting you say it was silver .. that's not a usual comment when the car starts smoking .. black , grey or white are the usual colours ..

So .. why is it silver .. hmmm ...

Fault codes and something to do with lambda sensors and Cat efficiency ..

That kinda gives the game away there !!

Why change both lambda sensors .. i can only assume they did both on Bank 1 .. kinda pointless as the post cat is only there to monitor the cat .. to replace that you would need to see switching on it .. ie it reacts to the exhaust gasses the same as the pre cat .. hence it tells you its either defective or the cat is !

Now then .. what causes a back fire .. its fuel in the exhaust igniting ..

A rich mixture ..
A valve timing issue ..
An ignition advance retard issue ..
perhaps a blocked cat causing back pressure issue ..


Rich mixture and yup .. blocked cat and yup .. others won't happen with the codes you have .. rich mixture and you will get lambda sensor faults .. same as with a cat ..

Silver smoke .. the cat is a ceramic material .. its a filter basically .. it has gold , platinum and many other components .. when it breaks up it's a silvery powder .. that kinda fits ..

lambda fault codes .. a restriction in the exhaust will cause this ..

A broken up cat can alter position and cause a blockage at times ..

You hit the exhaust .. these are fragile components and a shock to the casing can well damage them ..

IMHO you have a damaged cat and that's why the exhaust sounds louder !!

I would either remove the cat and inspect or remove the pre and post lambdas and stick a scope in there to have a look at it .

this may or may not be the fault as im only sitting here reading what you have written .. but it's something i would have checked in my garage had i had the same info ... assuming the other garages had the same then that's a bit of a shame really that it wasnt at least checked to rule it out .

I read your post and immediately thought its the cat .


I like long posts so don't say sorry for mine lol


EDIT ..

Just to say a crack in the exhaust near the pre cat sensor would cause similar issues but with 2 garages looking at the car i would have severely thought they would have spotted this if it was there so i discounted this fault from my above description .

 

[masher964]

A really nicely laid out response explaining logic and giving good guidance. Given the logic you express, it does seem a shame the other garages may not have investigated this possible cause. It'll be interesting to follow this one and find out the root issue opcorn:

 

[richieruss1]

Big thanks to all the responses, especially deMort, I have a lot of food for thought now. You are right it wasn't silver smoke more like a grey/white colour it was just dark and looked silver the way the sun was that evening and the shock that I was in as the thing always drove perfectly no bangs.

I need this backfire to be resolved so that I can hear the sound of that Flat 6 again without worrying about a bang. A point to note is that the bangs started after the AOS was replaced at my Indy. Prior to the AOS being replaced, the oil cap would not come off very easily when the engine was running.

You are right, as confidence is waning in my Indy, I'm not going to name anyone as he is a good guy but perhaps unable to fix this one.

Who would you recommend in the Bucks area to properly resolve this? I don't mind going further afield for some expert help! I just want my 911 back to full health!

 

[deMort]

If the oil filler cap was difficult to remove then it was a faulty AOS .. this may well have masked an issue related to the back firing .. also why it started after this was replaced .

The fault code is lambda sensor or the cat .. i would still have the cat checked as it fits some of your issues .

I'm afraid in this day and age a fault code or an eml light on which is basically a fault code is needed to try and diagnose a fault which may well be intermittent .

For me after confirming the cat is or isn't ok i would move onto the fueling adaptations listed on a tester and a basic look at general engine values .. how the lambda sensors are reading bank to bank .. is one side richer than the other etc etc .

If you drive the car and get an eml light on it will make it easier .. if it is the same code then we are back at a cat issue .

Id also like to know why the car sounds louder after it ground out .. something altered at that point !!

The Indy correctly diagnosed the AOS but maybe needs more time on this issue .. other than that its a case of looking around .. i'm afraid i don't know garages in your area so cant recommend anyone .. forum guys probably can though so perhaps a post asking ..

id put it in the general forum as not everyone reads the 991 section .

 

[TonyC911]

Who would you recommend in the Bucks area to properly resolve this? I don't mind going further afield for some expert help! I just want my 911 back to full health!

You have quite a few good Indy options within a reasonable distance;

• Northway nr Theale
  Autofarm nr Bicester
  Wrightune in Wallingford
  Porschetorque in Uxbridge

 

[coullstar]

Im assuming it only happens when hot?

Could the exhaust have a hairline crack from when you hit it that only shows up when hot (due to expansion) and is drawing in some air to cause the "pop".

 

[richieruss1]

Hi Guys,

Thanks again for your help. Quick update - I took the car to Devon last week for a staycation, and you guessed it, the amber light has come on again. I have been driving it like a nun and the MPG is amazing it's giving me almost 400 miles from a tank, so there is no efficiency issues, although I'm maxed out on the boredom scale!

Anyway, I've booked it into my Indy to investigate, hopefully with the help of this post we can get to the bottom of the issue!

 

[richieruss1]

I just came back from the Indy and the yellow fault code is now showing on Cat bank two. Indy suggests replacing Cat bank 2, but can't guarantee that this will help cure the backfire. Says any other issue like a coil or spark plug would show up in the diagnostics. The issue is that when it was in at Porsche they said that the issue was on bank one :?

I just want the thing to be fixed and back to normal, I'm fed up driving around in the slow lane even, although the mileage is good.

Should I just take the hit and send it into Porsche, or get a second opinion from someone else?

 

[TonyC911]

How about taking the opportunity to upgrade your cats to a sports version? :?:

If they don't fix the problem, at least you've benefited from a cool upgrade and not just back a square one?

 

[richieruss1]

That sounds like a good idea, do you know of any decent reviews?

 

[911Time]

Don't mean to steal Tony's thunder (sorry Tony) but I was looking into headers and sports cats a while back - either from Fabspeed or Topgear - the latter being UK based and far more cost effective!

Topgear webpage shows the 991.1 headers/sprts cats as currently out of stock but show 8 in production, so worth giving them a call.

Also worth asking for 10% discount (as they're a forum sponsor) and if you're prepared to create a before/after YT video you might get a bit more - never hurts to ask.

https://www.topgear.co.uk/porsche-911-991-gen-1-carrera-manifolds-with-sports-cat

Obviously can't promise this will fix your backfire problem but if you go down the sport cat route then they're worth a look.

PS: Don't forget, you may need to factor for replacement of the manifold bolts, which can shear and need drilling out, so that's something to be aware of! Some folk replace them with titanium studs to help removal down the line.