996TT rough idle, codes & MOT emissions problem

My car has had an erratic idle problem for a while now which I've been struggling to track down. It runs quite rough when first started and the rpm dips too low and almost stalls after revving the engine or when coming to a fast stop. The idle problem also seems worse once the engine is warmed up. I eventually got a CEL and read the following codes using Durametric: P0171 Porsche fault code 360 - Oxygen sensing adaption idle range bank 1 P0174 Porsche fault code 361 - Oxygen sensing adaption idle range bank 2 Things seemed much better after the CEL came on but went back to running rough and erratic idle once I cleared the fault codes. The car then ran rough for the next 30ish miles before the CEL came back on again with the same codes as before. This cycle can be repeated by clearing the codes again. I initially suspected unmetered air entering the system via a loose hose so did a boost leak test. However I pressurised the system from just behind the air box and it held pressure fine so I don't think that's the issue. The issue also goes away if I unplug the MAF which led me to suspect that was knackered. So I replaced with a brand new one which initially seemed to help but the problem quickly returned This has been a minor annoyance for a while but has now turned into a major problem as the car failed the MOT emissions test today with CO and HC values well over the limit and lambda of 0.947 which is just under the acceptable range. The car has been mapped and has a Speedtech exhaust with 100 cell CATs but this has been the case for the last 2 MOTs without issue so I'm sure the failure this time is related to the above problems. Anyone got experience of similar issues or any advice on what to check next ?

Unfortunately I don't know the answer to your problem, however I wouldn't boost leak the just after the airbox as that is the low pressure side. The boost leak test should be done and the outlet of the turbo. A good place to ask help on your issue is on the 996 turbo forum on FB, there is a guy on there called Markski who will know what your problem is. :thumbs:

Thanks for the advice. I've ordered one of the kits from Markski which will allow me to leak test from down at the turbos rather than behind the airbox. I also think this approach will identify problems that testing from behind the airbox doesn't (eg. #16 check valve).

Did you ask him what he thinks the problem could be? I hope you don't have to change the #16 check valve, I had to change mine 2 years ago and it was a pain to get to. Good luck :thumb:

I've asked the question on the FB group so will see if anyone's got any ideas there. Just waiting for the testing kit to arrive from the Markski so I can properly leak test. Fingers crossed it will be something simple!

The fault codes relate to a weak mixture on both banks , the car is adding more fuel to try and maintain Lambda but has reached the maximum amount of fuel it can add and so has flagged a code saying so. All cars compensate for small leaks or general wear and tear , owners will never realise untill they go out of limits . The car opens the injectors for longer to give it more fuel . Lambda = correct fuel / air ratio. So .. couple of things which you are already on top of .. Air leak and on a Turbo theres all the intercoolers to consider as well so a smoke test there . Maf sensor contaminated .. i would look at the voltage readings for the sensor but perhaps a clean of the sensor might be in order , actually you have replaced it so ignore that . Any type of oil based air filter can cause issues with the maf . Exhaust blow , air getting into the exhaust near the pre cat lambda sensors will confuse it . There are other things but we are moving into the unusual category so ill not go there yet . EDIT .. When you clear the fault codes the car resets the adaptions .. it WILL run like a sack as its way to lean at that point .. the car will slowly adapt and give more fuel untill it reaches its limits and flags a code plus eml light .

Hi guys, pleased to say that I have finally resolved this problem Turns out the issue with was the fuel pressure regulator (FPR) which was both clogged up with black gunk and also had a split o-ring. I could have just cleaned the filter and replaced the o-ring but decided to buy a new FPR as they're only around £40. Car now idles and drives perfectly Credit goes to Carl Henley from the 996TT Facebook group who pointed me in the right direction on this one. Looks like he had the same symptoms a while back and found his FPR clogged with the same black gunk as mine. Thanks also to the guys who responded on here....I really appreciate the advice and you've saved me a load of time (and probably money) chasing this down :thumb: I will post some pics of the old FPR below....hopefully this post will help somebody else with the same problem in the future.

Pics of the old FPR

Hi As a precaution, I might look to replace mine. Is the Fuel Pressure Regulator east to fit? Thanks

It's not too difficult if you have a decent set of tools. You have to unscrew the secondary air pump and oil filler neck bracket and move them aside and that gives you access to the FPR. Once i had removed the retaining clip, I levered it out with spanner against the air line barb. Reinstalling it is a little more difficult as it won't pop back in by hand. I had to use a long spanner and lever it back in against a solid section of the car body just behind it. The biggest issue I had was dropping things into the abyss which is the engine bay!!!! Lost a couple of sockets down there and had to retrieve with a magnetic tool. Also you need some extension bars as it's difficult to get the socket wrench in between the maze of pipes and wires in there. Definatly do-able by amateur mechanic

Thanks, I'll give it a go.

Hello there mate it's Carl from the 996tt FB forum,,so glad you got it sorted . I know what caused my problem with crap building up in the regulator or at least I think I do . I used a fuel line cleaner made by wings.I remember standing in a que in a parts shop and it was right there in front of me and I thought this won't do any harm just clean the fuel lines up😩. I believe that this solution created the problem. How about you haka did you add any cleaners in your tank a while back ?? Cheers Carl

Hi Carl, didn't realise you were on here too No I have never used any fuel cleaners on the car but the original FPR was put back on the car not long before the issue started. I was running a 5 bar FPR prior to that but put the stock one back in when we upgraded the injectors. I wonder if the o-ring got damaged at that point? Not sure where all that black stuff came from though! Just really glad it was such a simple fix...thanks again for your help :thumb: