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Hi all, I have what I could call an inconsistent engine idle with my 957 Turbo. The idle is not what I’d call rough, but there is an odd hesitation/miss which is always present on the first drive of the day and it is more pronounced in colder temps. Someone who is not particular would probably not be aware of the issue.
My shop diagnosed a hairline split in a plastic vacuum line that supplies the brake booster and this was treated with heat shrink. It did not fix the issue but my brakes got better!
I am wondering if there is potentially another or multiple vacuum leaks which could be contributing to this (hence it being worse when cold), or if anyone has a clue as to where I should start looking?
It may be worth noting that there is no issues with engine performance or power delivery whatsoever.
There is about half a mile of brittle plastic vac line and PCV line on your motor. It should be smoke tested to see if any of it is leaking.
But even before that, these things eat coils. When is the last time those were done? When's the last time you checked if there's oil in the plug bores? If you haven't done it yet, you're probably due for valve cover gaskets - which not only leak oil but also create vacuum leaks.
Have you replaced the coils and plugs? The CTTs eat them for breakfast. They are usually toast at about 60-80K miles, depending on your climate and driving style.
Plugs were done approximately 20,000 kms ago by the previous owner (less than 3 years ago). There is no invoice for coils that I have seen and he kept a lot of documentation from new so I'm guessing they've never been done. Would bad coils present this type of subtle issue?
There is about half a mile of brittle plastic vac line and PCV line on your motor. It should be smoke tested to see if any of it is leaking.
But even before that, these things eat coils. When is the last time those were done? When's the last time you checked if there's oil in the plug bores? If you haven't done it yet, you're probably due for valve cover gaskets - which not only leak oil but also create vacuum leaks.
Good advice. Looks like I need to pull the coils and have a look down there! Is it worth just changing the plugs while I'm at it? What type of plug is recommended for a CTT?
Good advice. Looks like I need to pull the coils and have a look down there! Is it worth just changing the plugs while I'm at it? What type of plug is recommended for a CTT?
If the plugs were done 20k ago they are probably just fine. I use the OEM Bosch FR6DPP332 in mine.
While the coils are out carefully inspect them for cracking. If they are original they probably are and should be replaced (they have been through something like 8 revision of these coils and they all still go bad eventually), but you might want to wait on plugs/coils until you replace the valve cover gaskets if you're finding oil in the plug bores. It's not going to destroy them but it does make a mess to be cleaned up later.
And to answer a previous question - yes, the coils can absolutely cause a subtle misfire that improves when it warms up. I've had exactly that and solved it with nothing other than coils. That being said, so can a bunch of other things so it's still a matter for diagnosis.
If the plugs were done 20k ago they are probably just fine. I use the OEM Bosch FR6DPP332 in mine.
While the coils are out carefully inspect them for cracking. If they are original they probably are and should be replaced (they have been through something like 8 revision of these coils and they all still go bad eventually), but you might want to wait on plugs/coils until you replace the valve cover gaskets if you're finding oil in the plug bores. It's not going to destroy them but it does make a mess to be cleaned up later.
And to answer a previous question - yes, the coils can absolutely cause a subtle misfire that improves when it warms up. I've had exactly that and solved it with nothing other than coils. That being said, so can a bunch of other things so it's still a matter for diagnosis.
Thanks. From my light reading on the Bosh plugs, they are pre-gapped? For the $90 odd I think I will just do the plugs anyway. Gives me a good opportunity to inspect what’s currently in there. I have had the car serviced twice at my local independent who specialize in Cayennes but they have yet to mention anything about the valve cover gaskets. Something to keep an eye on.
If you have a consistent misfire when cold on a particular cylinder when cold and after changing plugs swap coil with a cylinder that's not having misfires and see if the misfires follow the coil.
"For the $90 odd I think I will just do the plugs anyway." <<<This
FYI, while sometimes you can see cracking on a failed coil, they can fail without cracking.
Indeed, not even going to bother checking them. I’ll just order new ones off pelican then swap when they arrive. I’ve been putting it off for too long! The coils on my 996 GT3 failed after only 29,000 kms (but 17 years)!
Originally Posted by DarylJ
Very much so.
If you have a consistent misfire when cold on a particular cylinder when cold and after changing plugs swap coil with a cylinder that's not having misfires and see if the misfires follow the coil.
E: and yes, those plugs are pre gapped.
How would one determine which particular cylinder has a misfire?
How would one determine which particular cylinder has a misfire?
Even fairly basic OBD2 readers provide live data that will show you misfire counts per cylinder. On anything this "new" and electronic heavy you're basically guessing without a proper diagnostic tool.
Well after looking at walking past unopened parcel from Pelican Parts near the front door for the last month, I finally decided to swap out the plugs and coils yesterday. Glad I did. Every coil had at least one huge crack a couple of inches long. Im guessing they were original (128K Kms). Plugs had typical oil on the threads suggestive of valve cover gaskets leaking but the tips all looks good. The result is a smooth idle now. Throttle response is better too. Very pleased overall.
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957TT inconsistent idle when cold
