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Been getting my new (2 me) 996 C4 Conv up to snuff, but one lingering problem is an eratic idle. The car uses the electronic TB so there is no idle control valve to clean. I checked for vac leaks, cleaned MAF etc.

It will be fine when cold, but after a bit when you come to a stop light the idle will fall to around 900, then all of the sudden will spin up to 1800 or more. A quick blip of the petal will bring it down for a bit then it starts to slip up.

It will throw a 507 MIL code, etc.

Sometimes I get a bank x lean MIL, replacing all the O2 sensors is an option but rather pricey maybe this will fix it option.

Car had 109k miles and this is abouth the only problem I have not found a fix for.



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  • Admin

P0507 is almost always either a sticking throttle valve or intake air leak.

Even though you have no idle control valve you still need to clean the throttle body, the throttle butterfly - both font and back side. Use a good spray carb cleaner for this.

Also use some electrical contact cleaner on the throttle electrical connectors. Remove them and clean the contacts on both the plug and socket.

Finally make sure that the rubber intake boot is properly and securely fastened.

If you still have the problem after that you will likely need a shop with a smoke machine to look for intake air leaks.

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  • 2 months later...

Loren & all,

well 3 months later, new TB and seal, checked every place for intake leak. Cleaned MAF got better but still runs fine cold but after 30 min drive (my home trip) it gets back to the strange idle. You come to a light, it idles around 8 to 900 then will sneak up to 1800. Quick blip will bring it down only to start the event all overagain.

I am not new to Porsche problems maintained my 944,968, built my track car and build motors. I have duramatic tool and can get hard numbers. But this one only when it is up to full running temp has me stumped. Could it be a temp sensor? or O2's?

any advice other than look for intake leaks?


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No I had not checked the oil filler line, thanks I will track that one down.

As soon as it gets a bit warmer I will start collecting values for those data points. Trick is I need to drag my laptop along with me to catch it when it is playing the idle game.


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after you have cleaned the throttle body and verified you have no vacuum leaks don't forget to adapt the throttle body, i think you have to turn key on engine off for 60 seconds.. theres posts floating around about it.

keep in mind that throttle bodies do go bad occasionally and commonly the MAF / the oil fill tube is often rubbed through near the rear connection underneath the sheetmetal by the coolant bottle. its very hard to find without a smoke tester.

Check AOS bellows if applicable


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Check for a vacuum leak with the diametric. Check MAF values and fuel trims. If the MAF readings are below spec and the fuel trims are rich, then it's probably a vacuum leak. Could be a leak in the evap system. Pinch off the hose going from the evap system that connects at the throttle body. If the MAF and fuel trims go back to normal, it's the solenoid operated evap valve in the engine compartment.

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I still keep working this problem, will be check the leak potentials this weekend, but I have noticed one thing- We are having real cold weather and idle is fine until the water temp get to full running state (180). At temps below the full operating temp Idle is fine, sits around 600-700 after it gets hot it sits at 900 - 1000 and when sitting at a stop light it will randomly jump into its lets Idle higher and start moving up to 1200 then continue to 1800. A quick blip and it goes back to 900-1000

My logical brain suggests a temp sensor making system think it needs to be richer and the system just trying to balance the F/A mixture (Thoughts??)


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  • 3 months later...

i might suspect a maf...

they often fail without triggering a cel... i noticed you may have consider replacing o2's.. they are at the end of thechain as monitors and alering fuel trim.. mixture problems are often triggered further upstream ie. maf...intake leaks...

if, you have taken care of vacuum leaks tb cleaning etc...

you can see my recent post about a 996tt... http://www.renntech.org/forums/topic/38776-high-idle/

best of luck

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Hi Dan,

Did you ever resolve your high Idle? I have an '04 996 TT with the same problem... I have completely pressure tested it and done all the cleaning as you have and I just can't seem to find the smoking gun... EleCtriCT mentioned the MAF. I bet if the TB goes bad it is pretty obvious, but maybe not.

I'd love to hear your solution if you found one...



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All suggestions/recommendations are good things to check. I had erratic idle, floating and surging (or what I would call hunting). Cleaned MAF sensor several times. Finally bit the bullet and got a new MAF sensor. That did it for me. At 109K miles I would suspect yours needs replacement. They don't last forever and by replacing you will at least totally eliminate it as a potential problem.

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  • 1 month later...

I have as different slant on this rough running topic.

My car is a 2003 996 Targa tiptronic which I have had for a month as my first Porsche


It starts no problem and runs at 1200 rpm within seconds the revs start to drop , the engine starts to run rough, if you are really quick you can sometimes catch it on the throttle revving it to clear .


Otherwise the throttle position has no affect on the engine and the revs continue to drop with the engine misfiring badly until it finally stalls out at about 5-600 rpm.

The car starts immediately on a restart and can do this upto 3 times before it will idle.


You can sense a change in engine note as something either cuts in or switches off.


It only does it in the morning never in the afternoon having been parked all day.

There are no warning lights showing.

The throttle body has been cleaned and air filter changed.



You guys got any ideas on this one

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With the engine running remove your oil filler cap and note any changes in the engine idle.  My engine was idle hunting, misfiring, backfiring and running very poorly, rough and low power curve.  What I found was that my oil filler tube had split in two places in the corrugated section.  They were barely noticeable but were creating a vacuum leak in the crankcase.  If you don't notice a change in the engine idle with the oil filler cap removed, I would investigate the oil filler tube very carefully.  Previously I had replaced the AOS due to a split casing that leaked coolant, and the MAF sensor due to a CEL.  The other problems I noted were split and/or broken vacuum lines...the plastic ones running across the top of the engine.  Many heat cycles made those brittle so they break very easily.  Engine now runs perfect and the change is remarkable.

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