Jump to content

The RennTech.org community is Member supported!  Please consider an ANNUAL donation to help keep this site operating.
Click here to Donate

Welcome to RennTech.org Community, Guest

There are many great features available to you once you register at RennTech.org
You are free to view posts here, but you must log in to reply to existing posts, or to start your own new topic. Like most online communities, there are costs involved to maintain a site like this - so we encourage our members to donate. All donations go to the costs operating and maintaining this site. We prefer that guests take part in our community and we offer a lot in return to those willing to join our corner of the Porsche world. This site is 99 percent member supported (less than 1 percent comes from advertising) - so please consider an annual donation to keep this site running.

Here are some of the features available - once you register at RennTech.org

  • View Classified Ads
  • DIY Tutorials
  • Porsche TSB Listings (limited)
  • VIN Decoder
  • Special Offers
  • OBD II P-Codes
  • Paint Codes
  • Registry
  • Videos System
  • View Reviews
  • and get rid of this welcome message

It takes just a few minutes to register, and it's FREE

Contributing Members also get these additional benefits:
(you become a Contributing Member by donating money to the operation of this site)

  • No ads - advertisements are removed
  • Access the Contributors Only Forum
  • Contributing Members Only Downloads
  • Send attachments with PMs
  • All image/file storage limits are substantially increased for all Contributing Members
  • Option Codes Lookup
  • VIN Option Lookups (limited)

2008 C4S with Engine Management Light

Recommended Posts


My 2008 C4S (60k miles and fully serviced) has been giving me an issue for the last few weeks, prior to that no related problems at all.


It started a few weeks ago with an amber engine management light coming on around 2 minutes after starting the car from cold. It is fine at startup with the engine running nicely, but after arouind a couple of minutes the light comes on and the engine starts to run lumpy BUT when i give it any increase in accelerator, it becomes smooth and runs perfectly again - it just seems to be very unhappy with my foot off the accelerator and the engine at idle where it drops to around 500-600rpm and runs lumpy with the engine management light coming on. If i carry on driving and the engine gets 'up to temperature' then the idle comes back to it's regular 900rpm and it ticks over perfectly.


I have my own code reader and the car was showing the following codes;


P0040 - Oxygen sensors upstream of cat. conv. interchanged (Signal implausible)

P0300 - Misfire summation fault (exceeds limit value)

P0301 - Cylinder 1 misfiring (exceeds limit value)

P0302 - Cylinder 2 misfiring (exceeds limit value)

P0303 - Cylinder 2 misfiring (exceeds limit value)


So, based on previous research on this issue, I changed all 6 coil packs, all 6 spark plugs and all 4 oxygen sensors (both the upstream and the downstream sensors on both sides).


This has not improved the issue however, and still the engine starts fine and smooth, after it has run for a couple of mins, it audibly drops revs, becomes lumpy and engine light comes on, but if i give it some gas, it runs perfectly again and the engine light goes out.


I should also mention that during this time of issue, it has also had an MOT and passed with perfect emissions.


In my old fuel injection cars, they used a cold start valve, which was like a choke for fuel injection engines and gave the engine a bit of extra fuel to start and warm up - could this be something similar and the two minute period is when the cold start valve closes and suddenly the engine is starved of fuel and runs awfully until i use the accelerator to give it back more fuel? However, why only one bank of cylinders (1,2 and 3)??


I am struggling with this one and hoping someone may have had this or knows what's going on?


Thanks, Steve


Link to comment
Share on other sites

  • Admin



Possible fault causes:

- Electric fault in control line between DME and ignition coil
- Spark plug connector faulty
- Ignition coil faulty
- Spark plug faulty
- Secondary air

- Mechanical or electrical fault in injection valve
- Valve seat or valve faulty
- Valve lift control faulty
- Exhaust system blocked/narrowed
- Foreign bodies in intake tract, this can lead to sporadic misfires on changing cylinders.

Link to comment
Share on other sites

Hi Steve, I had a Boxster with these symptoms exactly, start fine in the morning, warm up and then idle roughly, give it some throttle and seemed to run fine. On the odd occasion it wouldn't do it at all. I think it turned out to be the AOS, if I remember correctly you can effectively blank off the AOS, there is a tube coming from it which you can disconnect and then blank off both ends to test it, not sure if its the same on the 911. I know that this might not explain the codes though. Only other thing that might be worth a go would be to clean/disconnect the MAF, but again not sure how this could relate to codes. Hope you get it sorted.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.