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Hello,

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

 

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:welcomeani:

 

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.

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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.

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