997.1 misfire in 5&6 only after 2 mins of idle from cold

[sneakybear]

Evening all.

My 997.1 C2S has developed an odd issue recently on startup.

From cold, the engine initially starts fine. It settles into an idle just above the 1k rpm mark happily. After a few minutes the idle drops to 750 rpm as per normal. Almost immediately after this, flashing CEL (plus a foreboding sense of dread) and it's incredibly rough. Rough to the point it almost cuts out.

About 20 seconds later it's back to idling fine and is perfectly normal elsewhere, it pulls fine, runs as smoothly as expected, no other weird issues, etc. This doesn't happen from warm start at all. Plugged in the OBD2 reader and I'm getting P0305 and P0306 codes indicating misfires in cylinders 5 and 6. But no other specific codes (it's not durametric).

Today from a cold start, I tried maintaining the throttle above 1.5k rpm for 30 seconds when the idle normally drops to 750 rpm. It never misfired and idled happily around 750 rpm afterwards


So, any suggestions on what should I be checking:

MAF came to mind - Can I unplug it and start from cold to see if it's the problem?
Coil packs - Previously they've been binary and either worked or not.
Spark plugs - Shouldn't I expect a consistent fail from cold?
O2 sensor - not sure about that but a bad reading could confuse the ECU perhaps?
Alternator - if not enough current, but I'd expect that to get worse when hot.
Engine earthing - again, expect hot to be worse.
Vacuum - if there was a leak that, when hot, sealed I'd understand, but it's fine from cold.

ECU?!

Appreciate any thoughts or input if anyone's had anything similar.

Cheers,
sb

[Boxstercol]

Try swapping coil packs around first to see if the problem continues but on different cylinders, if so - coil packs are the issue. If problem remains on same cylinders then not coil packs.

[IanLTV8]

I had exactly the same problem with mine a few months ago, would run perfect from cold and hot, when the car came off choke from a cold start, it would drop revs then throw up EML light with miss fire on left bank 4,5,6, when the car reached normal temperature it was perfect, this was after the big service with plugs and coil packs, turned out to be the cam valve sensor on the left bank, had that replaced but still wasn't perfect so the did the other side, total cost just under 400 all in

[sneakybear]

IanLTV8 said:

I had exactly the same problem with mine a few months ago, would run perfect from cold and hot, when the car came off choke from a cold start, it would drop revs then throw up EML light with miss fire on left bank 4,5,6, when the car reached normal temperature it was perfect, this was after the big service with plugs and coil packs, turned out to be the cam valve sensor on the left bank, had that replaced but still wasn't perfect so the did the other side, total cost just under 400 all in

This makes a lot of sense, many thanks guys!

Found similar symptoms on US sites reporting the same issue and where changing the solenoid and/or the cam valve sensor fixed it.
Researching how the cam valves work (aren't they clever!) and I can see why it would happen.

I'll post here once fixed.

Cheers,
sb

[jonno_]

I think it's probably the high lift solenoid.
Suspect once it's warm if you give it the beans to c5k revs you'll see the codes again.
Easy diagnosis is swap them between banks. If error codes follow, bingo.

[Pope]

My money is on inlet tappets.......

[sneakybear]

Evening all.

Turns out it was the camshaft solenoids on bank 2... new ones in and running very smoothly from cold through to idle and up to the red line... well nearly.

Chapeau to Sid at Porsche Torque who was able to diagnose and replace the parts super fast