Engine Roughness

[911_owner]

2013 carrera 4 PDK with 39 000 km (24 000 miles)

At times I do experience some rougher than normal idling or twitching while accelerating in the range of 1500 - 2500 rpm.

This happened happened again yesterday when I was driving back from a car meet, we had been driving on some country roads mostly between 3500 - 5000 rpm, now I was driving home on the highway ~ 100 - 120 km/h (60-70 mph) in comfort mode. PDK shifts to 7th gear and the engine runs smooth (1500 rpm or so), but it starts twitching every time I pushed the accelerator. So I put it in sports mode and PDK shifts to 6th gear and the engine runs perfectly again. No twitches on acceleration.

This only happens in comfort mode (and not every time) and never in sport mode. No fault codes or check engine light.

I ran a diagnostic but could not find any fault codes. I could se that there were logs of recorded misfires and an Engine Roughness Reference Value of 65513.

I can not fully interpret this data, so I need your help.

Are any of these numbers considered not normal?
What are the reference values?




Screenshots from some live data:





Thank you in advance!

[EGTE]

Does it happen when the roads are wet?

Misfires on 911s are often caused by cracks in coil-packs letting water in.

[911_owner]

EGTE said:

Does it happen when the roads are wet?

Misfires on 911s are often caused by cracks in coil-packs letting water in.

Yes, it was raining yesterday.
But I'm not sure that this problem only occurs when the roads are wet.
I'll check next time we have dry weather.

Thank you for your post!

[LennyM1984]

I don't really know how Sport mode works but if it was cracked coil packs you may expect it to happen regardless of the mode you had selected.

Nonetheless, are you able to take a look at the coil packs just in case? The cracks tend to be really tiny so you'll probably need to take them off the car to inspect.

[911_owner]

I brought the car to Porsche Center where they performed a standard trouble shooting and they couldn't find anything except that there were misfires on cyl 3 & 6. After some discussions, the coils on cyl 3 & 6 were replaced with new coils. Initially there result seemed promising, much smoother idling and more power (probably just a placebo effect) but after driving the car for a few days I can tell that the problem is not yet resolved.

There are still some misfires, but they occur less frequently and the car still occasionally twitches when accelerating on high gears in low rpms in comfort mode.

[Thehusky]

Have the spark plugs been pulled and inspected? That will give you a good idea on the health of the engine.
Have you done any logging of the lambda values when the engine is missifiring?

[Pope]

You may be experiencing valve lift switching - or lack thereof.....

If the engine is gently free-revved to 5-6krpm you can hear the step change as the valve lift switches from low to high lift - while driving it is load dependant so will happen at a different point.

If you are accelerating gently enough you can sometimes feel the change (it is normal to feel it briefly as it HAS to switch but generally as the car is accelerating hard it is not felt - not as aggressive as VTEC switching for example) but if your valve lift solenoid(s) are beginning to fail you'll feel it far more.

The valve lift solenoids switch the tappets using directed oil pressure; sometimes dirt particles build up under the solenoid or it can stick/fail completely.

If you have Warranty the solenoids can be replaced quickly/easily and the tech should flush out the solenoid bore and/or replace the oil if it's due a change (possibly at your cost). If not they are 100 quid each (ish) plus an hour or so to do both (bit of clean up from the flushing etc) + recommended oil change