Ok, don’t order random ECU’s


I don’t have any OBD info on 991.2 but major sensors may be the same. This is from a 991.1 TT so the cam values are going to be different. As a kinda educated guess check the intake cam sensor.

Check parameters of R&L Intake cam sensors while it’s misfiring. Compare the banks & see if one signal drifts. Monitor parameters and or wiggle wires & lightly tap on the sensors if you can get at them. See if you can make it miss while gently moving harness around.

Also compare L&R fuel trim and lambda values. That should catch an anomaly while it happens. If you can graph the parameters it may be easier to see.

(Note the orange highlight below, may this is it?)

 

thank you. Not ignoring, just a busy day and having a computer issue. I will study this and post today or tomorrow

 

When you have roughness / misfire at idle (vehicle stopped), was it in D?

Do you also have roughness /misfire at idle in N or P(vehicle stopped)?

I ask because you mentioned you initially had gearbox error messages. Please note that the engine ECU cannot actually know if there is misfire in the combustion chambers (since there is no sensor in the combustion chamber to detect misfire), it only looks as the engine speed trace whether it presents some abnormal oscillations and then infer whether there is roughness or misfire on going.
If there is a problem with the gearbox in D that makes the engine speed oscillates, the engine ECU will falsely detect it as roughness / misfire.
Potentially, any accessories or component messing up the engine torque resulting in engine speed oscillations can create a false detection of misfire.

 

How then is the engine CU able to count individual misfires on each cylinder?

 

Don’t over think it.

Aside from Ferrari, all manufacturers use this same crank acceleration method to detect misfire.

Now OP didn’t register any misfire errors but for a transmission to put a load the engine in such a way that it registers on 1 or 2 cylinders. Thats way more than a long shot. The window of time and acceleration it uses to detect crank acceleration is so specific that a loose part will not cause a misfire.


If you have a generic PDK fault and a specific Engine fault. The engine with a cam sensor fault will force the ECU to use a fall back method of looking at crank and exhaust cam to infer the position of the intake cam. The engine ECU’s changing to a fall back strategy will trigger other ECU’s that depend on engine parameters to flag a generic fault. These generic faults are tattletale errors that point to the engine.

So in a case of a car with 4-5 errors and only one is for a specific component and the others are generic, the specific fault has caused the tattletale errors indirectly. (PDK ECU is telling on Engine ECU)

 

Had misfires and rough running in P and D. Haven't tried to do any diagnostics while driving. Per your questions and 991Targa4S comments, I will look at data with trans in N, specifically the cam sensors. That said, since I am not well versed on these things, I am getting close to having it towed to a shop for professional analysis (I don't want to drive it the miles required to get there even though it still says driving permitted). Not going to admit defeat, just a strategic move so I get it back on the road sooner and I can go on to attending to other things. I like working on my cars but I also like driving them! I am pretty certain that it is not plug or coil relating since I switched those items on the # 2 and 3 cylinders with no change to the misfire and roughness readings. I will post an update tomorrow and thanks so much for your input.

 

been following this thread closely even though my wrenching days are 40 years behind me. interesting and puzzling. gotta tip my hat to you for even going this far. ditto the guys that chimed in with suggestions and instructions. pretty detailed and intense stuff but approached/explained in a way that most of us can grasp (kinda =)). learned alot here. thank you all

will be checking back to see how you get things sorted out. best of luck.
Cheers

 

Ran thru the data checks again today in P and N. No "P" codes, only data logs and still running rough. Unfortunately, my iCarsoft reads differently than the tool you posted but cam angle info consistent and lamda readings were "N/A". Given that I don't know where the sensors are located to rattle them, I'll be calling a local indie shop tomorrow to see if they are confident to diagnose and repair. I would like to establish a relationship other than dealer if possible (dealer OK, just expensive). I will continue to do my own PM but feel a little over my head on this issue. We live in the country and it's about time to set up the mower deck behind my Kubota, the pump for our reserve house water system failed to energize today so I have that to diagnose and repair and I need to get the '73 Corvette I resto-moded on the lift soon to do a brake bleed (it's been a while). I am going to the living room now to drink beer and eat junk food while watching Super bowl commercials and some football. Take care all, I will update soon as I have info.

 

After reading this thread.....it could become a parts replacement nightmare.....

Sounds like defective coil pack/s, ignition module that is regulating voltage incorrectly. and possibly a crank sensor gone bad...........

The person changing the plugs, etc...must have done something....hope it gets fixed without extensive expenses....

 

At your low mileage, a Fidelity Platinum warranty is less than $2500 depending on what term you want..

may be worth getting warranty and getting this sorted without throwing parts at the car

my .02 cents

 

not en expert but I think the engine ECU is able to calculate how much time in seconds it takes for a cylinder to move on a given angular window (in crank angle). If it takes too much time in seconds to let say move on a 20 crank angle window, it means there is no combustion so there is misfire. Since the ECU know exactly the angular position of each cylinder, it can detect misfire for each cylinder.

 

To your defense, after reading so many threads on misfire issues, when it’s not an obvious spark plugs/coil problem, I find misfire issues very difficult to diagnose. You have done already lots of tests so well done!

I agree best would be to bring the car to a savvy tech who can diagnose the car better.

I agree extended warranty is a good piece of mind on these modern 911s.

 

How does the ECU know if the cylinder is on the compression/combustion stroke rather than the intake/exhaust stroke?

Could knock sensors be used to assist in misfire detection?

Could modern wide-band, fast oxygen sensors be used for further confidence in misfire recognition?

 

- I don’t know the details. I suppose it is related to how the ECU synchronize the signals from the crankshaft with the camshaft.

- I don’t think so since the anormal combustion during knock results in pressure oscillations which is a cylinder pressure profile very different from the pressure profile when there is no combustion in the cylinder.

- I suppose it could be a useful info to use wide band O2 sensors since it’s value would change as a result of misfire (too lean for instance) but would be tricky to distinguish which cylinder is misfiring.

 

Not that it matters now for this problem, but for future reference and use, you toggle to:
1. settings
2. display
3. display menu
4. add/adjust available options to display on the screen (fourth line is usually blank, but you can add: voltage, compass, ETA, etc)