First time posting -- any thoughts / bits of advice are very much appreciated!!

Symptoms:
-Slightly rough-feeling idle, but not extreme by any means.
-No codes.
-Durametric shows anomalous roughness values for cylinder 3 / some misfires on cylinder 3 (intermittently up to count of 50, typically at idle, right after engine braking).
-Graph of roughness / misfires on all cylinders attached
-Scatter plot of rpm vs injection time colored by cylinder roughness attached (cylinder 3 roughness vs cylinder 1 roughness for comparison)
-Slight reverberation / periodic sound from engine when cruising at around 2500 rpm. Totally smooth sounding above 3k. Possibly just resonance from exhaust (Kline).

History of car / mods:
-2007 997 Turbo 6-speed with 77K miles
-Bought completely stock at 48K miles, some overrevs in range 1, none in range 2-6.
-I added kline exhaust, cobb accessport, with stage 2 tune for 91 octane from Sam@ByDesign.

History of problem:
-Engine dropped at 75K to replace coolant elbows (w/ sharkwerks), check coolant pipes and fittings (already pinned), and replace faulty aux water pump.
-Immediately after getting car back from mechanic, coolant pipe to expansion tank let go, dumping coolant all over engine bay. Quickly got it back to mechanic, refilling expansion with distilled water. Water temp seemed fine throughout, only threw "low coolant" warning as I pulled up to mechanic's shop -- hoping no overheat.
-Mechanic fixed coolant pipe, but upon returning car the idle was EXTREMELY rough -- idle felt like vibrating massage chair, but smooth at higher rpm.
-At this point I started logging with durametric and noticed abnormally high roughness value on cylinder 3 at idle vs others. Increasing rpm slightly brought cyl 3 roughness down, which correlated with my perception of reduced vibration while sitting in car.
-Coils and plugs are relatively new -- about 10K miles on them.
-I asked mechanic about coolant dump affecting coil on cyl 3 -- they told me very unlikely.
-Instead mechanic blamed engine mounts (I had RSS semi-solid), convinced me to return to stock.
-With stock engine mounts installed, perception of vibration / roughness at idle substantially reduced -- I let it go, and continue to drive the car.
-Now, 2K miles later I go back log with durametric just to see if anything has changed. Still seeing pronounced roughness (e.g. +5.0) on cyl 3 at idle and during engine braking (e.g. -5.0).
-Please see attached graph + scatter plot below, along with their captions which describe the anomaly

Questions:
-First, thank you for any time you spend looking at / thinking about this post. I really appreciate it!! I have scoured the forums and have found relatively little info about roughness values on durametric.
-The correlation of negative roughness for cyl 3 during engine braking vs. positive roughness on cyl 3 during idle / with most roughness only occurring at low injection time seems potentially informative, but I don't have the expertise. Does this look like a simple issue of faulty coil/plug or possibly clogged injector? If so, should my next step be to replace coil/plug on cyl 3 (and likely on whole bank 1 while I'm in there)?
-Alternatively, am I being a hypochrondiac and chasing ghosts? Again, there are no codes. Came from subarus where cracked ringland was always looming -- possibly has made me paranoid.
-Worth doing compression test / leakdown?
-Thank you again! Happy to provide any additional info you think might be useful!




Log of roughness values on all cylinders, misfire counts on all cylinders, along with engine speed, load, lambdas on both banks, and cam. Cylinder 3 typically has a +5 roughness value at idle (other cylinder's absolute value of roughness is typically around 1 at idle). Cylinder 3 typically has a -5 roughness value during engine braking (other cylinder's abs roughness value much lower during same conditions). No codes, by cylinder 3 misfire count sometimes ticks up to 50 intermittently.

Scatter plot of injection time vs engine speed, colored by cylinder 3 roughness value (left) and cylinder 1 roughness value (right). Cylinder 1 roughness value is low throughout most of plot. Cylinder 3 shows high negative roughness at high rpm / low injection time, high positive roughness at low rpm / low injection time.

 

My strange idle problem (idle would drop to 200, back up, then drop, etc) turned out to be MAF2.

Nothing showed in any of the testing I had done, I finally pulled the MAF and ran cleaner over it, idle smoothed out.

I am not sure how much similarity, but the car ran fine, juts idled like crap. One thing that kept it running OK at idle was the Sport Mode turned on. Turn it off and the drops started again.

Not sure this will help.

Ed

 

Thanks very much for the advice Ed! I have cleaned the MAFs relatively recently, but will do so again! However, I'd sort of expect issues on all cylinders in the corresponding bank if it were a MAF sensor -- has that been other folk's experience with MAF sensor issues?

 

This might be slightly more informative: bucketed graph of rpm vs ignition angle vs cyl 3 roughness. Roughness does get a bit crazy on this cylinder at high rpm WOT (e.g. +33 roughness), but maybe that's not unusual (other cylinders also show their most extreme values at WOT and I see no misfires at WOT).

Right now I'm thinking most likely culprits in this order:
(1) coil / plug on cyl 3 are faulty (they did have coolant dumped on them about 2K miles ago. im going to inspect and replace when i find a spare 8 hours (lol) to kill)
(2) injector on cyl 3 is faulty (injectors are original, but i have run a couple rounds of seafoam / injector cleaner through the gas tank)
(3) burnt exhaust valve on cyl 3 (is this consistent with symptoms? or would i be seeing codes / bigger problems at WOT? how do I test this?)
(4) intake manifold leak near cyl 3 (how do i test this? brake cleaner on cyl 3 manifold while idling and check log for AFR changes?)
(5) DME wiring issue (have seen a case like this on a subaru EJ causing misfires on one cylinder. where is likely culprit for a wiring issue? at the coil? at the DME? anywhere??)

Anything else I should consider with higher priority?

 

Managed to get a pretty good look at the back of the coil pack for cylinder 3 -- no discernable cracks .

Still having trouble figuring out whether these cylinder roughness values for cylinder 3 are even a real concern. Has anyone else with a 997.1 or a 996 turbo logged cylinder roughness values with the durametric? What kind of roughness values do you see at idle, during engine braking, and at WOT?

 

You should post this on Rennlist if you have not.

Good luck

 

And see if symptoms change cylinders.

 

@Gordon Thanks for the tip! I'll try posting on rennlist

@faiz Thanks for the advice! I think you're right, that's a solid next step -- just need to find some time to pull the bumper and ICs off.

 

Quick follow-up: Has anyone else logged cylinder roughness at idle with the durametric?

 

Well -- not without some trouble, I managed to replace coil and plug on cylinder 3 this past weekend. Unfortunately, no change in roughness values on cylinder 3 at idle. In fact, maybe slightly worse -- though possibly just temperature dependent or something. Now idle roughness for cylinder 3 sits at about +5.0. I've attached a picture of the spark plug I removed. What's next? Injectors I suppose? I still think the flywheel is a possible culprit. It is stock dual mass, replaced with brand new OEM part about 20K mi ago when I changed clutch. Any way to diagnose flywheel without dropping engine? Thanks for any advice!!!

 

Unless I am missing something, I am not sure what the flywheel would do to affect only 1 cylinder.

The injector would be my next step, unless you can pinpoint a wiring issue with that coil or a compression issue with the cylinder. To test leakage, build yourself a boost leak test kit and see what happens.

I will try and get you some logs next week (busy reffing this weekend with a tourney and normal youth/men's games).

Ed

 

Thanks Ed! Makes sense regarding flywheel. I'll start looking into injector swapping / compression and leakdown testing. Mechanic is all booked until Nov apparently, so will be trying to do it myself.