help with 02 996 misfires at 900-3000RPM engine speed

[garrett376]

Dear Renntech 996 community,

I am trying to solve a low speed misfire problem and hope someone can help since I am at the end of most everything I can think of keeping the engine intact!  I've worked hard trying to figure this problem out, as seen below.  Thank you ahead of time for any pointers you might have!!

 

The car: 2002 996 with an x51 engine with 29K miles.  It spends 99% of its time as a PCA J-class club race car but is street legal.  Only engine modification is a 200-cell cat system with no mufflers.  Gets oil changes every 500 miles or so and never requires top ups and has no smoke on startup ever.

 

Problem: low speed (900-3000RPM) engine misfires affecting both banks, primarily cylinders 1, 2, 4, 6, only occasionally resulting in P0300 and P030X codes.  Misfires can be felt and heard when idling up to 3K RPM, and rich exhaust mixture can be smelled from the car and backfires are prominent when shifting/letting off throttle.  At high load/engine speed it "pulls" strongly as it had in the past.

 

First incidence of the problem: Had a CEL with a Misfire cylinder 1 which affected the engine "feel" for about 5 seconds during a race practice session lap.  After clearing the code the issue never came back; ran the entire 3 day race weekend with no issues at all.  I chalked it up to the damp/rainy conditions and a possible bad coil.

 

Next incidence: drove the car around town and got a CEL with cylinder 1 misfire and misfiring could be heard and felt when engine is run at 900-3000RPM and exhaust smells rich.  Coil on #1 swapped with #3 coil; plugs inspected and looked OK.  Drove again and misfire #1 happened again.  Monitored engine running with PIWIS2 and misfires at low engine speeds occur across cylinders 1, 2, 4, 6 but not #3 or 5.  

 

What's been done (inspections, tests and parts replacement):

• New coils - prior ones on bank 1 had tiny 2-3mm cracks; bank 2 appeared normal.
• Spark plugs appear normal in color and wear (have 29K miles on them).
• New flywheel.
• Air filter new, filter box clean and not blocked, MAF screen clear, throttle body clean with zero oil in plenum behind throttle plate.
• New MAF installed.  Old and new MAF's read 10.8-12 kg/h at idle and only 28-29 at 3,000 RPM.  Running with no MAF plugged in doesn't affect misfires.
• Intake vacuum test reads stable 24mmHg at idle; throttle snap vacuum test is normal where vacuum goes to 0 and then back to stable 24
• Crankcase vacuum (Oil filler lid vacuum) test reads about 6 inches water
• O2 sensors swapped with known good units on stock cats with no effect
• Valve train high- and low-lift activations are occurring normally per the PIWIS2 monitoring while driving at various speeds/loads.
• No lifter rattles or noises that are abnormal.
• Purge valve test, and function valve lift control, function camshaft adjustment tests return normal/"function active" with PIWIS2
• Purge valve holds vacuum.  
• Manifold intake switchover flap holds vacuum and functions properly when tested with PIWIS2.
• Fuel trims appear normal (FR’s are close to 1.0 and RK’s are 0- 0.2% or so)
• injector shut off testing via PIWIS2 results in even RPM drop for all cylinders
• x59 connectors look brand new with no signs of corrosion (garaged california car)
• DME map re-flashed with factory map from PIWIS2 (original flash was factory programming map, too)

 

Current condition:

• Nothing changed after the above parts replacements/swaps.
• Misfires can be heard and felt and also seen when monitoring with a PIWIS2; they occur across cylinders 1, 2, 4, 6 with none logging at 3 and 5 at idle up to 3000RPM with no load despite P0303 CEL being logged occasionally.  
• Infrequent/occasional CEJ with P0300 and P030X codes stored; code usually occurs within a minute if not right after engine startup.
• Driving WOT under load the engine pulls normally with no issues and sounds normal.
• Variocam 3100 RPM "boost" sensation and noise change can be felt as normal when driving. 
• Cam deviations are 0.4 degrees from one another and do not budge with engine speed changes.

 

Any ideas short of pulling the valve covers to hunt for bad intake lifters?  Thank you!!  

Edited July 26, 2016 by garrett376

[judgejon]

I truly have no clue as to how to even begin to answer your question.

But, I'm curious.  This car you describe is not possibly "street legal."  At least I have a serious doubt about your comment to that effect.

Question:  where is the general area where you live?  I might want to move there.  

The car sounds awesome.  Good luck.  

[Loren]

Factory spark plugs?

[RFM]

Do a test drive with the OEM exhaust in order to know the difference in value, 200 cell cats can occasionally show strange problems when they are no longer optimally.

[DBJoe996]

I'm with Loren on this one.  You said spark plugs inspected w/29K miles on them, but you didn't say you replaced them.  Point in case, recent misfire on my car.  Determined to be bad plugs, and I only had  24K miles on them.  Replaced with new plugs and no more misfires.  Put in new plugs.

[garrett376]

On July 26, 2016 at 6:33 AM, Loren said:

Factory spark plugs?

Loren... Loren...  I should have posted this thread about a month ago.  The moment you replied I ordered a new set of plugs and the swap of the spark plugs performed yesterday eliminated the problem.  Thank you!! :)

 

The old plugs do not really appear worn and they are not shorted out when tested, but now I know they don't have to look bad to function improperly.  Thank you all again, I appreciate your input and willingness to help.

[Loren]