nine9six

Got a CEL today and performed an OBDII diag. read...

The following are the results from the scan

1996 Porsche 911 A = 3.6
I/M Monitors
Since DTCs Cleared
Misfire Monitor ok
Fuel System Mon ok
Comp Component ok
Catalyst Mon inc
Htd Catalyst n/a
Evap System Mon ok
Sec Air System ok
A/C Refrig Mon n/a
Oxygen Sens Mon ok
Oxygen Sens Htr ok
EGR/VVT Sys Mon n/a
=====================
DTCs (Codes)
P0300 MOD $17
Random - Multiple Misfire Detected
CONFIRMED
FREQ REPORTED FIX
1-Replaced Mass Air
Flow (MAF) Sensor
---------------------
P0306 MOD $17
Cylinder 6 Misfire Detected
CONFIRMED
---------------------
P0304 MOD $17
Cylinder 4 Misfire Detected
CONFIRMED
FREQ REPORTED FIX
1-Replaced Mass Air
Flow (MAF) Sensor
---------------------
P0305 MOD $17
Cylinder 5 Misfire Detected
CONFIRMED
FREQ REPORTED FIX
1-Replaced
Serpentine Belt
=====================
State OBD Check
MIL Status ON
Codes Found 0
Monitors OK 3
Monitors Inc 5
Monitors N/A 3

Can anyone help with some how to proceed from here? Would a PST 2 scan help in being more definitive to the potential issues?

Thanks in advance, gentlemen...

Sean E Yamuni

Just get a new cap for your ignition.....

Opo

Paul, what I will do first is reset it and see if it comes back on, you have been working on the seats and all that stuff might set off something, that's what I will do first but I'm sure some guys are going to think I'm LOCO

Jeff96-993

Search for "drive cycles", and then follow the procedure. You need to get all of the readiness monitors complete first, and then work from there.

Your random misfires can be tough to diagnose, but once you get all the monitors complete you'll know the extent of the issues.

nine9six

Rudy,
I'm sure the seat install has little to nothing to do with the random misfires on cylinders 4, 5, & 6...Maybe an O2 sensor on the right bank of cylinders, since the misfire is being recorded on the right bank.
I will however try to reset the CEL and go from there. car seems to run just fine.

Steve Weiner-Rennsport Systems

O2 sensor issues show up as short term fuel trim problems and you can watch them cycle in real time to see how they do. If they have 60K or more on them, its time.

Misfires really jack up the cat temps so this isn't something to ignore (unless you want to buy them, too). I'd be looking at:

1) Ignition: caps, rotors, plug wire set. If these items have more than 30K on them, they should ALL be replaced.

2) Failing dual-mass flywheel.

3) Non-factory fan & alternator belts.

There are more items, but this is a good start.

nine9six

Steve,
Thank you for your direction on where to start...Do you find it curious that all misfires are on the right bank of cylinders?

How does a failing dual mass flywheel affect misfires?

Lorenfb

"How does a failing dual mass flywheel affect misfires?"

The method, used to determine a cylinder's mis-fire, measures the small change
in RPM from cylinder to cylinder. When the DMF starts to fails, it causes the
ring gear for the speed sensor to not produce a stable speed signal which
appears to the DME ECM as a potential mis-fire.

"Do you find it curious that all misfires are on the right bank of cylinders?"

A bad O2 sensor on the right bank can cause that side to run lean causing
a mis-fire. All 993s run lean and are on the verge of lean mis-fires and all
it takes is a system element to 'push' the lean condition to cause a mis-fire.

Kika

I have had random misfires in the past. First off, don't worry too much about it. I would reset and see if it comes on again. If it becomes chronic, then I would look at ignition wires, and see if you have any with noticeable cracking. When was the last time you changed plugs?

Belts can also cause a misfire. A failing DMF, can also throw timing off, and throw a misfire.

Was the car hot? It as pretty hot in So Cal today, so a little extra heat could have caused a little more play in the DMF, which could affect timing.

Were you able to read the rpms? That could help to diagnose.

nine9six

Loren, Kika, Thank you for taking the time to share your knowledge!

Loren, if I understand you correctly, a marginal O2 sensor could be the culprit, since all misfires were noted on the right bank of cylinders, correct?

It was a hot (Santa Ana conditions) day yesterday, and although the car was not running "Hot" per se, it was warmer than usual, but not to midpoint of the temp gauge. Of course I attribute this to the increased ambient temps.
Could the increased ambient temp, exaserbated a marginal O2 sensor?

Car is shy of 72k, so I will check the records I have for plugs, rotor, cap, and wires. I have noted that clutch engagement is towards the middle and end of the pedal throw, but do not equate this with failing dual mass flywheel; but rather clutch wear.

I have since reset the fault codes, and will continue to monitor. At this point, rotor, rotor caps, wires, and possibly O2 sensors are probably prudent purchases if records do not show replacement within the recent past.

Thanks again for your knowledge and support! I'll keep my results posted...

nine9six

See responses within...


Kika, so what was the culprit to your "random" misfires?
I understand random, but when all read/recorded misfires are relegated to the right bank of cylinders; I find that pecular and of potential interest in a proper diagnosis, yes?

Otherwise noted, and will check...


Understood, but wouldn't one expect the misfires to be random I,E; both banks?

Operating temps were warmer than usual due to increased ambient temps, but never higher than midpoint on my temp gauge.

My code read was done with the CEL on and car off, so the short answer is, no, I dont have rpm readings. If this happens again, I will do a drive test and record the readings with my OBDII test unit. I have since reset the codes, and will continue to monitor.
[/QUOTE]

nine9six

Follow-up...
Records indicate that on 4/2010, half pulley, fan belt, alternator belt, A/C belt, and spark plugs were replaced.

edit:
O2 sensors replaced in yr 2000...So 13 yr old sensors could be the culprit.

Saw nothing in the way of rotor, rotor caps, or wires; so I think I'll replace the rotor and caps as part of maintenence and go from there...

csmab

When my flywheel went I got random misfires that triggered the CEL. If you clear the codes it might be interesting to see if your misfires are on the same bank when they trigger the CEL for a 2nd time? I cleared my codes several times and it seems the misfires were not consistently on the same cylinders.

Kika

Apologize up front for the long post: but a recap of my history regarding P0300 random misfires.

They started about 3 years ago, after I had chased down a P0410 and P1411 by having the SAI check valve replaced, and the bass guitar string treatment.

My P0300s were usually an entire bank, either cylinders 1-3 or 4-6. I did not notice it at the time, but the CEL was when the car was relatively hot, i.e. went to lunch somewhere, then on the second start up the CEL would come on. It was usually at start up. Since I had just changed plugs and rotors, and my wires were original (about 80K on the clock) I thought that was a good place to start, so I had the wires changed. It seemed like that took care of it, as the random misfires became much less frequent, or seemingly so. It is difficult to gauge, as I also was not driving the 993 as much. I haven't had a random misfire in over a year now, but I also only drove the car about 2000 miles in the last year.
I am in the process of having a LWF installed as we speak, so it remains to be seen if that will finally take care of it.

I know my approach is/was not very scientific, but I am hoping the LWF will completely solve the issue.

My theory is that I had 2 issues, failing ignition wires AND a failing DMF. Once I replaced the ignition wires, the random misfires became less frequent (before they wer about once every 5 times I drive the car, of about once a month) After that, it went to once every 3-5 months. It is just a theory, but the DMF has an inner section of silicon, and when it is hotter, there is more play, enough to throw the timing off and throw a CEL. I dont know if this qualifies as a "failure" or if it is just a situation that happens to throw a CEL, and is not really a problem. I was content to live with it, until it was apparent that my clutch needed to be replaced, and the time was right to replace the flywheel presented itself. I need to get the car back and drive it before I can report on whether or not my random misfire chapter is finally closed, but I certainly hope it is.

Hope this helps.

What this underscores for me is the importance of maintenance on these cars. There are some things that you can wait until they fail completely, but other things get out of spec and while the car will still run great, they will cause the CEL to come on as they start to fail.

nine9six

Kika,
Just for clarity, do you really mean less frequent after having changed the wires?

Thanks for your in-depth description!
Paul