How to correctly diagnose misfiring, vibration and torque loss?

[hsheil]

Hello all

Car information: 996 Carrera 2 coupe, model year 99, 6 speed, UK model.

Brief problem description:

Constant rough running, periodic engine misfires, reported P code associated with misfire events is P0134 (Oxygen sensor ahead of catalytic converter (cylinders 1-3) on Motronic 5.2.2 readout), noticeable engine vibration at circa 3,000 rpm. Advice needed on how to rule in / out MAF, O2 sensors etc.

More detailed problem description:

Recently my car developed a misfire. I ordered the Durametric hw/sw and saw that the EMU was reporting a P0134 code. I have read all the posts on this forum on MAFs and decided to clean the MAF using electronic contact cleaner.

This definitely helped - the misfires went away for circa 700 miles, however an engine vibration over 2,800 RPM remained. The engine also sounded rougher and I felt that the car did not have the power / torque it had before (but those are subjective opinions, it's easy to start imagining things!).

About 100 miles ago, the misfires returned, caused by the same P-code - P0134. I cleaned the MAF again and again this has temporarily solved the problem but I want to fix the problem properly and get rid of the noticeable engine vibration / noise and get the missing torque back. I'm asking for advice on:

* How can I identify if the problem is a MAF problem or an O2 sensor problem or indeed something else altogether;

* After I clean the MAF, does wiping the P codes using the Durametric package also reset the EMU or do I need to disconnect the battery in order to do this (and should I be resetting the EMU at all);

* If it is the O2 sensor problem, do all sensors need to be replaced together or can they be replaced individually;

* What features of the Durametric package can I use to help isolate the faulty component;

* will replacing a faulty MAF / O2 sensor (if it isn't anything else) solve the vibration and perceived lack of torque symptoms that I have also noticed.

Thanks in advance for your replies.

Humphrey

PS Here is the last output I read and saved using the Durametric package.

P0134

Factory Fault Code 10 - Oxygen sensor ahead of catalytic converter (cylinders 1-3)

Interruption of signal

P0300

Factory Fault Code 75 - Misfire damaging to catalytic converter

P0302

Factory Fault Code 64 - Misfire, cylinder 2, damaging to catalytic converter

P1319

Factory Fault Code 62 - Misfire, emission related

P1314

Factory Fault Code 51 - Misfire, cylinder 2, emission related

[Loren]

Any work done recently?

The misfire codes point towards wrong plugs, or bad coil packs (cracked), or bad connections.

I would solve the misfire problems and then clear all the codes and see what (if any) come back.

A bad MAF would not cause individual cylinders to misfire (at least not logically to me).

Clearing the fault codes with the Durametric Software should be fine. You could disconnect the battery also - that would cause the DME to "re-learn" your driving style.

If you do have a bad O2 sensor they can be replaced individually. BTW... How many miles?

Let's hope your vibration is misfire related - some of the other possibilities are not good. ;)

[hsheil]

Hi Loren

thanks for the reply.

Any work done recently?

The misfire codes point towards wrong plugs, or bad coil packs (cracked), or bad connections.

I would solve the misfire problems and then clear all the codes and see what (if any) come back.

No, no work done recently, this is a problem that just cropped up. Apart from needing a new coolant reservoir tank a few months ago, the car has been bullet-proof for the three years I've owned it - no problems at all.

The misfires always happen after the P0134 as far as I can tell. The reason that the log I posted has four misfire codes after the P0134 is because I had to carry on driving the car after the CEL came on. I will check the coil packs et al. as well though to make sure.

If you do have a bad O2 sensor they can be replaced individually. BTW... How many miles?

Let's hope your vibration is misfire related - some of the other possibilities are not good. ;)

The car has 110k miles on it, the MAF was last replaced at about 90k miles. The vibration started at exactly the same time as the rough running and intermittent misfire that's why I'm assuming they are all symptoms of the same problem.

Humphrey

[Loren]

P0134 Oxygen Sensor Ahead of Catalytic Converter (Cylinders 1 - 3) - Interruption of Signal Function

To diagnose the readiness for operation of the oxygen sensors, the position of the sensor voltage is evaluated (diagnosis is disabled in the case of certain faults or conditions).

Diagnosis conditions

Diagnostic Trouble Code P0134 is stored by the DME control module when the sensor voltage is continuously between 400 mY and 600 mY for more than 5 seconds.

Potential causes:

Open circuit in signal wire or ground wire of one oxygen sensor.

Oxygen sensor heating not functioning.

Output stage of DME control module faulty.

Open circuit to common sensor ground.

- Wiring harness

- Oxygen sensor

- DME control module

Look for loose connections and then replace that sensor of the connections are ok.

O2 sensors on these cars seem to last 60,000 to 120,000 miles. I've seen a Boxster need all for within a few thousand miles of 60,000. And, I've seen 996's with 118,000 and still no bad sensors.

[grahamtwatson]

The codes seem to point to a possible problem with cylinder 2. It may be a bit old fashioned, but have you thought of pulling the plugs and "reading" them. You can tell a lot from the color and condition of a spark plug (search the internet, there are several sites with good pictures of plugs and what the different appearances mean). I don't know how difficult is to get to the plugs on a 996 though.... If it is a problem with cylinder 2, I would think that narrows it down to a bad connection, bad coil, bad plug or bad injector, all of which you could test by swapping them with another cylinder and seeing if the fault moves as well.

Graham

[hsheil]

Hello all

just thought I would close out this thread by reporting the resolution to my problem. Turns out there were multiple underlying causes to the symptoms I reported at the start:

* a bad airlock;

* cracked coil on cylinder two; and

* an O2 sensor that was reporting incorrect readings (as reported by the Durametric package).

The car solved number one itself with a massive burp of coolant after a particularly hard drive, giving me a scare in the process (initially thought the cylinder liner had gone - engine cut out, the engine compartment was soaked and a big cloud of water vapour out the back).

Number two was solved as part of a 24,000 mile service. Number three (the O2 sensor) was solved by a fuel additive at the same time - I saw the outputs of the O2 sensors - the sensor ahead of the cat was continually reporting incorrect readings and oscillating back and forth - the fuel additive removed whatever buildup / something else and it's been fine since.

The car is running well now - all of the torque and power is back again. Thanks for all the replies.

Humphrey