uh-oh

You could have a failed/failing catalytic converter.

 

Time for a compression test. Could be a bent valve.

 

Fuel filter, dirty air cleaner, bad gas?

 

If you're saying that the wires (possibly the extenders) are arcing to the block I think you found your problem.

 

You said you saw some arching, have you tried swapping plug wires from one side to another to see if the problem jumps sides?

I would also have a look at the condition of the wiring on the back of the pins for the fuel injector clips on the wiring harness . Pull back the rubber boots and have a look. Ferrari wiring isn't all that great.

 

If you eliminate spark and fuel I would suspect a valvespring. They do not necessarily make noise and may even pass a compression test. Performing a running compression test (remove schrader from comp tester and monitor compression while running and compare to other cylinders) will likely confirm the concern.

 

Possible.

From the black that you observed in the throttle body I would suspect an intake valve.

 

+1
I had that same symptom on my 328 and it was an intake valve. No strange noises, no smoke, no oil consumption. Just an intake valve not sealing properly.

 

I would recommend a compression and leak down test. That's what I did to diagnose where the problem was. It will allow you to know if it is a valve problem (intake or exhaust), rings or head gasket. A confirmed valve problem will most likely require head removal. How many miles does it have?

 

Intake valve problem ... see the black intake?
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Have you checked exactly which cylinder is causing the miss fire? You can do this by removing the wires to the injector clips above the spark plug area just on top of the block. There is one per cylinder and they clip on and off. You will be able to tell which one is causing the problem by revs not dropping and then look to the cause.
i had a similar problem and it was as simple as some dirt lodged in the spark plug journal that was causing an intermittent short of the spark plug to the block. Mechanic cleaned this area out carefully and car was like new.

 

Are you saying the sensor is compensating and making it rich, or the sensor is defective, and causing the richness? Pre-cat O2 sensors?

 

If you are talking o2 sensors there are only 4 one before the cat and one after on each side from what i remember. There are standard bosch ones but they need to be retro fitted as they are 1 wire short but are a lot cheaper tham oem. A good autoelectrician/mechanic can retrofit for you. They are about $90 in australia about 1/3 the price of oem ones.

 

Lame! another 355 bites the dust.

 

See post #14 ...

 

Sudden loss of compression is likely a broken valve spring.