348 Spark Issue Unique Problem

Situation: On my '94 348 Spider I had a 1211 O2 sensor "check engine" error code on the right side of my engine. I swapped the O2 sensor from my left side to my right side and vice-versa.

Lo and behold, my 1211 code switched sides. OK. Easy enough. I replaced the O2 sensor, reset the ECU's when the car was cold, and no more error code after days of driving.

But...

I could hear a muffled popping sound in my cats at idle. Not loud. But there. To be fair, I probably noticed this sound even before I changed out the O2 sensor...just didn't give the sound much thought.

However, now this made me think that I had a real problem that killed my O2 sensor in the first place, so I investigated further.

For about 25 seconds I put my spark indicator tool on the wire for #1 cyl: great spark. Repeated for #2 cyl, lousy...only an occassional spark. Not good.

#3 cyl, great spark. #4 cyl, lousy. Hardly a spark at all.

#5, great. #6, great. #7, great. #8, lousy. It's like the #2 wire above.

#4 was by far the worst of the bunch.

So 2, 4, and 8 are problematic at this point.

OK. 348's have symetrical engines, so I swapped my spark plug wires from cyl #1, which had a spark, to cyl #8, which didn't.

Fired up the 348 and hit each wire with my spark indicator tool.

Now, cyls #2, 4, 6, and 8 had no spark. My even numbered cylinders aren't sparking!

OK. 348's have symetrical engines, remember...

So now I swapped the coils from left to right and vice-versa. During the swap it was obvious that the back metal plates of both coils were rusted, so I cleaned them thoroughly (what could it hurt).

Started up my 348 and checked the spark on all wires. Spark was now good on all except #8 and #4, which had almost no spark (intermittently would spark a single time then be quiet again for a minute or two).

Hmmm... progress, I suppose. Now I've gone from 4 cylinders with no spark down to 2 without spark, but each on opposite corners of the engine.


OK. 348's have symetrical engines (I sense a theme here)...

So now I cleaned and swapped the transistor power moduls from left to right and vice-versa.

Checked the wires and I've got a spark on #8 (though not perfect), but no spark on #2 and no spark on #4.

So 1, 3, 5, 6, and 7 are perfect...#8 has a mostly good spark (some pauses in the spark indicator light flashing), but #2 and #4 are DOA.

Pause

Since #4 has been bad all along, I swapped spark plugs between #4 and #1. No change. #4 is still not sparking, #1 still does.

(the odd #'s have always worked)

So it's not a problem with a spark plug.

OK. I got a used coil from a fellow 348 owner and installed it on my right side.

Progress! Now #2 is showing a spark. This means that I've got a detectable spark on all but #4.

Just to verify, I grabbed my infrared temp thermometer and measured my exhaust (shooting the laser beam straight into the exhaust pipes from outside in the rear).

156 degrees F on the left side, 280 degrees F on the right side (i.e. #4's side).



SOooooo... any thoughts on the temps (i.e. is 156 normal and 280 indicative of just one spark plug not firing)?

Should I be doing anything other than replacing the power transistor on the right side?

Any and all help is appreciated, thanks!

Maybe just a few thoughts could help:

The ignition wires are made of neoprene resistor wires. The resistor in ohms is per meter, so short one have less resistance than long ones. Even on a new set.

The outer material ages and the inner wire burns out. So time and mileage limit there live time. 10 years are the maximum IMO.

The ignition is of the wasted spark type. So two ignition wires have the same origin on the coil.
Or, one coil pack consists of two single coils, each connected to two spark plugs at the same time.

The compressed fresh charge in the cylinder which is supposed to fire at TDC has less electrical resistance to ionisation than the TDC cylinder which is filled with burnt exhaust gas. So the spark should always jump in the right cylinder by it self.

If the ignition wires have developed a gap inside or altered there resistance, it can lead to a situation where the spark does not jump in the right cylinder.
Using your tool, you may just see this as weak sparks.
Often this wire even discharges into water hoses or metal parts of the engine.

My next step would be : Get a few feet good silicon resistor wire an build new wires using the old connectors. If you are careful, you can re use them. Spray WD40 or silicon oil in the connectors before disassembling them. They are not glued. You can push in a small screwdriver and spray in the gap. Some are press fit, some are crimped and the ones at the coil might be turned in. If you have to choose wires, something like 1k ohm per meter should be fine with the Motronic.

Please don’t use solid copper core wire. It will not work with the old connectors. If you prefer these, you have to use resistor type connectors at both ends and “R” type spark plugs.

Conclusion: If the compression in both cylinders is ok and the injector in working order, the spark plugs perfect and only one cylinder has misfire, it can only be the connecting ignition wire.

A swap of the wires from left to right might cure the symptoms for a short time, because you change there position. If the resistance measures ok, the neoprene can still be porous. It is not OK to get electric shocks when touching the wires on a running engine, even if some consider this normal!

Chris