Misfire under boost: plugs, coils and sensors

The problem

A Z22SE running boost (close to 2 bar, according to one reply) on an aftermarket standalone ECU stumbled under full, hard acceleration, though not every time. It didn't happen on the rolling road. The owner had thought he had fixed it, and it came back, so he asked whether another coil pack could be fitted.

Can the standard coil pack cope?

• One owner said a coil pack can cope with almost any power, because power has little to do with the spark voltage needed. Compression and boost do, though most of that is sorted with plugs and gaps. Revs, he said, tend to define the limit of a coil pack, because of how saturated it gets.
• Another had never heard of the original coil being a problem, with people pushing over 400 bhp on turbo conversions. But if you are running the original 10-year-old unit it is probably worth a swap.
• A third said the 20,000-odd volts will jump the gap even at 3 bar, so the answer is a smaller gap, a strong battery, clean contacts and good engine earths.
• A blog post about a tuned Saab with a misfire that later coils fixed was also mentioned.

Plug gap, heat range and type

• Gap. One owner suggested tapping the gaps down to 0.7 or even 0.6 mm, which is easily done and reversible. His own were 0.7 mm and were fine up to 2.1 bar of boost. A Turbo owner in 2009 also said to gap to 0.7 mm, and that the larger-gap iridium (1.1 mm) risks blowing out on high-boost cars. Another said that others run Harrop superchargers without problems and suggested plain BKR7E plugs gapped at 0.8 mm.
• Heat range. One owner suggested a colder plug, because they can get too hot and crack or crumble the ceramic. He runs the coldest, which he thought is a 9. The owner with the problem was running an 8.
• Electrode type. One owner said that he has heard of misfires with single electrode plugs on the Z22SE even on a standard engine, and suggested trying standard Vauxhall triple electrode plugs or Bosch Super 4 first. Another asked whether that is marketing and suggested plug indexing, which is free. Another asked whether people are using the longer taper-seat plug or the Turbo crush washer short plug, and what heat range, because the ignition system is most critical at the point of highest BMEP. The owner with the problem was using single electrode plugs.
• Plug depth and swirl. A reply suggested trying deeper or multi-electrode plugs to ignite the mixture in a different place, but warned to turn the engine over by hand first to make sure the piston crown doesn't strike the electrode. He also floated that the inlet charge might not be swirling into the top of the cylinder as the OEM one does at high boost, and suggested deflectors in the inlet runners, which he called not simple and possibly risky.

Coil packs and alternatives

• Replace the old one. One owner said to buy a new coil pack, because a new ECU may run wasted spark ignition and hammer the coils four times more. He swapped his 10-year-old pack on a wasted spark ECU and thought it smoothed things a bit. His advice was to be prepared to change coil packs and plugs four times as often. That is one owner's experience.
• Heat. Coil packs can heat saturate, which can explain differences between a rolling road and the road. He suggested taking off any cover above them to let air flow.
• Other packs. A Saab LSJ unit could be used if you aren't using the ion sense with a standalone ECU, though it might need a slight modification for the standard Z22SE cam cover (one owner wasn't sure). A Z22YH coil pack can be made to fit, but not with a Z22SE ECU. Another owner suggested a normal four-cylinder twin coil unit with HT leads, or coil-on-plug. Nobody reported having done any of these.
• The standard ECU and cam position. One owner said the standard ECU uses ion sense in the coil pack to find cam position, and asked how a standalone ECU detects it, or whether it uses the crank signal alone, because of an accuracy issue at high rpm. The owner thought it was crank only.

Misfire detection

• One owner said the standard misfire check doesn't work once supercharged, so it is disabled in the OBD. Another said his misfire fault went away after changing to different software because he could switch the misfire check off.
• A reply said turning off the misfire check doesn't help you, because it just hides the problem. It also asked which crank sensor, and how clean and shielded the signal wiring to the ECU is.
• Another owner with the same misfire under heavy load or at wide-open throttle was fitting a full 3 inch exhaust, hoping the restrictive one was the cause.

What fixed it

The owner fitted a new crank sensor and said the car "hasn't missed a beat" afterwards. That was on 17 July 2014, four days after the main discussion, so it is a short-term result. On 9 August he posted again with a link to an article on a supercharger site that he thought described his problem. In reply, another owner suggested checking the coil pack, the wiring and the crank sensor if it was still misfiring, then the MAP sensor at higher boost, and that the exhaust isn't blocked by a broken catalytic converter. He said a log file is very helpful in these cases. The thread doesn't say whether the misfire stayed away.