Yet another idle misfire fix

Over the past couple of months I had noticed the idle quality deteriorating on my 88 Mondial 3.2 with 23K miles. Strange thing was that it was only when the car was warm, and then only about 5+ seconds after settling into an idle from running at higher revs. The idle would hunt over a range of 100-200 rpm centered around whatever point I set it and you could here an audible "poof" or "thud" sound in the exhaust every time there was a miss, say 1-5 times per second when warm. Also, accelerating out of idle would momentarily cause what felt like a 1 or 2 cylinder skip maybe 1 out of 5 times on average. Not show-stopper stuff but definitely annoying!

Some theories I gathered from the archive of Ferrarichat.com messages were spark plug extenders, plug wires, and an aging oxygen sensor (typically shows up as idle hunting). Considering the 5+ second delay before the missing started and since it didn't miss under load, the extenders and wires didn't seem logical to me at first.

It's easy enough to test whether or not the O2 sensor is a problem by disconnecting it and forcing the controller to run open loop. This can be done by lifting off the air cleaner box and temporarily disconnecting the single green wire that normally passes through the firewall in that corner. The car will default to a slightly rich mixture and the frequency valve will still be buzzing. In my case it definitely did not hunt any more, but the slight miss at idle and when transitioning from idle to acceleration at low rpm was still there. Also worth noting is that you can connect a digital voltmeter's + lead to the oxygen sensor signal lead and its - lead to the chassis and read your mixture (0V = very lean, 1V = very rich, with 0.25-0.75 range close to stochiometric). It won't oscillate as it does when you're in closed loop.

Even though I had looked at the plug extenders 3-4 months ago I decided to take a look again and this time revealed a couple of visible punch-through pinholes in the extender insulator, formed by arcing from the metal clip to the cylinder head spark plug tunnel. The first place to check seems to be the front bank since it is the hardest to get to! With the extenders fixed, the idle is now perfectly smooth, no hunting and no miss when transitioning off idle.

Post-analysis: In this case the hunting probably arose as a side effect because the occasionally missed cylinders passed their unburned fuel through to the exhaust and the closed loop system would then lean out the mixture to all cylinders in compensation. Then a period with little or no missing would get the mixture too lean and start the cycle again. That the miss occurred only at idle was probably because under load the desired path for the spark voltage offered less resistance due to the presence of more fuel/air in the combustion chamber. As for the ~5 second delay observed I can only guess that is a delay associated with the primitive computers that run the mixture and ignition.

Bottom line: Plug wire extenders did it again! Hopefully one or more of these symptoms will match someone's problem in the future and save some time.