This one puzzles me... I was on the highway in my 1982 Mondial QV on the Oresund Bridge on my way to Sturup Raceway in Sweden. The engine was warm and running perfectly so at 120km/h I geared down for accelerating past another vehicle. At roughly 7700rpm though, just before shifting, i noticed in my rear view mirror a short burst of black smoke out one of the exhausts, and instantaneously the engine got a slightly unenven torque feel and clearly reduced power. Today I looked at the spark plugs. Seven of them were fine, whereas no.8, from the front row, looked like this. I inserted a new spark plug and the engine seems back in perfect condition. Does anyone have a idea what may have happened? Why did this electrode bend? The sparks are quite new, having done about 2000 km. Image Unavailable, Please Login
Can you be sure the plug wasn't bumped before it went in? It could just be that the plug went in like that and finally fouled. Might be a good time to check the valve timing though.
You are right, I cannot exclude that the plug was already deformed when mounted. Furthermore, the other plugs look a bit too cold, so gradual fouling of the deformed spark plug is likely. The final "shorting" then happened during the high revs. This seems plausible. My only remaining puzzle is how such a "shorting" could provoke a burst of black stuff out one of the exhaust pipes....?
How about a good sized chunk of carbon coming off the top of the piston? That would explain the black smoke and the deformed electrode. It looks like its running cold so carbon buildup could easily be happening. Check through the spark plug hole with a borescope to see if you have a nice shiny spot on your piston.
Well, it happened again: A spark plug electrode bent and short circuited after a short highway acceleration when I geared down and the engine did 7700rpm under very hot conditions. This time located in cylinder No 5. After a quick and rough use of a pair of pliers, the plug and the car was back in trim. I shortly thereafter spoke to one of the Porsche racing guys here in Copenhagen, and he told me that this phenomenon is seen in 911 engines when running too lean & too hot. None of us had any idea however what physical or metallurgical mechanism lies behind the bending. If anyone in this forum can explain the mechanism, please do so - I'm puzzled! Since the spark plug bending now has happened twice, I can exclude the "oops-I-dropped-the-plug-theory" and after a visual cylinder inspection also the "a-carbon-chunk-loosened-in-the-combustion-chamber-theory" can be excluded. The "running-lean-theory" actually makes good sense: I know my air fuel ratio is bad at high revs, and yes, on both occasions the sparks bent at high revs under hot engine conditions. Action for the comning weeks: Reduce supercharger compression and re-check A/F ratio.
One assumes the correct reach plugs are being used and that there were no internal modifications made to the engine... I used to see this occasionally on modified performance (US v8) engines though I have never heard of it on a "stock" engine. It happened with hi compression pistons and a some carbon buildup on the piston domes. At max RPM occasionally the carbon build-up on the piston would hit the electrode doing exactly what you saw. You could disassemble the engine and see the mark on the carbon. It only happened at high RPM - I don't recall ever seeing it at under 7k RPM. We just "assumed" that the high RPM condition allowed a bit more "stretch" of the rods but I must admit we never did any research into it at all, just cleaned up the pistons and that took care of it. But it's hard for me to imagine that any oem engine would have so little piston to plug clearance where this could ever be an issue, even with a substantial carbon buildup. But, as I have said before, I have never worked on a Ferrari engine or even seen a Ferrari engine apart.
That's some carbon build up letting go. The black stuff you saw coming out of the rear when you were accelerating was what it looks like when it blows through the exhaust. The carbon is going to let go most likely when approaching redline and tends to come off in a chunk (which can interfere with piston crown/cut-out v. plug clearance) and then is broken up into smaller pieces as it makes it's way out of the CC and through the exhaust. Carbon clearing itself can easily compress the electrode on a spark plug but rarely does more damage than that (dent the crown or bend valves) because it will break up fairly easily through the course of a few revolutions. One thing you could try is installing a plug with the same heat range but having a slightly shorter nose - perhaps 1-2mm less. Then, take the car for a couple "Italian tune ups" and see what else you can shake loose. The porsche folks use techron poured directly into the combustion chamber as a means to shake it loose. Put it in one afternoon, let it sit over night, then take the car up to redline in a spirited drive the next day. There's actually a porsche factory tech bulletin recommending this approach. Good luck!
You mention supercharger. I'm guessing that isn't stock. 1st. Unburnt fuel will come out black. Fuel = black Oil = blue White = water. If you have a plug foul at max RPM's and don't have a catalytic converter all the fuel dumping into that cylinder will turn into black smoke. A theory, and just a theory based on similar but different experience. If an engine is allowed to "ping" too much (pre ignition) it can eat a hole into a piston. I'm wondering if at top RPM your engine leans out too much and has max boost as the piston still has too much rise you are getting pre ignition and the force of the blast is bending your electrodes. I'm thinking a force that could blow a hole through the top of a piston could bend an electrode. Yeah, I'd like one of the techs to chime in.
I would say it is the piston. Are you using correct washers spacers at thread end of plug, are they the correct spark plug, I used to have an aston martin the big end bearings were worn and under decellaration the piston load is stretching away from the bearings and the piston would gently tap the valves, it sounded just like a tappet noise, under accelaration nothing.
