I had my first Marelli meltdown just now… Classic symptoms… the car got kinda slow. Pulled over, looked under the car, and the cat was bright red. I guess the only thing surprising was how fast it went from me noticing the car was down on power and the cat being red.
Anyway, do the ceramic internals literally melt or is there a possibility the cat is still serviceable?
I’d look myself but it’s still red while I wait for AAA 
-John
There are still people on here who think it’s a myth.
It’s a fairly safe bet that your cat is toast. Of course, the fix is to just gut it. If you must have a functioning cat for some reason, others have had good luck with installing generic cats in place of the OEM.
My 88 with the Lucas system - when I inspected my Cats, there were holes in the middle with chunks stuck in the exhaust pipe following the Cat. So yours being about the same age as mine, and also having a meltdown, I don’t see your Cats holding up.
You can count on finding pieces of the melted cat downstream in the pipes and Mufflers.
It has nothing to do with “melted” ceramics. Ceramic substrate inside the can is a porous carrier of the washcoat with Platinum group metals which are doing the job in flow-through converters. Higher temperature makes aforementioned washcoat to disintegrate/being blown away. If the ceramic substrate melts, it’s already waaay too late to save anything.
I’ve seen on Japanese market cars the requirement for a cat overheat sensor, which is basically a temp sensor stuck on the said catalyzer, shorting to ground when a specific temp is reached,
it might not be this simple, as the internals of the cat need to be at about 500°C what is the “normal” outside temp of a cat ?
The temperature of the can should be somewhere between 250-300 Celsius. It has nothing to do with the working temp of flow through substrate. The inlet should always have lower temp than the outlet (can), at least if there is some PGM there left to work. Infrared thermometer is the best tool, thermal camera is much better to “observe” eorking condition.You can diagnose engine fuel/timing problem via lambda sensor installed.
Hmmm, apologies, not chasing the v12 issues as too many, what is the root cause of this? In short…
Cheap plastic. Eventually arcs right through, then you lose spark, and gasoline keeps pouring into the side of the engine.
Kirby suggests shortening the Center shaft in the distributor, and putting a potting putty underneath the rotor. Of course obviously replacing it often makes sense too but recently it has been very hard to find a Marelli rotor for under $100.
The one closer to the manifold is even more melty looking. I’m really surprised the steering boot didn’t melt. If you leave off the heat shield they will melt with just regular use.
The plastic is 0.150" thick. Obviously it needed to be thicker. My suspicion is that 0.150" was the time-tested thickness for dielectrics in ignition system use, but then the Marelli proved to be a hotter ignition than those that had successfully used that thickness before.
My suggested fix of shortening the top of the distributor shaft and potting the underside of a brand new rotor is basically adding dielectric to that 0.150" of the rotor itself. One user tested it on a fried rotor and it rendered it functional, so hopefully it’ll do well on a rotor that’s good to begin with. I dunno if we’ve heard of any failures of potted rotors yet.
But we certainly have heard of other Marelli failures. Basically, anything that disables one bank can cause that cat meltdown – bad coil, bad amp, even a bad HT wire or two.
Oh, and remember to properly gap your spark plugs. Running the gaps as the plugs come out of the box is one of the common causes of rotor failure.
My 1992 facelift car has just that - an exhaust overheat warning light on the dashboard, but it is only connected for the Japanese market.
It’s required by law in Japan.
I always find it strange when car manufacturers have to provide something like this in one jurisdiction, but remove it from the rest of the world when it is something that would benefit everyone. The warning light is already there, would the extension of the loom and the sensor cost that much, compared to the inventory control for two separate wiring systems.
They probably had committee meetings about it, and eventually decided having to explain to potential buyers why a warning light when your cats are overheating is a good idea would just scare customers off.
I guess the same principal as swapping the oil pressure sender for a switch + resistor as customers didn’t like the low-high gauge swings.
here you can’t even find one, all are poor aftermarket copies, sold at the price of original ones
could you post a picture of the said light ?
my car should have one, as it was sold originally to Japan
