that's a cap to reduce noise in the electrical system from the pulse of the ignition coil maybe? Pretty sure i've seen the cars and trucks run without that little bugger. Do you hold more knowledge on that little black box?
It's a cap, and it's there to reduce noise. Since we usually associate noise with sound the most common assumption is that it's there to prevent noise in the speakers through the radio. But that's not it, the power supply in the radio has its own filters, this thing is part of the engine management system and would be there without a radio.
The cap that's connected to the ignition coil ignitor is there to limit the amount of electrical noise present in all of the engine management system (the ECU and all the sensors drink from the same straw..) and the coil ignitor & coils are the rowdiest assholes in the circuit.
In that sense it serves kind of like how a torque converter in an automatic transmission eliminates jerky starts like when a novice is driving manual - which is more comfortable. But the cap is much too small to be there for the purpose of absorbing that draw & shielding the rest of the electrical system for it, waste spark coils are going to eat almost 20A when they fire. The only reason a capacitor that small would be implemented is to drain out
other noise, apart from the expected ripples when you simply switch on and off a ~15-20A instantaneous load. It's also not sized to concern itself with frequencies as low as engine RPM, it's an open circuit at those frequencies.
The guy on that forum is speculating that the other noise could be from back-EMF - the electrical equivalent of your engine getting spun up by engine braking down a hill - because ignition coils go through a noisy stabilization phase while building up their magnetic field (known as dwell) and in some systems that can be a significant amount of voltage getting shoved into the "exit" of a transistor - if you look at that circuit diagram the first place a spike like that would go is into the base terminal for the engine speed bus, as in the same one that the crank position sensor cares about.
If that capacitor's job is to drain off transient high voltage from the noisy dwell phase of the coils (it is positioned in the circuit to be effective in that role) and then it is removed from the circuit or fails due to age/whatever, the next place that energy is going to get dumped is into the same part of the coil ignitor transistor that the ECU cares about for timing. If it's robust enough to handle that without burning out, it may still introduce enough noise to run badly. If it's not robust enough then the weakest link burns out, either coils or the ignitor or the ECU or the crank position sensor, they're all in the path.
So here's my angle: It might not be a problem. It might be a problem. The cap is not there for radio noise it's there for the ignition system to work correctly and since the vehicle is 20+ years old and we've all seen our share of forgotten connectors, forgotten ground straps, and "fuck it this thing is just there for the radio to sound purty you don't need it!" attitudes there's a possibility the thing is simply not connected or not present. If it was my truck I'd at least include that in my inspection and if there wasn't one, I'd make sure to get one since the things it's there to protect are the ECU, ignition module and crank sensor all of which are more expensive than $12. The part number from Mitsubishi is MB363405
