Dahiatsu Terios Codi P 0350 and is cut + 15

Good evening, I need help with a Daihatsu Terios. The problem I'm having is that when it reaches operating temperature, the truck makes a kind of backfire through the intake and exhaust simultaneously, as if the current is cutting out and then returning. The check engine light comes on and goes off when the problem occurs. I've checked the ignition coils, ECU, wiring harness, and ignition switch. The only solution is to bridge the positive terminal of the coils directly from the battery, the ground directly to the chassis, and the signal from the starter to the ECU. This clears the problem. The diagnostic tool is generating code P0350 (main ignition system failure). I hope someone has experienced this problem and can help me with a solution, as this issue has been bothering me for over a month. Thank you very much.

Hi TOTORACING2.
You mention that the problem occurs when the engine reaches operating temperature, which makes me wonder. It's possible the problem could be due to faulty valves in a cylinder, an incorrect electrical signal, or ignition timing outside the acceptable range—even a crack in the cylinder head that isn't connected to the cooling system. However, these causes can lead to the problem at any operating temperature. You have a good investigation ahead of you, the kind some of us old mechanics enjoy. Good luck.
Miguel

Thanks Miguel, yes, I did remove the cylinder head, reseated the valves, looked for cracks, and resurfaced the surface, but the problem persists. Today, while examining the entire wiring harness (which I have completely disassembled), I noticed that the voltage on the multimeter fluctuates between 13.8 and 14.5 volts, with peaks of 19.6 and 18.3 volts, and so on. It has even reached peaks of 8 and 9 volts. I disconnected the alternator, and the voltage fluctuation continues. When the fault occurs, it's as if the positive voltage cuts out and then returns. I've already ruled out the negative voltage because I installed new, independent grounds for the engine and chassis. It's a very mysterious problem. The truck was referred to me from the dealership because they couldn't find the cause of the fault. Thank you in advance for your help, and I hope to find out what's wrong with the truck so I can post it here.

Hey friend, my experience with this vehicle tells me it's less trouble than you think.
To start, I recommend changing the spark plug wires, and if that doesn't solve the problem, replace the entire fuel pump.

Well, my friend with the Terios... Daihatsu, I've had this happen with Charade models, 1.6 with fuel injection, 2003 models. Two in particular, but since they're similar engine displacements, I think my troubleshooting of the problem might be helpful. On the cylinder head, hidden near the intake ducts, there's a ground wire coming from the wiring harness from the ECU. These are very important because they're the computer's operating grounds, independent of the chassis/engine ground, which is used to connect the main power negative to the starter motor and the 12-volt power supply. Since the reading is a "voltage drop in the ignition supply," and the voltage of your supply is also fluctuating, this is the instrument's natural response to a power supply affected by spurious oscillations from the ignition high-voltage unit. I managed to fix both cars with that problem (in one, the mechanic who rebuilt the engine didn't tighten the connector properly to the bolt, and in the other, one of the ground wires was definitely cut a few centimeters outside the ring). I hope my experience helps guide you, and good luck with your repair. Regards, Valdiviano.