build diary

April 17, 2006: Exorcised!
Jeremy figured it out. With one of the ECUs attached to the simulator, he started twiddling dials. He poked one and the injectors woke up. What was it? The throttle position sensor (TPS) signal. I hadn't played with it because I "knew" it couldn't be the problem. Above a certain value, it was preventing the injectors from firing. With the wiring for the TPS removed, there was a phantom signal that was changing with heat. With a cold ECU, it would sometimes be low enough to let the injectors fire. As things warmed up, it would rise to the point where the car wouldn't get fuel. See, the ECU has a setup where if you crank on wide open throttle, it won't fire the injectors to help clear a flooded condition. This is what was being triggered. I solved the phantom signal problem by installing a jumper between the TPS signal input and the reference voltage coming out of the ECU. No more heat-related variance and a nice consistent start. The odd thing is that the TPS has been removed from the car for months. I trimmed some wires back when cleaning up the engine bay and that's when the signal must have started to creep. Very odd. I must admit that if your car is going to develop this problem, it should always develop it at the Flyin' Miata shop where there is more gathered Miata wiring knowledge and diagnostic tools than anywhere else on the planet. A big thanks to Jeremy for finding the trigger for the problem and an even bigger thanks to Bill who spent hours with me on trying to chase this down.

So, with two more evenings before I leave, I have only a few things to do. First is to bend up the new hood. The flat, louvred sheet arrived today. I may chicken out and get a local fabrication shop to help me as I'm scared to screw it up. I'll also cut a new stone shield for the driver's side, as the new one on the passenger's side makes it look battered. Pack the trailer, gather the tools (the garage is a little, umm, chaotic) and I'm pretty much ready to go. I hope so anyhow.

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