![]() ![]() ![]() What we're ideally looking for in a trace of a signal like this is clean regularity and sharp edges and the traces here are not those things. The signals we've scoped here are the high-frequency engine speed signal (Trace A, Blue), and the low-frequency engine phase signal (Trace B, Red). The bit that takes some experience is interpreting this picture to figure out what it's trying to tell you. They show us an image of what the voltage level is right now, but also how it has changed over time. A quick explanation of oscilloscopes? They're just volt-meters. Now, don't get scared off by the oscilloscope traces, the squiggly lines can be a bit confusing if you're not familiar with them, but they tell a great story and if you stick with us we'll break it down. We hooked up an oscilloscope to the trigger signals so we could get a good look at them, but even with the engine at idle, the alarm bells started ringing. As we blindly assumed it was this same issue we were having, we thought it a great opportunity to get some detailed data and analyze what was really going on. The exact mechanics behind the nature of the problems encountered when using the original Nissan signal pattern are often up for discussion. It's common practice when tuning an aftermarket ECU fitted to a Nissan engine of this era to modify the engine speed sensor to output a lower frequency signal which seems to interface better with aftermarket ECUs. Without getting into the nitty-gritty of the operation of these Nissan trigger sensors, they output a very high-frequency signal, which only gets higher as the engine speed increases. It's reasonably well known that the triggering pattern Nissan chose to use on their engines of this era can give issues with aftermarket ECU's at high engine speeds. We're guilty of the cardinal sin of automotive diagnosis here we made an assumption. Arguably, the engine speed signal is the most important input to the ECU if it can't keep track of the engine speed it can't fire the fuel injectors or ignition coils at the right time and hence it can't run the engine correctly. Looking at the datalogs, they showed a very erratic engine speed signal as the engine approached the 6400 RPM point. Pops and bangs can be great when they're asked for, but if you get them when you're not expecting it, you have a problem. ![]() While performing the full power tuning however, the engine would refuse to rev above 6400 RPM, it would instead buck and misfire. The low to mid RPM steady state tuning was a breeze, as the modifications undertaken are a pretty well trodden path. It came to us to be tuned for the modifications, and initially the process was going very smoothly. The engine has some mild bolt on modifications, with a plug in Link G4+ ECU. The car in question is a 1994 S14 Nissan Silvia K's, with the factory fitted SR20DET engine still in place. The subject matter here is pretty dry, but we'll try to spice it up where we can, and there is some really great learning ahead if you stick with us. It was a perfect opportunity to document the diagnosis process we went through so you can follow along with us. We recently came across a car with a really interesting grounding issue. In this article: Misfiring | Assumed Problem | Actual Problem | Determining the Cause | Effecting the Repair | Proving the fix It's important to get it right though, or you'll end up chasing those gremlins in circles till you're frustrated enough to part with your money and pay someone else to chase them for you. ![]() Once you delve in, and start analyzing the grounding requirements of an EFI system, it can get rather complex. This is because on the surface it seems like such an easy thing, just ground everything to the chassis of the vehicle, the more grounds you run the better it'll be, right? Not quite. Grounding problems are without a doubt the most common cause of electrical gremlins in the automotive performance world. ![]()
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