Recently, I encountered a perplexing issue with my 99 Miata after performing some routine maintenance. Thinking back to the most recent work on the car, specifically the engine, I recalled replacing the oil pump and engine bearings. Startup after the maintenance seemed perfect initially, but shortly after, I began noticing extended crank times and sluggish throttle response. Furthermore, an engine miss became apparent when revving above 4000 rpm. Eventually, starting the car became a real struggle.
My first suspicion fell on the fuel pump or relay. However, when I used a diagnostic tool, it only reported a knock sensor fault and a pending camshaft discriminator fault, with no indication of fuel pump problems. What followed was quite interesting.
After consulting wiring diagrams and even trying a new fuel pump relay, the problem persisted. However, I observed that when the diagnostic tool was connected to the OBD2 port, the car would start and accelerate under load without any issues. Disconnecting the diagnostic tool immediately brought back the original symptoms. To rule out a ground issue with the fuel pump relay, I grounded the fuel pump using the diagnostic connector under the hood, but this made no difference. This pointed towards a potential issue with the power supply. It turns out that the fuel pump relay receives power directly from the engine side injector harness B+ feed.
While carefully inspecting the harness for any signs of a power feed wiring problem, I noticed something unexpected. The bolts securing the intake manifold harness grounds, located near the throttle body, were surprisingly loose – less than finger tight. Upon tightening these two bolts, the engine problem was completely resolved. It remains unclear why the system seemed to find adequate B+ voltage when the diagnostic tool was connected. Intriguingly, I even tested a second diagnostic device plugged into the dash side OBD2 connector, and it yielded the same result – the car ran normally with it connected.
This experience underscores a crucial point in automotive repair: when a new problem arises, it’s essential to thoroughly review the most recent work performed on the vehicle. Often, we ourselves are the source of the most baffling issues. In this case, a seemingly unrelated maintenance task led to a grounding problem that significantly impacted engine performance. While in this instance, the OBD scanner didn’t directly pinpoint the loose ground bolts, having access to even a basic OBD2 scanner was invaluable for initial diagnostics and ruling out other potential issues like fuel pump failure. For comprehensive diagnostics and deeper insights into your 99 Miata’s health, investing in the Best Obd Scanner For 99 Miata can be a game-changer. These advanced scanners offer features beyond basic code reading, providing live data, component testing, and more, which can significantly streamline the troubleshooting process for various car issues in the future.
