Dealing with a car that’s not running smoothly can be incredibly frustrating. Like many Subaru enthusiasts, I’m facing a perplexing issue with my 1998 Impreza, and I’m determined to get to the bottom of it. I’ve been systematically working through potential causes, and I wanted to share my troubleshooting steps and experiences so far in hopes of finding a solution and perhaps helping others facing similar problems with their 98 Impreza.
My initial focus was on the pressure switch. Based on the information I found in a Hanes manual, I understood that the pressure switch output voltage should behave in a specific way: around 3V at idle and decreasing to approximately 2V during acceleration. However, my 98 Impreza was showing the opposite – the voltage actually increased when accelerating. To rule out a faulty component, I swapped out the pressure switch with a known good one and even replaced the pressure source switch solenoid. Unfortunately, after these replacements, the voltage output remained unchanged, indicating that the Hanes manual’s specifications might not be accurate for my specific model or situation. This highlights the importance of cross-referencing information and potentially seeking out more specific resources for the 98 Impreza.
Moving on, I considered the Engine Control Unit (ECU). Sometimes, ECU issues can manifest as rough running conditions. To test this possibility, I swapped in another ECU from a 98 Impreza, ensuring compatibility. However, this ECU swap didn’t resolve the problem. In fact, the car started and ran even rougher with the replacement ECU, suggesting that the original ECU might not be the source of the issue, or perhaps the replacement ECU was not functioning correctly either. This test, while inconclusive in fixing the problem, helped narrow down potential areas to investigate further in my 98 Impreza diagnosis.
Next, I decided to check the fuel injectors. While the engine was running roughly, I cautiously disconnected the connectors to the injectors, one by one. I observed that disconnecting the injectors did cause the engine to bog down slightly, and it immediately triggered a Check Engine Light (CEL) code. This response suggests that the injectors are at least partially functional and contributing to the engine’s operation. However, it doesn’t definitively rule out injector problems; it’s possible that one or more injectors are not performing optimally, even if they are working to some degree. Further, more detailed injector testing might be necessary to completely eliminate them as a potential cause of the rough running in my 98 Impreza.
Another area of concern was the exhaust system, specifically the catalytic converter. I suspected a clogged catalytic converter or exhaust restriction because the exhaust components were getting excessively hot when the car was running roughly. A clogged exhaust could certainly lead to poor engine performance. However, a contradictory piece of evidence emerged when I took the 98 Impreza for a California emissions test on a day when it miraculously ran well. It passed the emissions test with excellent results. This result is somewhat puzzling because a failing catalytic converter or a problematic oxygen sensor typically leads to emission test failures. The fact that it passed the emissions test suggests that these components might be functioning within acceptable parameters, at least intermittently.
Finally, I turned my attention to the Mass Air Flow (MAF) sensor. Using a used MAF sensor from a 1991 Legacy, which shares the same part number with my 98 Impreza, I performed a swap. Using my OBD scanner, I checked the MAF sensor readings. Interestingly, the readings remained consistent at around 2.6 grams per second, similar to the original MAF sensor. This consistent reading, even with a different sensor, raises questions about the accuracy of the Hanes manual again, as it lacks output specifications for the MAF sensor. The downloaded Factory Service Manual (FSM) also unfortunately doesn’t provide specific output specs for the MAF. This makes it difficult to definitively assess whether the MAF sensor is functioning correctly based on these readings alone.
To further evaluate the impact of these adjustments, I plan to take my 98 Impreza for an extended drive. Hopefully, one of the numerous adjustments and component swaps I’ve carried out will have addressed the underlying issue and improved its running condition. I remain hopeful that a solution is within reach and appreciate any further insights or suggestions from the community as I continue to troubleshoot this 98 Impreza.
