It’s a frustrating situation when your project car is almost complete, yet a persistent fueling issue is holding you back. Many DIY mechanics encounter challenges when mixing and matching parts across different engine models, and this case is no exception. Let’s delve into a perplexing problem: an engine that starts and runs briefly with a 5SFE fuel system, but refuses to sustain operation, particularly when paired with a 3SFE ECU. This article breaks down the potential culprits and theories behind this issue, aiming to provide clarity and direction for troubleshooting.
The current setup involves a 1994 5SFE fuel rail and injectors, modified for a returnless fuel system configuration. The original fuel pressure regulator was removed and replaced with a cap from a 3SFE injector rail, adapting to the RAV4’s in-tank regulator. Initially, this seemed viable as the engine did start and run. However, the engine abruptly shuts down after a short 15-30 second run, and frustratingly, refuses to restart immediately afterwards, often requiring days of rest before another brief start is possible. This behavior strongly suggests a fueling problem.
To diagnose this, several theories have emerged, each exploring different facets of the fuel delivery and engine management systems. Let’s examine these potential causes:
Theory 1: Fuel Pressure and Returnless System Incompatibility
One possibility is that the 5SFE injectors (part number 23250-74100) are not behaving correctly within this modified returnless fuel system.
Even though fuel pressure should theoretically remain consistent regardless of the return system, there might be subtle differences in how these injectors function in a returnless setup compared to their original design. The solution proposed here is to revert to a return-style system. This would involve reinstalling the original fuel pressure regulator on the fuel rail and running a return line back to the fuel tank, while simultaneously removing the in-tank fuel pressure regulator. This change could potentially resolve inconsistencies related to fuel pressure regulation and injector performance in the modified system.
Theory 2: Injector Impedance and ECU Signal Mismatch
Another theory centers on electrical compatibility between the 5SFE injectors and the 3SFE ECU. It’s speculated that the 74100 injectors might not be responding correctly to the electrical signals from the 3SFE ECU, possibly due to impedance differences (high vs. low impedance injectors).
While both the 5SFE (74100) and 3SFE (74140) injectors measure the same 14.3 Ohms of resistance, indicating they are likely high impedance injectors, there could still be nuances in the ECU signal or injector driver circuitry that are causing issues. The engine might run briefly, but the signal might not be sufficient for sustained operation. The question arises whether an inline resistor could correct a potential impedance mismatch, although the equal resistance readings suggest this might not be the primary issue.
Theory 3: Faulty Oxygen Sensor Signal
A malfunctioning Oxygen (O2) sensor in the exhaust header is also considered as a potential cause. A bad O2 sensor could send incorrect or erratic signals to the ECU, disrupting the fuel mixture and causing the engine to stall after a short run.
The fact that resetting the ECU doesn’t immediately resolve the issue and allow for another 30-second run is puzzling. If an O2 sensor were the sole problem, clearing the ECU’s learned fuel trims might be expected to allow for a slightly longer run time before the faulty sensor signal skewed the mixture again. However, this is not observed, making the O2 sensor theory less likely as the primary cause, although it could still be a contributing factor. Identifying and replacing potentially faulty O2 sensors is advisable as part of a comprehensive troubleshooting process.
Seeking Further Insights
These theories provide a starting point for diagnosing why the engine turns over with the 5SFE fuel system but fails to run consistently with what is suspected to be related to the 3SFE ECU or its interaction with the 5SFE components. Further investigation and expert opinions are needed to pinpoint the exact cause and implement the most effective solution. Could there be other factors at play, or are there nuances within these theories that need deeper exploration? Community input and shared experiences are invaluable in resolving complex automotive issues like this.
