The Importance of Checking EFI Sensors

Electronics may be the wave of the future in performance and racing, but they are still very much unknown quantities for many enthusiasts, especially those who have spent the majority of their lives tuning on carburetors. The worst place of all to have a problem or miscue is on the road or at the track, yet that’s where they always seem to happen. Here are some ideas on how to check the various EFI sensors from the staff at FAST. Hopefully it will keep you from problems down the line.

The very first troubleshooting procedure is one that should be performed before there is any type of “trouble” at all. Before heading out, a pre-run inspection is always a good idea. To do this, check all sensor readings and compare them to good, “normal” readings. You can either use a laptop, external readout or display unit, or both, depending on what your particular EFI system allows.

Since any EFI system relies on inputs to do its job, always verify that the input readings are legitimate beforehand. If an issue is encountered at the track (or on the street for that matter), go back and verify your inputs before doing anything else.

Inputs refer to the standard sensors most systems rely on. These usually include: TPS (throttle position sensor); MAP (manifold absolute pressure, which measures manifold vacuum or boost); CTS (coolant temp sensor); ATS (air temp sensor); wideband O2 sensor (tells the ECU how rich or lean the engine is running by measuring the oxygen content in the exhaust); and crank signal (generated by the distributor pickup or crank trigger).

Each of these sensors has a “normal” reading or range for both engine on, key off and for engine running.

If an issue is encountered, one of these readings will likely reveal the root cause of the issue, or at least narrow down the list of suspects.

For instance, if the engine will not run, and you log into the ECU and see that the RPM stays at zero as the engine is cranked over, don’t bother checking the fuel pump or injectors. If the ECU doesn’t know the engine is turning over, then it doesn’t know to operate the injectors. In this instance, you know that the issue lies somewhere between the crank sensor or pickup and the ECU.

EFI Sensors

A fuel pressure sensor is an optional, but beneficial component to utilize.

Another sensor that is technically optional, but many consider necessary, is a fuel pressure sensor that feeds constant readings to the ECU. While not used in any of the base fuel calculations in the ECU, troubleshooting a fueling issue requires the ability to monitor (and preferably datalog) fuel pressure so you can know exactly what it is doing under load (or whenever the issue is occurring).

More advanced systems like the XFI 2.0 have the ability to use this real-time fuel pressure reading and make compensations for fluctuations in fuel pressure from the base pressure setting. This helps to stabilize the actual fuel delivery to the engine, even though the fuel pressure is moving up and down or dropping. You must bear in mind that this function can only provide so much compensation. If the fuel pressure is dropping, the injectors will eventually go to 100% duty cycle (wide open) and after that point, they cannot provide any more fuel even if the fuel pressure compensation function is asking for more.

In order to advance your EFI troubleshooting skills, we recommend researching and studying exactly what function each sensor or input performs in the operation of the system. By doing this, you can gain a mental picture of what is going on with this symphony of signals as an ECU is controlling an engine. Then, just as an orchestra conductor can identify if an instrument is missing or out of tune, you will be able to know which input is malfunctioning by the symptoms of the engine’s performance.