Fuel Injection Pressure

Fuel Injection Pressure

OSSA's Troubleshooting Procedures document states that the fuel pressure should be 3.5 ±0.3 kg/cm2 (that's 47 to 54 psi for us metric-impaired Americans). Note that this is to be measured with the engine running. If you are careless enough (as I was) to measure the fuel pressure with a 12-volt battery powering the ECU, it will only produce roughly half that pressure.

Fuel pressure is critical to correct injection volume. Flow is proportional to the square root of the pressure ratio. For example, if you only have 25 psi when you should have 50 psi (25/50 equals a ratio of 0.5) you will only get about 70% of the intended fuel flow.

Excluding external leaks, there are two places the system can lose fuel pressure – the fuel injector itself and/or the fuel pump. If the fuel injector is dirty, it may not close completely (both when it's injecting and when the engine is stopped). A leak in the injector can result in a rich running condition.

Semi-permanent tee tap in injector hose

You can learn a lot by “teeing” a pressure gauge into the fuel injector feed hose. Results from my TR280i during diagnostics.

It's also illustrative to measure the pressure as the system “leaks down” after the engine has stopped.

I think there is always going to be some leakage from the fuel pump assembly back into the fuel tank as the seals there cannot be perfect. For example, it takes about 7 minutes for my system to leak from 47 psi (running pressure) to 34 psi (pressure achieved by operating the pump via a 12-volt battery alone). The rate of leakage trails off exponentially, so 20 - 24 hours later the pressure is still about 7 - 8 psi.

I discovered my 280's injector was leaking by comparing the rate of leak-down with the injector installed versus a solid plug installed in place of the injector.

Fuel injector cleaning apparatus

Here is a photo of an injector cleaning apparatus I made. It was originally intended to characterize the flow rates of various fuel injectors, but I made an adapter that allows the introduction of cleaning fluid. It is made from an old PC power supply and a MegaSquirt JectorRate (basically you just need something that will pulse the injector on and off a few thousand times). I used a 5-ms pulse width.

Since then, I have heard that back-pressurizing an injector (on the outlet side) via a hose filled with cleaning fluid and then applying compressed air works wonders. Obviously, you need to open the injector, but this may be accomplished with any 12-volt DC power source. Injectors are not polarity sensitive.

Seems like a good idea to me, but I have not had a need for it.