By Jim Kerr

Another vehicle wouldn’t start this morning. It was close to starting but the engine would pop and buck but to no avail. Repeated attempts to start it only killed the battery.

This is a common problem with vehicles towed to repair shops, and our fleet of nearly 70 vehicles has experienced this problem many times. The culprit? Low fuel pressure.

Carburettors required only 5 to 7 PSI (pounds per square inch) to operate properly and most vehicles used mechanical engine driven pumps to supply fuel to the carburettor. With the introduction of fuel injection, much higher pressures were required. Throttle body systems where one or two injectors supply fuel for the complete engine typically operate with 9 to 35 PSI, depending upon the manufacturer. Electric pumps controlled by the engine computer now supply the fuel pressure.

Port injected gasoline engines dominate the industry now. With one injector per cylinder, fuel delivery is more even and accurate throughout the engine. Fuel pressures range from a low of about 30 PSI to well over 70 PSI on different systems. This higher pressure pushes the fuel through the injector tip in a finer spray, enabling it to vaporize quicker. Gasoline engines won’t run on liquid fuel. The gasoline must turn to vapour before it will ignite. By using higher pressures, most of the gasoline can evaporate as it enters the engine cylinder, producing maximum power and efficiency.

When fuel pressure is low, the fuel spraying out of the injector tip comes out in a stream of droplets instead of an even mist. Dirty injectors have the same effect. Droplets of fuel take much longer to evaporate. Some of the fuel doesn’t have time to evaporate and passes through the engine in a liquid state and is burned in the catalytic converter. This may overheat the catalytic converter and cause it to fail too if there is excessive fuel burned there.

Once the engine is started, the metal surfaces are warm enough to help evaporate the fuel and the engine may operate fairly well. You may notice a drop in fuel economy or a slight decrease in performance. If the problem becomes worse, the vehicle may start to hesitate too.

A cold engine presents a different problem. The fuel can’t evaporate and fouls the spark plugs. The engine tries to start but misfires. If you are lucky, you may get it started. Once the engine is warm, it starts fine a second time. Next morning, you have the same problem all over again.

Testing fuel pressure is usually a quick procedure on most vehicles. Hook the pressure gauge up to the test port and turn the key on. There are two pressure tests to make: one at initial Key On before cranking the engine, and a second check of pressure with the engine running.

In my experience, most times the pressure is only checked when the engine is running. Fuel pressure may be within specifications then because the fuel pump has warmed up and charging system voltage is higher than cranking battery voltage, enabling the pump to produce higher pressures. To test for difficult stating, pressures need to be checked before the engine is started and during cranking. Battery voltage is much lower and weak fuel pumps won’t develop enough pressure.

With current systems, minimum fuel pressure specifications are just that – a minimum. If the specifications state for example, the minimum is 65 PSI when you turn the key on and your gauge shows you have 63 PSI, that won’t be close enough. Even a couple PSI low on fuel pressure changes the spray pattern from the injectors, and it can be enough to cause difficult cold starts.

So what’s the fix? Fuel pressure tests should always be one of the first tests for any driveability problem. If pressure is low, check for fuel flow. A plugged fuel filter can cause low pressures, but be careful about fires when testing flow. An electric pump can dump a large volume of fuel out when the key is turned on and it only takes one spark to cause a fire or explosion.

Next, test voltage at the fuel pump while it is operating to ensure there is not a wiring problem with the vehicle. It should be close to battery voltage when the engine is cranking. If voltage is good, then you will usually require a new fuel pump. Most electric fuel pumps are mounted internally in the fuel tank and are not easy to change unless you have the proper lifting equipment. A tank full of fuel weighs a lot! Changing an internally mounted fuel pump is best left to a repair shop.

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