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Fuel Pressure Leak-Down Test

An area often overlooked is the performance of the fuel system not while the engine is running, but when it is shut off. If you experience hard starts especially restarting a short period after shutting off the engine, then one possibility is a leaking fuel system. We’re not talking about external leaks such as joints or cracked fuel lines, but instead, we’re talking about internal fuel leaks. While it’s important to know how your fuel system pressure is performing properly during engine operation, it’s just as important to know what the fuel system is doing right after the engine is shut off.

During engine operation, the fuel system is maintaining the required fuel pressure to meet the demand of the engine. When the engine is shut off , the fuel system should be at or near the same pressure level. The rate at which the fuel pressure drops can be an indication of an internal system leak (assuming no external leaking occurs).

Two most common areas of concern are the fuel injector nozzles and the pressure regulator. If the regulator valve has a poor seal, fuel will leak past the valve seat area and, since the pump is no longer running, pressure will quickly drop off. The causes for this could be a worn valve, weak spring or defective diaphragm. An internally leaking regulator can cause long crank times as it takes the pump longer to build pressure in the system.

The other potential problem area could be the fuel injector nozzles. The nozzle may be leaking due to a deposit build up on the nozzle or a worn out internal valve seat. In this case, fuel leaks directly into the engine’s cylinder. Depending on the number of injectors on the engine, this could be a real problem if more than one injector is leaking fuel into the cylinders. This is a common cause of long crank times and hard starts after a short engine shut down. It’s never good to have fuel filling hot cylinders which can result in an over rich condition during startup.

Checking for an internal fuel pressure leak down problem is very simple. On engines that have fuel service port schrader valves, installing a Performance Fuel Systems EFI Gauge Kit is a quick an easy way to perform a fuel pressure leak down test.

EFI Fuel Gauge Kit

Simply install a Performance Fuel Systems EFI Gauge Kit on the fuel service port schrader valve (follow the instructions supplied with the kit). Start the engine and let it warm up at idle. Note the fuel pressure while the engine is running. The pressure should be holding steady in the range of 35-50psi for most applications. Shut off the engine and look for a rapid drop in fuel pressure. In a properly sealing system the fuel pressure reading should hold at or near the running pressure although there may be slight change when the engine is first shut off. This is ok due to some regulators being affected by the manifold pressure while the engine is running. The key is to look for a rapid decrease in fuel pressure. A 2psi drop per 5 seconds, for example, is considered a rapid decrease and indicates a problem. If the rate is much slower then there is no need to worry. The fuel pressure will eventually bleed down over time.

Once you identified an internal pressure leak down problem you can take the proper steps to identify the leaking component. Isolating which component is the leaking will be covered in a separate tech article at PerformanceFuelSystems.com.

 

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