Thursday, August 11, 2022 Detailed Auto Topics
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Extended cranking time, or a hard start is a common problem with modern vehicles. Most times the problem is with the fuel system and often it is NOT the fuel pump. Learning how to test this problem can save an expensive mistake.

Many home mechanics have a fuel pressure tester in their arsenal of tools. With the proper adapters this is a very valuable diagnostic device. Using the gauge is straightforward, but a lot more information is available than many people realize. For instance, in testing for our hard start problem, we attach the gauge to the fuel test port.

The basic fuel pressure test, except direct injection

The fuel pressure reading is only part of the information

When we switch the key to on, the computer will turn on the fuel pump for a few seconds. The pressure should immediately rise to the specification. In the picture above, we assume 47 PSI is normal, but the actual reading varies greatly from one vehicle to another. We have to consult service-data to confirm the proper reading. This simple test shows the fuel pump can build pressure. This does not mean the pump is good.

The basic leak-down test

A second test is to turn the key to off and see how long the pressure holds.

The system should maintain pressure with the key off

For the specification we must consult service-data, but the system should hold some pressure for about five-minutes. In the example above, from a late-nineties model Jeep, it should hold at least 30 PSI for five-minutes. If the system maintains the pressure, a third test is in order.

A volatile fuel test

Pressure that rises with the key off may mean volatile fuel

With the fuel pressure gauge attached, run the engine until it reaches full temperature. Turn the engine off and carefully watch the gauge. Pressure rising significantly may show fuel boiling in the fuel rail. One cause for fuel volatility is excess alcohol in the gasoline. This can occur with too high of an ethanol mix and sometimes if we use a Winter grade fuel in the Summer. Another cause may be A fuel additive, such as injector cleaner added to the gasoline.

Boiling fuel will vaporize and make the vehicle hard to start especially in hot weather. They easily test fuel to learn alcohol content. Any well-equipped diagnostic-shop can check the gasoline.

More thorough fuel pressure drop tests

Dropping to zero indicates a problem

In checking our hard start vehicle, suppose the fuel pressure drops close to zero. This could mean the fuel pump check valve is bad. It could also show leaking fuel injectors or a bad fuel pressure regulator. We need additional tests to isolate the actual cause.

Hose set for extended diagnosis capability

A major enhancement to the basic gauge is a hose-test-set, with shut off valves. This extra usually does not come with the gauge. Unfortunately, not having this limits the information available.

For instance, to test the fuel pump, we switch the key to on. The fuel pressure rises to specifications. We turn off the valve in the hose, leading to the engine. This stops fuel from reaching the fuel rail. If the pressure still drops, this points to a bad fuel pump.

We should repeat this test. This time we turn off the valve to the pump and leave the one to the engine on. Fuel pressure that drops, points to leaking injectors or a bad fuel pressure-regulator. As a follow-up test, we can unplug the injectors and repeat the test. Pressure that no longer drops, means an electrical problem. A transient ground, bad PCM driver or even a wire shorted to ground can cause an injector to remain open.

Fuel in the vacuum hose means a leaking regulator

Often, removing the spark plugs shows one or more wet with fuel. We would remove and test the injectors for leakage on the cylinders with wet plugs. We can inspect external fuel pressure regulators for leakage. Fuel in the vacuum hose, leading to the regulator, shows a ruptured diaphragm.

Testing fuel pressure requires many adapters for the many vehicles on the road. Owning all of the adapters needed may not be cost effective for the home-mechanic. In these cases hiring a good diagnostic shop to test the pressure may make more sense. Understanding these tests quickly points to the cause of a fuel-related hard start.

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