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Rest Pressure - Hot Restart Issues

Having a hard time restarting your fuel injected car after a hot soak? You know the scenario: you drive your car to the store on a hot day; spend a lot of time idling in traffic with the a/c on; and then park in a large asphalt parking lot in the midday sun.

After shopping, you return to find that while the engine cranks over, it takes a long time starting. In extreme cases, it may not start at all until it cools down. Even if the engine catches and starts after extended cranking, it runs poorly for several seconds before smoothing out.

The problem may be a loss of rest pressure. Fuel rails are designed to trap residual (rest) pressure in the fuel rail after the fuel pump stops running. This ensures that there will be fuel readily available at the injectors for a restart. Just as importantly: rest pressure raises the boiling point of the fuel. Low pressure fuel in that scorching hot fuel rail will quickly percolate, turnig liquid fuel into millions of tiny vapor bubbles.

Hot, "carbonated" fuel causes hard starting.

Maintaining Rest Pressure

The Fuel Rail is designed to keep liquid fuel inside the rail pressurized after the fuel pump stops running. It does so by creating a sealed length of fuel line between the fuel pump and fuel pressure regulator. This length of fuel line includes the rail to which the injectors are attached.

A one-way check valve at the fuel pump outlet and the return path at the fuel pressure regulator both snap shut. This traps fuel in the rail between the two shut-offs, maintaining pressure for the next start.

Leaks and Pressure Loss
• If the check valve or the fuel pressure regulator leak, pressure in the rail falls, and the fuel can "boil." These "internal" leaks allow fuel pressure to bleed back to the fuel tank.

• Leaking injectors are double trouble. If injectors leak, they flood one or more cylinders with raw fuel after engine shut down. This fouls spark plugs and lowers rail pressure.

Finding Leaks

• Attach a fuel pressure gauge to the fuel rail. Operate the fuel pump to pressurize the rail and then stop the pump. Rest pressure will fall gradually in many vehicles after shutdown, but the rail pressure should hold steady for a manufacturer-specified time, and any observed pressure loss should be very gradual. If OEM rest pressure test specifications are unavailable, you may need to take a reference reading from a known-good vehicle.

• If pressure drops like a stone as soon as the fuel pump stops running, alternately pinch off the supply and return lines (points A and B) and repeat the test to isolate the exact location of the rest pressure leak. Just be careful not to pinch off metal or solid plastic lines or you'll ruin them.

If pressure falls quickly regardless of which hose you pinch, pinch both at the same time. With both supply and return lines pinched, loss of rest pressure indicates that one or more of the injectors is leaking.

Carbon Deposits

If no rest pressure problem is uncovered, intake valve deposits or clogged injectors should also be considered. The carbon deposits act like a sponge, absorbing fuel. A top engine cleaning with carbon removal chemicals and an injector cleaning may both be effective cures for this hot weather malady.

It never hurts to check TSBs, either. We know of at least one OEM that has used PCM reprogramming to lengthen injector on-time during cranking, to compensate for fuel starvation caused by deposits that restrict fuel delivery.

©2009autofixworld.com
Return to June 2009 Issue.

» 1 Comment

1Comment
at Friday, 19 June 2009 20:50by lustysam

And for owners of older cars, don't forget the ever popular fuel accumulator, a type of rest-pressure retention device used in some CIS and CIS-E (or K-Jet, as it was also known) equipped vehicles. This device resembles the fuel pressure regulator shown in the cutaway, but was installed in-line between the fuel pump and the filter. Some accumulators had a nipple for a vacuum or vent line, while others had a block off screw plug. If you find any fuel under the screw or in the hose, the accumulator diaphragm is faulty and must be replaced.
Some CIS cars, notably the Rabbit, were updated to pulse the cold-start injector during prolonged hot cranking to vent boiling fluid.

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