All Rabbit, Scirocco, Jettas, Dashers, Golfs, Quantum, Audis, etc. from ‘77 through approximately ‘88 have CIS fuel injection (continuous injection). One of their classic problems is hot starting. After you turn off the car, the heat from the engine and exhaust pipe soaks into the injectors and injection lines and boils the gas into vapor. Then you have vapor lock. It’s kind of backwards vapor lock because vapor lock is usually an air space in the line between the fuel tank and the pump. And the pump can’t suck a vacuum, so it spins in the air “vapor” and you sit there and grind and grind and go no place. Bosch installed a number of solutions to this problem, one of which was the fuel pressure accumulator.
The fuel pressure accumulator is like an orange juice can with a big spring and rubber diaphragm. When the pump is turned on, the fuel pump pressure compresses the spring, fills the accumulator with (liquid) gas, and holds this pressure. When the pump is turned on, this reserve tank of pressurized fuel keeps constant pressure on the fuel at about 50 pounds. And as you remember from your high school physics class that you slept through, a pressurized liquid be it water/steam or gas/vapor boils at a much higher temperature if its pressurized. Therefore, it won’t boil in the lines and stays liquid and your pump is able to pump it and pressurize the fuel in your system to allow your injectors to overcome their internal pressure valve in operating. As you know the CIS injectors have a pressure valve in them which keeps them from leaking and dripping fuel into the cylinder when the engine is off. But this check valve will not open to the pressure of fuel vapor. And therefore will not open and let you start if the fuel has vaporized due to the low pressure caused by a faulty accumulator.
Accumulators fail externally and internally. In all Volkswagen applications the fuel accumulator sits under the rear passenger side seat and it is exposed to salt and external corrosion. Many older cars in the salt areas develop leaks due to external rust through of the fuel accumulator. This is usually very easy spot by the puddle of gas under the car. Check out your rusty accumulator in advance because it’s not cool to drive down the street with gas from a pin hole in an accumulator spraying on a nice hot muffler. You don’t want to look like one of those fake car crashes in movies where everything burns up.
Defect number two is internal. The internal diaphragm springs a leak and the accumulator gets “water logged” with gas on both sides of the diaphragm and is less effective. This is more common.
When the accumulator is not working it contributes to the hot start problem. If you have a hot start problem, the very first thing you should probably do is replace the rubber O rings on the injectors, as they only cost about a buck a piece. This causes vacuum loss so the throttle plate doesn’t lift, so no start. (A different problem but common.)