One of the reasons I love the Rabbit so much is because of it's easy to diagnose and work on, and relatively CHEAP, Fuel Injection system, The Bosch (Mechanical) CIS. Is it the best? Hell no. Is it the highest performance? Definitely NOT! Is it the most efficient? Not really, but follow:

Given you already know the basic principle of a gasoline engine, with the CIS system the engines vacuum flows through an intake tube, regulated by the throttle. Vacuum lifts up on a metal plate called the "flow plate" on the end of a see saw mechanism and pushes up a plunger in the middle of the "see-saw" thus opening orifices to the injectors. Delivered is properly metered air/fuel and the little bunny goes.
 

Cut away of Flowplate
When the plate rises, it lifts the plunger on the same side of the pivot (1). Photo is ©Robert Bentley.

Sure this is over simplified but the rest is not too difficult either.

The fuel distributor is the block that sits on top of the air box. The plunger goes through the center of this to control distribution of fuel to the four injectors. The fuel distributor also distributes fuel to the pressure regulator, that adjusts the fuel pressure to accommodate different temperature based (and altitude based in some models) fuel needs of the vehicle. Still with me?

Pressure is maintained at a high PSI with a high pressure fuel pump (always replace with a Bosch unit, please) located on the passenger side outside of the fuel tank. There is an accumulator here also, to maintain a consistent pressure in the system since the fuel pump can cause surges and low spots in the pressure. Now follow closely.

Pressure is regulated through the control pressure regulator mounted to the left of the block (front of car). The CPR is a thermal activated device similar in principle to a choke. The fuel is delivered to the CPR, and if the CPR is cold, the control pressure drops sending less fuel back to the tank and the mixture enriches. If the CPR is warm, the pressure is increased. If the CPR is clogged, the pressure stays high and virtually all of the fuel is returned to the tank. This happened to me recently and I thought the fuel pump had died. Thanks to my friend Pat's pressure gauge, we figured out the problem was only the CPR. (Whew!)

Any fuel that can not be used by the motor is returned to the tank by a spring actuated check valve in the fuel distributor that is opened by the pressure built up by the CPR.

The cold start valve is energized by a thermo switch in the block. However, there needs to be adequate fuel supply. This is why the CPR needs to be operating properly. If the CPR is not accurate, it may return too much fuel to the check valve in the fuel distributor returning the fuel you need right back to the tank.
 


 
 
 
 
 
 
 
 
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