Originally Posted by ctaylor738

Fuel flows from the fuel distributor (fd) to the warm-up regulator (wur). The wur has a valve which bleeds fuel back to the tank.

System pressure is measured by blocking the flow to the wur by closing the valve on the wur side of the gauge. The fd has a relief valve that maintains 5.8 bars of system pressure, by sending some fuel back to the tank.

So then you open the valve, and that allows more fuel to flow through the wur and back to the tank. This reduces the pressure to about 3.8 bars for a warm engine. But to provide enrichment when cold, the wur increases the flow back to the tank, which lowers cp to 1.0 - 1.4 bars depending on the temperature. Now, there is an bimetallic strip in the wur that gets current from the fuel pump relay. As it heats up, it bends and increases the pressure on the valve, which reduces the flow back to the tank and increases control pressure, stabilizing at 3.8 bar.

The reason that you disconnect the electrical from the wur is so that you can get a stable reading with the engine cold, like if you are adjusting the wur, you don't want the reading changing because the strip is heating up! So you test the constant reading and then do a second test to let the strip heat up and bend.

The key to understanding this is that the lower the control pressure , the richer the mixture. This is hard to get your head around. But it happens because as control pressure is reduced, a given movement of the air sensor plate moves the piston in the fd a greater distance, increasing flow to the injectors. Higher control pressure works in the opposite manner.