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Trying to work out what these are and how they work. I can't find anything in the Haynes book.
Seems there are 2 x vacuum operated fuel solenoids of some kind mounted at the front of the engine on each bank.
The fuel line from the supply pump enters at one point and appears to feed the fuel rail from there. I am presuming that the inlet manifold vacuum is allowing it to open (or close) and allow fuel to the rail but it seems a bit odd. What am I missing here, what are they for?
The hose from the fuel pump is under the intake manifold delivered via the A/C heat exchange ports.
The D-Jetronic XJ-S fuel system involves a fuel tank with a drain that feeds a small surge tank. A screened pickup in the surge tank leads to a soft-mounted fuel pump just below and to the right of the spare tire. The line leads to a fuel filter and then to two separate fuel rails on the engine, one for the left bank and one for the right. A pressure regulator on each rail directs excess flow through a fuel cooler and then back to the tank. The Digital P uses a similar system except that the two separate fuel rails were replaced with a single rail. There are still two pressure regulators, but in this case one is a supply regulator and one is a return regulator. Supposedly the return regulator, the one on the LF corner of the engine, is the only one that actually does anything. While the D-Jetronic regulators held rail pressure constant, the Digital P regulators vary rail pressure with intake manifold vacuum and therefore have vacuum lines connected to them.
They are the fuel pressure regulators. Port injection ( as opposed to throttle body injection) needs the vacuum reference to maintain a constant flow rate of the injector. The flow rate depends on the pressure differential across the injector, so by applying the vacuum reference to the regulator it adjusts the fuel pressure automatically in response to the throttle opening, then the pressure differential across the injector remains constant, no matter what throttle position is. Since a TBI injector sprays into atmosphere above the throttle plates, it doesn't need the vacuum reference.
The regulators are set for a different pressure, the first one drops the high pressure from the pump to a more manageable pressure for the rail, and the last one maintains the correct pressure for the injectors. It's generally preferable ( and gives finer control) to have pressure drops done in multiple stages, rather than in one large drop.
Last edited by Jagboi64; Aug 25, 2018 at 10:33 PM.
Fuel Valves x 2
