Fuel system - Digijet - Brief overview

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Revision as of 16:08, 30 January 2007 by HarryMann (talk | contribs)
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The injection system on the DH is Digijet, much the same as the 2.1 DJs (the later MVs are Digifant) 

This is a system that controls the fuel injection, but not the sparks, which are provided courtesy of a distributor with cent adv. and vacuum adv/retard unit. A Hall sender in the dizzy tells the Ignition Control unit to produce a spark and that unit fires the primary of the HT coil, rather than the conventional Low Tension circuit breaker points - so don't play with the ignition as freely as you might have done on older cars. So that's electro-mechanical, works fine, but is not integrated directly into the ECU (Engine Control Unit, brain) on your DH.

The injection is controlled by the ECU, which listens to a few signals, mainly the AFM (Air Flow Meter, big black box, right rear of engine bay; a basic sprung vane/potentiometer device). Also listens to temperature senders in coolant and in AFM , throttle position (demand signal) and opens the injectors up (non-sequential) so that they flow fuel into the ends of the manifold quite near the cyl head ports. These are pressurised with fuel from the fuel pressure regulator which itself, is fed by the fuel pump. The pressure regulator (about 30psi) sits atop the engine, as also does a mysterious silver cylindrical thing called the Idle Stabiliser Unit, controlled by a black box full of relays that people argue over all the time, like where is it? On the roof, under your seat? No! Behind the r/h taillight or on the l/h eng. bay flitch panel, depending on model. Anyway, the ISU Control Unit is wherever the wires lead to on your particular vehicle. N.B. This is the unit that has two plugs entering it, that have to be disconnected and shorted together before checking or setting the ignition timing.

So get the idea? Two fairly good (when they're new) but independent systems to sparking and fuelling on the DH/DJ. Lots of nice thwacky sparks, and suitable fuel quantities if the engine's turning and the ECU's reading the airflow into it (correctly).

HarryMann