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Vacuum Routing

Vacuum routing is slightly different between certain model years (and US/UK models). Most of this is US-centric information. This page attempts to disentangle all the vacuum lines in the engine compartment.

The US cars had air pumps equipped for emissions compliance. Most owners removed this system due to the fact that it would frequently break. The diverter valve and the associated plumbing is usually removed. My car kept the diverter valve to make use of the vacuum connecter on the top.

As explained below, the two red boxes are the vacuum solenoids for fuel pressure and ignition. When the solenoids are not energized:

  • Purple and Magenta lines are connected (ignition)
  • Blue and Cyan lines are connected (fuel pressure)

When the solenoids are energized:

  • Purple and Yellow lines are connected (ignition)
  • Cyan and Green lines are connected (fuel pressure)

Vacuum Solenoids

There are a pair of solenoids mounted under the intake plenum for fuel pressure and ignition. Lotus refers to these as part #A910E6634F.


Located beneath front of plenum chamber. Function is to advance ignition on operation of choke. A micro-switch operated by the dashboard end of the choke cable (which also operates the choke tell tale lamp) energises the ignition solenoid. The signal to the distributor vacuum advance capsule (normally connected to No.1 cylinder throttle edge drilling) is then switched to the vacuum pump, and thus full ignition advance is achieved, together with an increase in engine idle speed.

Fuel Pressure

Located beneath rear of plenum chamber. Function is to reduce fuel pressure on 'overun' and idle. A micro-switch, mounted on the front of the front carburettor, and operated by the throttle lever in its fully closed position, energises the fuel pressure solenoid. The signal to the fuel pressure regulator (normally connected to boost pressure in the plenum chamber) is then switched to No.1 cylinder inlet manifold depression, thus reducing fuel pressure to the carburettors and mixture strength under closed throttle 'overun' conditions.


I took some time to test and investigate these solenoids (they are identical parts, but serve two different purposes).

These solenoids are a German Pierburg part (#7.12942.00) they are not necessarily an easy part to find (right now, sourcing one is more than $50 USD each).

I tested the solenoid to see the internal routing when energized and when not. The solenoid switches between connecting the top outlet to the bottom-side outlet when energized, or connecting the bottom center outlet to the bottom-side outlet when NOT energized (polarity doesn't matter).

The least expensive replacement unit is an A/C Solenoid Valve part 1470004 or SW9000. This operates in a similar fashion to the stock unit as long as you set up the vacuum lines properly. These can be purchased for about 1/2 the cost of the Pierburg unit, however, note that the connections are 5mm which are bigger than the stock vacuum lines.

engine/vacuumrouting.txt · Last modified: 2023/12/10 02:54 by