Fuel Injection Vacuum Hoses and Pressure Regulator Series 3

First of all : What a fantastic forum and such helpful people ’ I have just purchased a 1985 series 3 Australian car , it had a broken vacuum hose that goes from the inlet manifold to what i presume to be a vacuum switch on the fuel rail and then to the fuel regulator at the front of the fuel rail. There are no wires going to the switch or the pressure regulator . I would like to know :
What does this system do in regards to the FI system , and what would of it been doing with the broken vacuum hose at the intake manifold en , other than sucking in air . Thankyou in advance , this is my first post . Michael

Manifold vacuum is applied to a membrane in the fuel pressure regulator, Michael - modifying fuel rail pressure according to manifold vacuum. This ensures that the pressure difference between the fuel rail and manifold is ‘constant’ - which is important for how much fuel is actually injected into the manifold.

The vacuum switch is operated by fuel temp; with high fuel temp the switch opens to ambient air - removing vacuum from the pressure regulator. This increases the amount of fuel injected, compensating for less density of hot fuel - it’s basically an ‘on/off’ switch, not graduated with temp. The ECU compute fuel amount based on constant fuel pressure - it has no input for actual fuel pressure or fuel temperature…

Incidentally, the description in the manual says that it is to flush out fuel in the fuel rail, to prevent vapor lock. Which it cannot do, but the resultant increase in rail pressure will counter fuel boiling…

xj6 85 Sov Europe (UK/NZ)

Thank you Frank : My Australian sense of humor would say I was hoping for a more descriptive answer :grinning:, That is a way of saying that is an excellent reply . I hadn’t found anything on this so I really appreciate your input. Im also presuming because there would of been no vacuum with the broken hose , fuel delivery would of been higher . It would presume wide open throttle , low manifold vacuum . Thank you again.


The main factors in fueling is manifold vacuum, sensed by the absolute manifold pressure sensor at the ECU - connected by a vacuum line to the manifold. And the engine temp as given by the coolant temp sensor, electrically connected to the ECU. Neither of these has any relation to the distributor’s vacuum advance items.

As the vacuum lines related to the latter are usually small; their leaking has minimal influence on manifold vacuum - the engine gulps in vasts amounts of air through its normal channels, and broken vacuum lines adds but a little. However, leaking dist vacuum lines or faulty components here will influence ignition timing - which in turn will influence engine power and fuel consumption…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

The switch can be bypassed but the regular must be connected.

There is a pressure drop between fuel rail and intake air pressure. The system tries to maintain that pressure drop so the vacuum signal ensures that the pressure drop is not higher (=too much fuel injected) at idle or during overrun as compared to full throttle, no vacuum.

Which applies to the V12…which Michael is perhaps fortunate enough to own.

I’ll just mention that, with the 6-cyl 4.2 FI system, 'tis the Air Flow Meter that provides primary fuel metering control, via the ECU


I didn’t even see that. Per his bio he has a 6 cylinder.

That means no vacuum signal to the ECU, just rpm, water temperature and air temperature, air flow information from the AFM (and some other load switches, possibly o2 sensor Info etc)

The cold start injection is a stand-alone circuit.

The pressure regulator maintains the constant pressure drop across the injectors and is bypassed by the superfluous brown switch to combat rare heat soak.

To everybody , Thank you , You guys are really appreciated . This is an 85 Australian series 3 , It does not have an oxygen sensor , and I dont believe it has an vacuum lines to the ECU , I havent looked hard enough at that yet . Im certainly feeling a little more confident , and getting a better understanding , thankyou , cheers all Michael

Happy to help Michael, and feel free to give us some details about the car, like what condition it’s in and how it runs, what works and what doesn’t etc.!
Vacuum to ECU is a V12 thing so don’t worry about that, the XJ6 doesn’t have it.

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Team : I have owned 2 jags a series 3 for around 10 years , sold it 10 years ago . A series 2 we have owned for 25 years : Im a carburetor type of guy and Fuel injection I know very little about . Our series 2 is at the point where its going to need a reasonable amount of work , so we started looking for a replacement . We purchased a series 3 very recently and that is why I joined the forum . Its in above average condition for its age , we purchased the car because we have known it for quite some time. It does need some maintenance which im starting. I have hardly driven so not sure yet what else it will need . The one issue I " Think " is wrong is it has no Low fan speed on the heater or cooling side . Im yet to confirm however I feel it doesn’t work on low . The car is a white 4.2 Sovereign and very original. Thanks again

I can’t understand while I in one post ‘referred’ to V12, Doug (and David) - senior moment ?!

The ‘only’ vacuum commonality is the function of the pressure regulator and even that, the V12, hasn’t got the vacuum switch…:slight_smile:

xj6 85 Sov Europe (UK/NZ)