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Having tested my way through various possible reasons for the car not starting (petrol, pumps, spark plugs, battery, etc)
I have got to the stage where I can say for certain that petrol is reach the fuel rail end to end at pressure but not getting as far as the return line and that the spark plugs are sparking but remain dry after several attempts at turning over accompanied by that ra-ra-ra-ra-ra sound.
the injectors seem to be getting about 2.0v at their doubled-pin terminals (2.0 at each pin).
I assume at this point that I have a problem with the ECU computer unit in some regard that the injectors are not getting the 'fire' instructions.
what are the test check points and deductive elimination methods best used to evaluate an Lucas 5CU for an EFI system? Where should I look for problems and how best to test them?>

The S3 EFI is Dougs domain.

The fact there is NO fuel exiting the return line is NO good, full stop. That is either a jammed FPR (Fuel Pressure Regulator), or an insufficient fuel pump, as in not enough GRUNT. Rule of thumb on these early systems is about 60% of the incoming fuel is returned to the tank, and even more at idle.

As I said, the testing for what does what is a mystery to me, as we only ever had 1 S3 6cyl, and it always ran as designed, and I never did get wrapped up in the under bonnet stuff.

hm, seems to be a good deal of pressure at end of fuel rail so maybe jammed FPR then?
return line is entirely dry as are the spark plugs, not a drop of fuel.

KOEO---Key on, engine off--- the injectors should have 12 volts. In an original Ser III installation this supply voltage comes from the injector ballast mounted low on the RH inner fender.....which in turn is supplied by 'main relay' on the firewall. 'Tis brown/slate wires that carry voltage between the two....if your installation still has original wires and you wanna look for poor connections, etc.

The ECU grounds one side of the injectors to make 'em fire. Two common faults can prevent this from happening:

1) The 'trigger wire' (black/white) from the coil negative post to the ECU is faulty. In an original installation the wire runs aft along the water rail.

2) The main system grounds are faulty. Look at the rear of the water rail. There should be a bundle of black ground wires under one of the bolt heads. These are the grounds for the entire system, including the ECU itself. Make sure it's all in good order

As for a dry return line....

Have you actually measured fuel pressure? The pump should produce about 75 psi outright, which is then regulated down to about 36 psi at the fuel rail. If the regulator is stuck shut, and the injectors are not opening, you'll see full pump pressure at the rail.....as it is pumping and pumping but the fuel has no place to go.

OTOH, maybe the problem is just the opposite.....not enough oomph or pressure, as Grant mentioned.

What pump are you using?

First get the injectors working and then measure fuel pressure, IMHO

Cheers
DD

the injectors click as if active with a direct 12v feed to the terminals, but the engine terminals that supply them are not getting more than 2v.
traced the earth wires at end of water rail and their are 5 of varying then and good condition coming up to a bolt on the rail from a cable bundle below and a 6th small wire emerging from the bundle that travels along the back of the water rail, this 6th wire is very frayed and will need repair.
the pump is one which came with the engine from the 1985 Jag from which the engine was transplanted, not sure of brand, will check.
https://cimg0.ibsrv.net/gimg/www.jag...323c700b4b.jpg

NOW FINDING fuel in the return line, so assume that is progress.
re-checking wire.
any further ideas? is there a way to force the injectors open momentarily?

forced open an injector and get a putting rev'ing noise, a growl and lots of exhaust smoke. that brings me back to the wires and the computer again I guess.

If you spray some form of ether into the intake, while cranking and it runs, you can then completly rule out the ignition system, and focus on why the fuel side of things is faulty.


Restrictions in the fuel system, like a blocked filter/rusted tanks clogging fuel pump or even seized pump or injectors all are issues that can come with a 30 year old car.


Remove the fuel line feed hose, and make sure a steady amount of fuel is being supplied when the ignition is on and being cranked (or in drive so it wont actually crank) that should show that at least the supply side of things is fine.


Then there can be problems with the return check valves, and tank vent system, possibly limiting return to either tank.


Injectors can gum up or become clogged with foreign particles, if they are partially blocked or not operating correctly, that's another issue.