But only because the engine ecu cant speak - or shout.
The long journey bit may be true as most of the single point systems go lean rather than rich under highway conditions. Lean running on a long motorway journey is another nail in the coffin of a Rover V8 that is probably past its best anyway.
All good stuff,but as you are not familiar with Gems - it uses a pair of 5-0v Titania oxygen sensors,one for each bank and also a knock sensor for each bank.Right, well what's shown in that picture is an 'Closed Loop - Duel Point' system, not an open loop system; bit more complicated to fit, but not much.
Vapouriser in that pic IS a duel diagphram unit, or 'Two Stage' vapouriser. As far as the plumbing's concerned, works just like a single stage unit, hard line 'in' for LPG, soft line 'out' for the vapourised gas. Then a hose port 'in' and 'out' for the heater water supply.
Ie: they ALL would look to have "2 outlets and 1 inlet" for 3/4" hose, one pair is for the water, one is for the boiled fuel!
Advantage of a two-stage vapouriser is that it can usually handle more gas, for bigger engines, and second doesn't boil it all in one go; first chamber pre-heats it, second chamber boils it 'as required' so there is less gaseouse fuel in the system at one time, hence makes the engine a tad more responsive.
Now, the intricacies; That Closed Loop system is that it has an electronic 'brain' controlling a stepper-motor actuated mixture valve. This sits in the pipe from the vapouriser to the mixer and controls the mixture strength according to a signal from a Lambda sensor in the exhaust.
Identifying whats in that picture:-
On the Left, two black boxes with tags on the top; Injector Emmulators
Moving to the right, and just above, big aluminium thing, is the duel-stage vapouriser
Beneath that, shing round thing with brass bit stuck out the side; Mixer.
Beneath that, the thing that looks like a funny shaped bolt with a tail of wire coming out of it and looped round with red ends; Lambda Sensor
Next to that is a small black box..... No idea, pictures not clear enough but at a guess, probably the 'change over switch & LED Guess Guage'
Next to that, brass thing with a black plastic bit on the top; fuel shut off solenoid.
Above, sitting in the coil of wire, Black Box with a tag, Control Unit
Thing sat next to it, black plastic moulding with some kind or metal cap on the end; stepper mixer valve.
Reason its called 'closed loop' is becouse the Lambda sensor reading CO levels provides feed back areound the engine giving a 'closed loop' feed-back to correct mixture strength.
The 'open-loop' systems work purely on er....... well in some cases NOTHING! There's just a preset 'tap' in the hose between the vapuriser and mixture to adjust the basic mixture strength, and it then relies on the venturi in the mixer flowing more gas into the air stream the more air that passes through it.
Crude but effective! if correctly set up.
As comments elsewhere; on a 'pre-set' stystem, you set the pressure regulator on the vapouriser for peak fuel flow, then adjusted the mixture on the mixture scre to the mixer, erring on the side of ritch, as you had little compensation to ritchen the mix for acceleration. BUT, if the regulator was set too low, then on load, the mixer wouldn't be getting enough fuel and the mixture would weaken off, and engines could start to knock.... not a problem of LPG.... a problem of bad setting up and tuning.
anyway, slightly more refined systems have a vacuum controlled mixer valve; this takes a vacuum line from the plenum, some-times the same coupling as the distributor's vacuum advance (Not that you'll find one of those on a distributor-less 4.6!) to a diagnrphram arrangement similar to the vacuum advance on a dizzy, which opens the mixture valve under vaccum, ie acceleration, to provide enrichment for acceleration.
Makes the system a bit more 'responsive' as well as a tad more ecconomical..... in theory!
But same problems as pre-set system, in getting the initial delivery pressure and mixture right AND setting the compensator valve to compensate by the right amount, at the right time! Bigger ball-ache to set up, and even more chance to balls it up, I'm afraid. Again, though, not a fault of LPG or even the LPG system, just ignorance or lack of expertise in using it.
Now, since your mate doesn't have a mixer, I'll mention the NEXT advance in open loop systems, and that is the BLOS carburettor. Big news at the moment, becouse some-one has just started selling them at realistic prices (under £100).
Basically this is a propper 'carburettor' which does the job of both mixture valve and mixer, and it works like an old fasioned SU carb, with a slide lifting and falling under manifold vaccum to keep the air velocity through it constant, while metering the fuel via... well and SU carb did it with a needle and jet, but on the BLOS its a shrouded spray rail to handle the larger volume of gaseouse fuel.... but the ideas the same.
Reports on these things have been promicing, and I'd be VERY tempted if your mate needs a mixer, to look at one of these set ups. Open loop, doesn't need the exhaust drilled for the Lambda, and theres not so much complicated electronics involved, as with the closed loop system in the example you've shown.
I came accross the LPG carb, about seven or eight years ago, and the engineering studies I read were very promicing, but it was a commercial non-starter, as it was too expensive to make, compared to more sophisticated electronic systems..... except that's changed now this chap has found a way to get them to market for under a ton...... provided cost cutting hasn't severely compromised them......
ANYWAY..... back to the topic; 4.6 is a GEMS motor. On the earlier 3.5 & 3.9 systems with Lucas Flapper or Hotwire injection, you could fit the mechanical bits of the LPG system, and dissable the injection simply by killing the power supply to the injectors. ECU would keep 'firing' the injectors, but since it did that by switching the earth side of the injector, it would just be turning on and off a dead circuit! Didn't even matter if it was a Lambda system...... it could adjust the fueing as much as it liked.... without power to the squirters, wasn't going to put any in!
Meanwhile it would keep sparking the ignition, so if you had the electric bits of your LPG system in place, to turn on the gas....... off you'd go.
On a GEMS engine, though, you NEED the ECU to control the ignition for you, as you dont have a seperate distributor. Switch off the injectors, whole system will go wampy, hence you need those injector emmulators, to 'fool' the ecu into thinking its putting fuel into the engine when its not, and to keep the sparks flowing.
Which just leaves the matter of the Lambda, if its a closed loop system. GEMS engine SHOULD already have a Lambda, and the sensible thing to do would be to take the feedback signal that the ECU is providing for the injectors to drive the mixer stepper, but most systems find it more convenient and easier to by-pass it and put in a second Lambda and control loop for the stepper.
Dont know if any of that is of any help what so ever...... but the simple answer is it's NOT quite so straight forward on a GEMS engine as it is on the earlier ones, and without the detailed instructions that (hopefully!) would have come with the original kit, plus a bit of know-how and some ideas of the variouse regulations, standards and basic operation of gas....
Its NOT a job for the feint hearted to fit an LPG system! Especially a second hand one, worse an INCOMPLETE second hand one!
My advice? Find some-one that knows gas, if not a pro, a trusted authority....... failing that, stick it back on e-bay!
And use the dosh for
a) A pro-fit conversion
b) a 'New' kit with instructions, and ALL parts sized properly for that engine
c) buying 'full-Price-Fuel'!
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