Thread: Idiot's version of engine management gas vs petrol

Traditionally the only thing turned off to run LPG on a D1 is the injectors. All other components of the petrol system should remain connected and functional just they don't affect the LPG fuelling in any way. Idle speed is still controlled by the ECU as per normal. Ignition timing is never controlled by the ECU as the distributor timing is by mechanical devices only. LPG mixtures are controlled by knobs on the Impco mixer.

There are two common ways to turn off the injectors. One way is to use two relays (or a dual relay) to cut the two wires that fire each bank of the injectors. The wires are both yellow, one has a blue stripe and the other has a white stripe. The other way is to cut the power wires to the injectors. Some fitters don't understand that the MAF is also supplied by the injector power feed. A new wire has to be run from the live side of the cut to the MAF power input. The wires in question are brown with an orange stripe, easy to identify at all injectors, in the loom, and at the MAF. They are all joined together in the loom somewhere under the dash. If the MAF isn't powered when on gas the ECU goes into limp home mode when you return to petrol. Then you have to switch off the engine and restart on petrol only until you fix the wiring as it needs to be.

So to complete the picture, as the injectors are turned off so too presumably a relay turns on a gas feed valve ... at the tanks? And vice versa.

Last bit to my training request:
So my gas idle is terrible … sometimes. Sometimes its perfectly fine but at others it chokes and stalls unless I keep the throttle open a bit and the revs up. Petrol idle is a bit all over the place but always settles. Now I've just discovered a workaround .... if I start the car on petrol then switch to gas after a minute or two (standard procedure), then gas idle is bad, but if I start on gas in the first place, then gas idle is good. So from what I now understand from Bee-Utey of the basic principles, it could be the ECU but unlikely, it could maybe be something to do with the gas/petrol changeover, but is most likely a sticky or lazy IAC. I'm thinking the stepper is correctly retracting on engine shutdown, so starts fine on gas or petrol, but does not retract correctly for idle situations - petrol can cope, gas can't. Agreed?

Originally Posted by Dannyroger

So to complete the picture, as the injectors are turned off so too presumably a relay turns on a gas feed valve ... at the tanks? And vice versa.

Well yeah, it helps the gas run if all the solenoid valves are turned on. One for each tank and one under the bonnet next to the gas converter. There should be a safety circuit in the wiring that shuts off all the solenoids when the engine stalls. Turn off the power to the gas solenoids and the gas in the converter runs out a second or two later.

As for the gas idle, that could be any number of things. The ECU won't affect the idle unless the IAC is totally cacked. You do know you can manually adjust the idle speed up? There's a 3/16 allen keyed screw in the top of the manifold throttle body that sets the minimum idle speed. Wind it out to increase base idle.

A more common problem is a failed fuel pressure regulator. It is connected by a small vacuum hose to the manifold and a holed regulator allows petrol to dribble into the manifold making it run rich as hell at idle.

BTW you should always start a dual fuel engine on gas,the rich running will quickly foul the colder running plugs you need to make the gas run properly under load.

Thanks Bee Utey If I'm ever in SA and find out who you really are, I'll buy you a beer.

Why should you start a dual on gas Bee? Should I change mine?

Originally Posted by xstriple

Why should you start a dual on gas Bee? Should I change mine?

You can use colder running plugs in a dual fuel car so that the plugs cope with full load running on gas much better. Many older cars run pretty rich on petrol when cold and they fouls these plugs quickly. There's no problem using petrol to swing a motor into life first thing but its better for the motor to run on gas as soon as possible. It's kinder on the rings too as the cylinder wallls aren't running wet with extra petrol.

Most cars with mixer style dual fuel run quite well on gas once started cold. All you have to do is avoid full power until the engine has warmed a little. Your vapour injection system on the other hand is usually set to run on petrol up to around 40C coolant temp as they have trouble following the cold run fuel map built into the petrol ECU. Having said that modern vehicles are designed to have the O2 sensor warm up really quickly so the plugs don't have too much trouble coping. D1s on the other hand are just filthy old beasts and petrol should only be selected on start or a warm engine if possible. Your oil will thank you for it and stay cleaner twice as long.