Hunting Engine

[Dave Rogers]

I've got an RRC 3.5 efi, converted to LPG on which it runs great, economy isn't great but the performance is good, well, it goes where I want it to!
However, when it runs on petrol at idle, the engine hunts, tho when I went out yesterday laning, it didn't hunt on petrol, until toward the end of the day so I switched back to gas, I'd used petrol just to give the injectors some exercise as it usually runs on gas. When it's hunting the exhaust give out a dark hued smoke like it's a diesol!
On gas the exhaust is clear of smoke. The gas system is the single point type.
It ticks over at just under 100rpm on gas, but very slow on petrol, 500 rpm or less which rises or falls, if I change the idle speed on the plenum chamber it doesn't tick over on gas.

Any ideas??

[Range Rover Blues]

How does the LPG system dissable the petrol EFi? it's possible something likje an injector has worn whilst running dry.

Anyway, it sounds like it's running way too rich.

[Guardian.]

MINE IS SET UP TO RUN ON GAS, ITS A 3.9 BLOCK, BUT WITH MY OLD CARBS, THIS RUNS MINT ON GA BUT IDLES VERY SLOW AND CAN PINK A BIT ON PETROL, YOU CAN NOT HAVE IT RUNNING PERFECT ON BOTH, UNLESS YOU BUY ONE OF THOSE ELECTRONIC BOXES TO CHANGE THE SETTINGS WHEN YOU SWITCH BETWEEN FUEL.
IM GUESSIUNG YOURS IS THE SAME, SET TO RUN 100% ON GAS.

[Dave Rogers]

The switch over happens via a dual changover switch, the original iwema system gave up the ghost so I make a new one, electronics being my job, it's done in this format, starts on petrol, then on changover, the fuel pump switches off, then using a capacitor to delay the gas relay until the petrol has had time to go so to speak, the gas valve is opened and the engine runs on gas, The rest of the electronics are not touched. The cold start works as it should tho sometimes there is a strong smell of petrol but not always, the engine hunts when smelling of petrol or otherwise.

[Range Rover Blues]

I might suspect injector problems then as I'm told they don't like to run dry.  It's more common to shut off the injectors and leave the pump running.  Another benefit of this is that the fule is always circulating so you don't get a pipe full of hot fuel near the engine, oh and it keeps the fuel filter busy.

Aparently one of the few exceptions is Jeep, who only put a fuel feed intot he tank not a return? the pump doesn't like to run when the fuel isn't moving.  So I'm told anyway.

[Range Rover Blues]

MINE IS SET UP TO RUN ON GAS, ITS A 3.9 BLOCK, BUT WITH MY OLD CARBS, THIS RUNS MINT ON GA BUT IDLES VERY SLOW AND CAN PINK A BIT ON PETROL, YOU CAN NOT HAVE IT RUNNING PERFECT ON BOTH, UNLESS YOU BUY ONE OF THOSE ELECTRONIC BOXES TO CHANGE THE SETTINGS WHEN YOU SWITCH BETWEEN FUEL.
IM GUESSIUNG YOURS IS THE SAME, SET TO RUN 100% ON GAS.

My LSE runs on a multi point system, it's about as good as you'll get an engine to run on dual fuels.

The ideal C:R for LPG is around 12.5:1, so it's not just the static advance but the whole advance curve that's wrong for LPG if not the whole engine.  Probably one of the reasons we get worse economy on LPG even though it burns hotter :?
I'm told that in the 'states it's common to have a dual-mapped ignition for LPG.

Megajolt anyone?

[Coastcard]

What Mr. Blues says. On Injection engine it is normally the injectors that are disabled so the pump runs and in the process cools the injectors. It would appear that you are compounding the problem by letting the injectors run dry and still firing them all the time with no fuel.

Having said this, if it has always done it, you can try to adjust the idle CO (bypass) on the AFM (hex screw). Screwing anti-clockwise should lean it up. As a base point, start at 2.5 turns out from fully home (having measured how many turns in it was originally, so you can go back if it makes no difference).

A third option is the old favourite, the coolant temperature sensor?

[Dave Rogers]

Cheers all, food for thought anyway, will check the injector dc feed. will add to the relay set up.

Odd thing is, on sunday, it didn't play up at all until much later on........

Confusing!

[Rossko]

This'll be the flapper type ECU system then .  My guess is it'll never idle quite right unless you set the base idle speed correctly, at the plenum as you say.  The reason why it won't then idle on gas is that the flapper AFM needs a little gizmo to prop the flap open a bit.

If yer too skinflint to buy a proper switch it shouldn't be too demanding to make one   it only needs to be open a few mm.    On the homemade switch side, you have rigged it so it will shut off gas if the engine stalls?  This is in important safety featue of commercial switches.

cheers, Ross K

[Range Rover Blues]

Actually that's a good point, on SU carbs and flapper EFis system you need to iliminate the variable vent or VAF flap when running on LPG, both to protect it from damage and to stop it messing up the mixture.

[Dave Rogers]

Hi, thanks for your comments, checked the wiring, I did wire the injector dc feed to a relay which turns off when the gas is on, the comment about the flap valve set me thinking and it seems to me that the anti blowback flap might be the cause, checked it out and the spring had broken, replaced the spring and hey presto..................... running better already

The switch I put in to do the change over is a 'soft' switch using a ttl chip, that then activates a pic programmed chip to activate relays via some 1A drive transistors. It's then a simple matter of triggering some relays, tried to find some solid state 1A switches but couldn't find one robust enough. One tap of the finger sees the relay go through a process with led's showing progress and status of the relays. The engine if it stops, automatically switches off the gas as the default state of the relays are normally open, a voltage regulator looks after the gas, when the voltage drops below a nominal value (not 12vdc), it turns the gas off.  
As I said, I work with electronics.................. not that good at setting up fuel systems/air/fuel mix tho so thanks for your help, sometimes the simplest of causes........