740 & 760 Unusual no start issues on a dual Gas/Petrol 740

edited May 5 in Troubleshooting

This one's got me stumped. 740 16V with extremely low KLMS and done just a handful of KLMS in last decade.

FPR replaced after finding that was a source of rough running when petrol selected.

Now when it is running it runs well, but it doesn't want to start hot or cold (seems worse hot, but still very difficult cold). Seems slightly better to start on gas, but again difficult on both and once running it runs fine on both.

So I'm thinking it can't be;

- The AMM, since the the gas system uses some other method of fuel control I believe.

- The Fuel Injection Relay, since it's not required for gas and once running it runs fine.

- Can't be the CPS, since that would cause no run / intermittent no run issues, this is a start issue only.

Any thoughts on what can cause a no start / extremely difficult start on both gas and fuel then run fine on either once running?


  • volvodrivermanvolvodriverman @volvodriverman Redcliffe Queensland

    Weak spark whilst cranking maybe?

  • edited May 5

    Sounds logical, ignition amplifier?

    Weak spark from some poor condition distributor or rotor another possibility I guess. Has fresh HT leads, so that's unlikely an issue.

  • volvodrivermanvolvodriverman @volvodriverman Redcliffe Queensland

    I'd start by probing the battery on crank. It might have enough juice to wind over but not enough left for injectors, pump, spark maybe. Then my next thought is crunchy wiring to the distributor but I'm not sure if you're lh2.2 so that might not apply.

  • Don't completely rule out crank angle sensor as the signal could be weak at the lower cranking rpm.

    Ignition system problems are usually more obvious on lpg

    What kind of gas setup is it?

  • edited May 5

    She's an 89 model, so Def has the crank position sensor rather than hall sensor. Thanks @timbo for confirming this may still be an issue. I've only experienced these causing intermittent complete failures, both during start and during driving. I've got a few spares and they aren't to hard to change so I'll swap it out just for general reliability. Till J get the chance though I'll try the next few starts with a jump start connected and see if the extra voltage does anything. Battery seems very healthy, and I've had other good batteries in the car and had same issues.

    RE the gas setup, I know very little about it and it looks similar to one I had on my 83 244 with a ford 5.0 :), I'll add some pics of the (untidy) engine bay with the main gas components. I'm thinking it's unlikely gas or fuel since the symptoms are almost the same (except on a cold start, pretty much gas is the only way I've started it the last few times, switch straight to fuel immediately and she still runs though).

  • Check the wiring going directly into the radio suppression relay on the right inner fender

    They often slightly pull out causing a disconnection, they need to be fully inserted otherwise the injectors will not ground

  • If it was done as an aftermarket thing then I'm going to guess it's an Impco or Parnell system that uses the type that fills the manifold with LPG through one of those gas ring style setups.

    Agreed that if the ignition was at fault it would be very obvious on LPG because it would most likely backfire through the intake. I wonder if it's a fuel pump problem in the tank from a blocked up filter sock. It could explain why it starts easier but still not easy on gas as from memory those older systems will first crank on petrol and switch over to gas once the motor starts to prevent excessive cranking.

    At least that's how the Parnell system should have worked on my Rodeo but the gas ecu was broken so it was metered through a shrader valve and starting on gas when hot could be tricky as the gas ecu that would normally prime with petrol was gone and it had to wait for the manifold to fill with gas before starting.

    I think it's worth going through the fuel system in the tank and checking that filter.

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