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Two weeks ago I was stopped at a traffic light and my engine shut down. It took 4 tries to get it back running. People behind me got pissed when they missed their green light.

This morning the same thing happened. I was waiting for a traffic light and a ton of cars when the engine stopped. The same thing happened when I tried to start it. This time the engine shut down on 4 different occasions and it took 2-4 tries to start it on every one of those occasions.

I found a website explaining that if the engine stops on gas, some gas will remain in the engine. When starting again, the mixture of gas and petrol will be difficult to ignite, so that could explain the difficulty in starting after the initial shutdown.

What could be the cause of the initial shutdowns? I would love to be able to solve this one myself. If there is more info needed I'll provide it, but I'm clueless about what you would need to know.

My car is a Volvo 850 2.5 from 1996 with 550k+ km and I don't know how long this vehicle has been running on gas. The car always starts on petrol and switches after a second or 2. It doesn't seem like I can prevent it from automatically tyring to switch to gas.

  • what do you mean by "during LPG conversion"? If the car switches to LPG 2 seconds after you start the engine, it should be running on LPG already by the time you get to the traffic light. - I'm asking this because some problems are caused by trying to switch from petrol to LPG at the wrong moment. – Hobbes Jun 13 '17 at 10:08
  • The residual gas will be pumped out after a few revolutions of the crank, so shouldn't be causing problems. – Hobbes Jun 13 '17 at 12:39
  • By conversion I meant when it switches from petrol to LPG... – Scuba Kay Jun 14 '17 at 7:11
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Most lpg systems - ones that I have had have a switch either gasoline / lpg or Auto / gasoline / lpg that enable you to switch to gasoline if you are out of lpg or in situations that require lots of stop / start. It may not be directly visible, but most do have them.

  • Yeah, there is a switch, but it switches to lpg automatically after starting. Also when I don't want it to. My question was more about why the engine is shutting down in the first place. – Scuba Kay Jun 13 '17 at 9:25
  • Hmm, usually there is an "lpg off" position somewhere.... Especially since some tunnels and boats prevent the use of lpg (or did - they may have changed...) and require the tanks to be closed, curious as the lpg tanks are better protected compared to gasoline tanks but that is another discussion. – Solar Mike Jun 13 '17 at 10:03
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    Have you checked the idle speed? with and without A/C running... – Solar Mike Jun 13 '17 at 10:06
  • The button is an electronic push button with a light indicating petrol (red) or gas (green). There is no 'position' the button can stay... What do you mean by checking idle speed? Checking if the rpm stays high enough? (I read somewhere some systems shut down <1000 rpm?) I'll check in an hour.. – Scuba Kay Jun 13 '17 at 10:50
  • Most engines need an idle speed 750 to 900 rpm... Higher when cold. – Solar Mike Jun 13 '17 at 10:58
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Many LPG systems contain an evaporator: this uses engine coolant to warm up and evaporate the LPG before injection. If the evaporator is broken or the engine switches from petrol to LPG too soon (before the coolant has warmed up enough for the evaporator to function), you can get these symptoms.
Newer liquid LPG injection systems (LPi) don't have an evaporator.

Things to check: the evaporator itself, the thermostat in the cooling system, temperature sensors.

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