lpg - exhaust backfire
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Thread: lpg - exhaust backfire

  1. #1
    FRESHMAN ROVER
    Join Date
    September 1st, 2009
    Location
    Cheshire, UK
    Posts
    16

    lpg - exhaust backfire

    Hi

    I'm getting a major backfire through the exhaust on overrun. Typically, this is going downhill with foot completely off the throttle, after ~5-6 seconds (guess), BOOM!! Otherwise, the engine seems to run well, on petrol and LPG.

    I believe its an SGIS system, with 8 injectors visible, plumbed in to the 'bananas'. Should they not shut off when throttle is not applied, in the same way petrol injectors do? If so, does that mean my injectors are leaking?

    I'm assuming my cats have been gutted, due to telltale weld marks on both, so no damage done there, but I'm obviously keen to resolve this as I'm sure it can't be healthy and only time before something gives out.

    Is there anything I can test for? I've seen a couple of error codes relating to O2 sensors, but assume thats because the cats arent there. I can't tell you specific codes right now, I didnt write them down and I'm 1/2 way through replacing head gaskets etc, hopefully built back up and running this weekend!

    Reason for headgasket change, I was getting pressurised coolant system, which meant coolant was being dumped leading to eventual overheating. One of the head gaskets was on upside down, but other than that, didn't look in a bad shape and heads nice and flat. bores look ok too, so hoping new gaskets has solved my problem (fingers crossed!)

    Eventually I'll get the necessary bit to plug laptop into SGIS so I can check that out and make make any adjustments as part of the LPG service
    Land Rover series IIA 2286cc petrol 1962MY, Range Rover P38 4.0V8 Petrol/LPG auto HSE 2000MY

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  3. #2
    FRESHMAN ROVER
    Join Date
    September 30th, 2016
    Posts
    17

    Re: lpg - exhaust backfire

    SGIS is a multipoint system that slaves off the petrol system so when the petrol injectors shut off on the over run, the LPG injectors should do the same. If one or more are not shutting off properly and allowing gas through, it will ignite in the exhaust system as soon as the air/fuel ratio is right for combustion. Do you know what injectors are fitted and how long they have been on there for?

  4. #3
    FRESHMAN ROVER
    Join Date
    September 1st, 2009
    Location
    Cheshire, UK
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    Thanks for the info, is there an easy way to test if they are shutting off? I presume that could be done with a multimeter? They are stamped with valtek 3ohm and are bright red. From a quick search, looks like 2 type 30 rails in series. Is that enough info? The previous owner had it less than 12 months and reckons he put them on. Reason for sale, alleged an L322 came along that was nicer.

    Any way to know if 3ohm is suitable for my particular application?

    Thanks

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  6. #4
    FRESHMAN ROVER
    Join Date
    September 30th, 2016
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    Re: lpg - exhaust backfire

    Valtek type 30 are about the cheapest injectors you can get so aren't brilliant but should last longer than a year (unless it has done starship mileage in that year). With it running on gas, use a long screwdriver as a stethoscope to see if one is sounds different to the others, that will be one showing signs of wear. The other thing that is necessary for the fuel to ignite in the exhaust is a source of air so a leaking manifold to head or manifold to downpipe would make it much more likely.

  7. #5
    FRESHMAN ROVER
    Join Date
    September 1st, 2009
    Location
    Cheshire, UK
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    Cheers! Hopefully I'll get it back running this weekend! No, not done many miles, I'm assuming they are 30's, the only way to tell seemingly is by colour. They are bright red on a square section rail, so either 30 or 33, hard to tell. Yes there's a bit of a blow, but i think that's as a result of the backfires, need to be sorted either way! I'm just hoping I've fixed the coolant pressurising issue i had by changing the head gaskets and is not something worse like a cracked/porous block!! Thanks for the info.

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