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Thread: LPG. Missfiring, rough idle and rough running.

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    Patrol God mudnut's Avatar
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    LPG. Missfiring, rough idle and rough running.

    A dual fuel engine has its own set of factors which may cause problems to deal with. First, and foremost, the ignition system needs to be in excellent condition to handle both fuels.

    Running constantly on LPG, for long periods of time can cause some components in the carburettor to dry out and become unserviceable.

    If after checking for vacuum leaks, faulty ignition components, freeing up any sticking carby linkages and running a carby cleaner in the petrol, the problem still persists, you have a few options.

    You can do a carby kit change out, replace the carby with an aftermarket unit, or get it reconditioned. Once the problem has been fixed it is a good idea to start the engine on petrol, then switch to lpg, at least once a day. This will ensure that the carby components will be kept in working order.

    The quality of the gas conversion plays a big part in how the engine will perform. I have found that some (not many) of the mechanics that do the gas conversion can sometimes use of inferior wiring connectors (scotch locks and cheap lugs), and also they may run poorly routed wiring and heater hoses.

    Some mechanics also use pliers instead of proper lug crimpers when doing the conversion. This can cause high resistance joints and/or intermittent faults, and might also be a factor in making the engine backfire, which, in turn can cause more problems.

    Routing of the electrical wiring over sharp metal, near the exhaust, or not securing the wiring properly may also result in short circuiting or open circuits. Also wiring that is under tension, has a knot in it or is flexing constantly can become open circuit. Sometimes it a can also be the cause of an intermittent fault.

    Loose or poor quality spade lugs can develop a film of carbon under them. Although they may look okay it is a good idea to check them and use a pair of pliers to tighten them.

    The cab heater hoses are often used to supply hot water to warm the mixer and prevent it from icing up and starving the engine of gas.

    The use of "Y" pieces instead of "T" pieces at the junctions where the mixer heating circuit is piggybacked to the vehicle's heater is recommended, because they prevent cavitation, which may result in poor water circulation.

    I have also seen problems caused by metal filings or swarf from the gas piping or from the service station get lodged in the gas components, such as solenoids.

    There has also been a few cases where the solenoids and control components have failed.


    The Diaphragm in the gas mixer can "go slack" over time, too so the mixture may need to be adjusted. I recommend taking the vehicle to a mechanic who can tune the engine properly.



    Some relevant threads;

    http://www.nissanpatrol.com.au/forum...t=idle+mixture

    http://www.nissanpatrol.com.au/forum...t=idle+mixture

    This is a scotch lock. An easy way to join wires, but they sometimes fail, especially if subjected to moisture and vibration.
    I strongly recommend soldering all joints and then sealing them with heat shrink or brush on electrical tape.
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    Last edited by NissanGQ4.2; 24th July 2016 at 04:27 PM. Reason: Removed sentence as requested

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    dom14 (5th July 2016)

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    Administrator AB's Avatar
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    Quote Originally Posted by GQ TANK View Post
    Missfiring, rough idle and rough running

    This can also be caused by the a loose carby. in the base of the carby there are 3 screws that join the butterfly assembly to the rest of the carby, they can come loose (especially if you are using a impco carby mixer) - as they have a lot of top loading on the carby.

    To detect remove the air cleaner, try moving the top of the caby - any movement you have this issue.

    To fix - remove the air cleaner - remove the 4 mounting bolta at the base of the carby, flip the carby and tighten the 3 large screws.

    While you have the carby out and if you don't live in cold climate consider doing the carby heat mod for a little bit of extra power - remove the honey comb element.

    Note:- the Loose carby issue affects the RB30s and TB42s - I have had it occur on both motors

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    Patrol God threedogs's Avatar
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    I have heard some use Araldite on the tank wiring
    to prevent it coming loose and possible causing sparks.
    All wiring should be done by a qualified gas fitter IMO
    04 ST 3lt auto, not enough Mods to keep me happy, but getting there

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    Patrol God mudnut's Avatar
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    Some of the worse workmanship I have seen is by qualified people, but they're a minority. The examples above were done by qualified mechanics.
    Last edited by mudnut; 3rd June 2016 at 03:52 PM.

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    Patrol God threedogs's Avatar
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    Also using "Y" pieces instead of "T" pieces will decrease the chance of cavitation.
    04 ST 3lt auto, not enough Mods to keep me happy, but getting there

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    Quote Originally Posted by mudnut View Post
    [COLOR="#FF0000"]
    This is a scotch lock. An easy way to join wires, but they sometimes fail, especially if subjected to moisture and vibration.
    I strongly recommend soldering all joints and then sealing them with heat shrink or brush on electrical tape.
    Evil creatures for lazy people. Strip, solder and tape/heatshrink.
    There is no legal cure for STUPID!

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    Patrol God mudnut's Avatar
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    I must point out that LPG is a dangerous fuel and all repairs to the gas equipment and tuning should be done by a Qualified person.

    LPG is heavier than air and will settle in low areas, thus can accumulate and become a hazard.

    I have also found one fitter had run a pipe from the back of a gas diaphragm to the engine. This caused the engine fumes to eat away part of the diaphragm.

    A very dangerous scenario. Luckily the fault was found and rectified before anyone was hurt.

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    Quote Originally Posted by mudnut View Post
    I must point out that LPG is a dangerous fuel and all repairs to the gas equipment and tuning should be done by a Qualified person. LPG is heavier than air and will settle in low areas, thus can accumulate and become a hazard. I have also found one fitter had run a pipe from the back of a gas diaphragm to the engine. This caused the engine fumes to eat away part of the diaphragm. A very dangerous scenario. Luckily the fault was found and rectified before anyone was hurt.
    Too TRUE Mudnut !! I foolishly mucked around with my gas tuning when I was 18 and melted the guts out of my perfectly good Quadrajet. Was a disaster, inlet manifold was caked in the crap :-(

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    Quote Originally Posted by threedogs View Post
    Also using "Y" pieces instead of "T" pieces will decrease the chance of cavitation.
    Good to see you read the op!

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    Patrol God mudnut's Avatar
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    Dirt clogging the fuel pump is another cause for rough running and cutting out. This Thread may offer a solution.

    http://www.nissanpatrol.com.au/forum...Petrol-problem
    Last edited by NissanGQ4.2; 7th January 2017 at 05:24 PM. Reason: As request by OP

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