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Thread: V8 on LPG - 'popping' on deceleration

  1. #1
    Join Date
    Jan 1970
    Toowoomba QLD

    V8 on LPG - 'popping' on deceleration

    My 3.5 has recently started popping from the exhaust pipe on deceleration, particularly when running downhill on a closed throttle. It has twin SU's.

    Could it be due to the idle mixture (too rich/too lean?)
    or possibly the timing? I'm just guessing at the moment and would be keen to hear some more informed opinion.


  2. #2
    Join Date
    Jan 1970
    stockton nsw close enough to the beach to hear the waves and smell the sea :)
    sounds like the slides arnt likin the air speed goin by em check ya oil level in the dash pots and then check the diaphrams arnt torn as this may be the drama and its usualy only one side

  3. #3
    Join Date
    Feb 2004
    Williams West Aust
    Exhaust manifold/pipe leak will cause this.
    Midlife Crisis.Im going to get stuck into mine early and ENJOY it.
    Snow White MY14 TDV6 D4
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    2003 Stacer 525 Sea Master Sport
    I made the 1 millionth AULRO post

  4. #4
    Does it happens on petrol too?

    If lokka nd LandyAndy's suggestions don't work, then perhaps some unburnt fuel is getting into the exhaust system, suggesting a valve problem or extremely worn camshaft (exhaust valve late to close on intake cycle). Another possibility is incomplete combustion in one cylinder.

    I'd check the state of your plugs and do a 2-stage compression test to get and idea on the valve sealing - a first reading on each cylinder and a second reading after a squirt of oil into the cylinder to improve the sealing of the piston rings.

    If your timing is very retarded I suppose it would be possible at low revs (minimum ignition advance) for combustion to be taking place as the exhaust valve is opening, but you would be very down on power and running warm. I'd be surprised if the motor would even start for you.

    LPG can backfire if ignition is sub-optimal but it doesn't sound like this is your problem.



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