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Thread: 4.6 low compression in one cyl, chugging through oil filler

  1. #1
    Join Date
    Jul 2016
    Location
    adelaide
    Posts
    7

    4.6 low compression in one cyl, chugging through oil filler

    hi guys,

    18 months ago i purchased a p38 on a whim. i was instantly rather taken by it, and it became my daily driver. i have driven it nearly 20,000km in that time. other than a water pump, the usual blower relay issue, and a battery drain because of the remote locking antenna, it has been pretty much faultless.

    it has always had what sounds like a noisy lifter - silent when warm, but quite a clack as it warms up. i was going to get around to it some day - cam and lifters looks much easier on a carbie 3.5!

    in an effort to improve its LPG mixture, some months ago i had it put on a dyno and one of the oscilloscope readings was said by the specialist to indicate good (or at east even) compression.... but i have never had cause to do a compression test.

    in the last few days it has started running rough at idle - narrowed it down to cyl 5, found compression down to 60. removed the oil filler cap, and there is a rhythmic chuffing sound distinctly like compression leakage. i think it is in time with cyl #5. not a great deal of blowby as you'd expect in a holed piston scenario.

    it does not use ccolant, doesn't get hot, and doesn't pressurise the cooling system.

    if, say, a lobe has worn to nothing, or a lifter has worn/collapsed, and not opening the valve sufficiently, could this cause compression loss into the crankcase? is the 4.6 prone to the top of the cylinders leaking into the valley, like the old 3.5s?

    i am trying to find other scenarios to the piston/cyl damage thing. the apparent blowby is not nearly bad enough to suggest something like a hole in a piston. and if that were the case i'd expect 0 compression.

    any ideas welcome


    thanks in advance
    Davide

  2. #2
    Join Date
    Apr 2009
    Location
    Crafers West South Australia
    Posts
    10,044
    The noise is probably a loose cylinder liner, the chuffing a broken compression ring or two or a leaking head gasket. It's possible the two are related. You can pull the heads and sump in the vehicle if needed but replacing the liner is expensive and requires the engine out and full reconditioning.

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