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combustion leak / head gasket leaking


  • failure of the sealing at the head gasket may result in:
    • combustion gases leaking into the coolant system
      • this is evidenced by air bubbles rising up when the radiator cap is off and the engine is started
      • it can be proven by using an Head Gasket Test Kit analyser which changes color if the bubbles are exhaust gases
    • coolant leaking into the combustion chamber causing:
      • unexplained coolant loss and resultant car over-heating or losing coolant
        • rate of loss is dependent upon:
          • size of leak
          • combustion chamber pressures - the harder the car is driven the faster it may leak
          • coolant pressure - a faulty thermostat is likely to exacerbate the loss and over-heating
      • white smoke (if using anti-freeze) / steam from exhaust
      • water in cylinders or on spark plugs when you pressurise the coolant system
      • cylinder misfiring due to transient spark plug issues
    • possible leaking of coolant into the engine oil
      • this is evidenced my the oil appearing milky and is not a good sign
  • definitive repair is VERY EXPENSIVE (thousands of dollars - parts alone will approach $AU1000) as it usually requires removing the whole engine and thus on an old car this may not be cost effective
    • labour is usually at least 10 hours and you should have a machinist magnaflux check it from hairline cracks in the head and re-surface the head to make it smooth to reduce any warps, and while they are they they should do the valves
  • an alternative is to try repair using head gasket sealants which are poured into the radiator and which then hopefully solidify in the head gasket crack and stop the leak (see below)
    • this may give your car another year or two before it recurs or has a catastrophic head failure / blown head gasket

Diagnosis of a combustion leak / head gasket leak

  1. there must be unexplained coolant leak
    • no obvious coolant leaking onto the ground
    • pressure testing fails to reveal coolant leak
  2. there should be exhaust gases escaping into coolant and coming out of radiator when cap removed
    • this can be a visible test with engine idling
    • can be confirmed with an Head Gasket Test Kit analyser which changes color if the bubbles are exhaust gases
  3. severe leaks may also cause:
    • obvious white smoke or steam from exhaust
    • engine misfiring
  4. a leak allowing mixing with oil will additionally show:
    1. foamy/milky oil (although this can occur at places other than the head gasket)

Possible causes of failed sealing of head gasket

  • cracked head gasket as a result of prior over-heating (eg. stuck thermostat)
  • warped engine from overheating (even one thousandths of an engine warp will create sufficient leakage at the gasket)
  • cracked engine head
  • cracked intake manifold gasket

Gasket sealant repair options

  • DO NOT use silicate-based sealants if water is getting into oil - it may destroy your engine by locking up the bearings!
  • these are generally based upon using sodium silicate products (+/- other additives such as microfibres)
  • the sodium silicate hardens in areas of high temperatures (eg. 1000deg) such as at the gaskets to hopefully form a seal which is generally temporary although additional additives aim to provide a more long lasting seal (at the added risk of clogging up the thermostat and radiator, and possibly the oil and oil filter)
  • thus generally the engine needs to be run at optimum temperature for these products to work and a second application may be required
  • follow the instructions carefully
  • choose product taking particular note of the recommended idle time required and whether your car can go that long
    • Blue Devil silicate only sealant requires 50 minutes
    • most others require 15 minutes
  • consider what they are made of and risk of clogging coolant system / heater core:
    • sodium silicate only eg. Blue Devil
    • sodium silicate with fibres
      • microfibres
      • carbon fibres - eg. Bar's Leaks - pours like a smoothy and even thicker than K-Seal
    • copper with ceramic fibres without silicate
  • you may need to flush the coolant system first esp. if it is dirty or you are using a silicate sealant
    • silicate sealants work best in clean water as the coolant and work best after a few hours of flushing the coolant system 1)
  • shake well before pouring
  • make sure before adding sealant, radiator fluid is above internal radiator core
  • generally pour into radiator, top up coolant if needed, then start engine and turn heating system and fans all the way up
    • usually run for 15 minutes until it gets to optimum operating temp so thermostat opens and the sealant can get to the gaskets and they are hot enough to alter the sealant - usually 105degC, then turn off to cool it down and leave for a few hours or overnight so the coolant can solidify in the gasket
      • some recommend repeating this 15minute idle then several hours cooling several times if using a silicate sealant and for bad cases consider removing the affected sparkplug to reduce pressures while it is sealing 2)
  • take great care on 1st run to ensure it doesn't over-heat due to clogged thermostat
    • check coolant level before driving

microfiber options

australia/car_headgasket.txt · Last modified: 2022/03/20 17:07 by gary1

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