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I have a VW Citi Golf, and the Welch plugs on the engine block suddenly started leaking water. The engine sounds fine, but it obviously leaked a lot of water and although I didn't drive far, I'm not sure how long ago this happened.

The mechanic seems to think they just need to be replaced, but what would cause this to suddenly happen? I'm imagining the the block heating up so much it warped. Are there any less possibilities?

  • Wikipedia says they can corrode and fail from improper engine maintenance. However, I'm unable to come up with a really good explanation for why. I suppose not servicing the cooling system ever could lead to an issue, but you'd almost certainly see it elsewhere first. – Brian Knoblauch Sep 12 '11 at 18:09
  • This should be posted as an answer, not a comment. Put it under "answer this question" so it can earn votes and maybe even get accepted :) – pufferfish Sep 12 '11 at 18:56
  • I didn't think it was worthy, but done. :-) – Brian Knoblauch Sep 12 '11 at 19:46
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Wikipedia says they can corrode and fail from improper engine maintenance. However, I'm unable to come up with a really good explanation for why. I suppose not servicing the cooling system ever could lead to an issue, but you'd almost certainly see it elsewhere first.

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    Turns out it was not enough antifreeze, which has anti-corrosives in it too – pufferfish Sep 14 '11 at 6:38
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Welsh plugs are mainly made of carbon steel, and with out antifreeze in the coolant water, they corrode quite fast with the engine heating and cooling. My brand new 3 year old Citi Sport has a leaking head welsh plug because I did not top up with antifreeze.

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Welsh Plugs are in engine blocks to protect against freezing (expanding) coolant. If you only have water and not anti-freeze in coolant system this can happen, warmer than 0 Deg C climates excepted. If they suddenly leak after years of motoring you should probably be thankful and get them replaced. If they leak a week after being replaced I would take back to garage and get them installed properly free-of-charge (FOC).

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Replacement should solve the problem if it is done correctly and with the correct part. As mentioned they can corrode even over a short time but also can just leak due to poor fitment. They are usually a 'tolerance fit' so are just knocked in without any sealant needed.

The Welch plugs main function is to block the holes that are used in the casting process. The term 'freeze plug' is a misnomer, however if your 'coolant' freezes it may allow for the expansion of the iced up coolant, popping out the plug in the process. If the plug doesn't 'give' then your engine may be irreparably damaged. Best advice is to maintain your cooling system as per your manufacturers service intervals.

  • Why is "freeze plug" a misnomer? AIUI, they're designed to pop out of the block if the weather's cold enough that the coolant freezes and expands: better to lose the plug than crack the block. – DavidSupportsMonica Jul 7 '18 at 15:49
  • They're a misnomer, because they aid in casting the block, which is their first function. The design could have been selected because they help in alleviating expansion when the cooling system freezes (which would be an extreme case). – OptimizePrime Mar 9 '19 at 5:48

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