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hissing from coolant expansion tank ford fiesta?

the other day when i was driving steam appeared from under the bonnet and it was coming from my coolant expansion tank, i slowly opened the cap and released the pressure when i got home i topped it up with coolant and water but after i drove it i could hear a hissing noise, ive bought a new cap but it still continues to make the noise the system is not leaking coolant and the temperature gauge stayed on normal. i was wondering if maybe its a water pump problem not circulating the fluid quick enough which is allowing it to boil.


i dont think it is a blown head gasket the car never overheats, and there is no sign of water getting mixed with the oil

4 Answers

  • Robert
    Lv 4
    9 years ago
    Favorite Answer

    Firstly, lets get our terminology straight.

    The expansion tank, top up tank, the one with a high mark and a low, should have no pressure. It merely supplies the radiator top header with coolant or takes coolant from it.

    The cap you bought would be a rad cap, this is responsible for the pressure in the radiator.Below this is the tube which is connected to the expansion tank

    The hissing is the air escaping from the cooling system.(air lock caused by boiling)

    I would drain the system, flush it, with a flusher, refill system, fit new correct pressure cap. It, for sure, is not water pump noise. Bye

    • Bert5 years agoReport

      Clarification: This system IS A PRESSURIZED COOLING SYSTEM WITH A PRESSURIZED OVERFLOW JUG. I'm not sure what you part failure is, but be careful removing the overflow jug cap, because it is under pressure.

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  • cutter
    Lv 4
    4 years ago

    Ford Expansion Tank

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  • 9 years ago

    if its boiling why is temp guage not showing, ist open expansion tank feel into it against wall and check for a slightly oily substance then check your dip sticck if oils anything other thanbrown (slightly cold tea or milky color} your head gaskets away, if oils okay color but oil stuck to inside wall of exp tank head gaskets prob on way out,

    Open hood start engine and wath as thats quikest way to heat up egine, when it gets hot it has a temp sensor which switches on fan or fans to cool water down,you might not notice this has prob been happening so as it gets hot and according to guage in car reached just shy off normal go bak and watch if fan comes on then chek guage in car to see where its position is to trip on fan,it should go back off within minutes. if it doesnt come on atall. and maybe need new sensor,any friend that works with cars can bypass(not recommended for any lengh o time just till replacement from salvage yard) sensor so fan runs all the time when engines running,highly unlickly to be water pump, they either leak from bottom (theres a little hole on stem of pump if inner bearing fails water will cim out there, check pulley on pump to see if it moves,) or bearing goes and youll hear that,head gaskets worst case sinario butpossible, Maybe air in system,get someone to check. (you fill up exp tank and squeeze fat hose that goes onto radiator and youl see water and hopefully air cumin bak up way and as you release hopefully water level goes down and you have to repeat process few times till nothing but watter glugs up in tank and back to full line,dont overfill go with min/max lines after youve tried de airing it, and if it does push air out few times and then stops and just water comes up and down.take a run with crossed fingers. PPS 1 other thing is car heater workig? if not could be simpal thermostat problemm, tom

    Source(s): self
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  • Anonymous
    9 years ago

    Blown head gasket and/or cracked head. It's a FORD! Found On Road Dead!

    Source(s): Pro mechanic.
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