Re: Coolant Boiling Upon Shutoff

b1377@xxxxxxxxxxxxxxxx wrote:
>
> I have a friend with a 2005 GTO whose coolant is boiling when the
> vehicle is shut off.
>
> The engine temperature varies (per the Driver Information Center - in
> diagnostic mode) between 190 and 194 degrees when the vehicle is
> moving. When idling, with one fan running, the temperature goes as
> high as 229, then as the second fan kicks in and brings it down to
> about 206, then it climbs to 229, repeating the cycle. The temperature
> gauge (in the tradition of modern temperature gauges) does not move
> from its position of 3/8ths of full-scale during this temperature
> cycling. So far, so good.
>
> The problem is when the vehicle is shut off. After about 5 minutes,
> the coolant can be heard to boil - if you (carefully) grab a radiator
> hose, you can feel the bubbles. Sometimes coolant comes out of the
> overflow bottle. The overflow from the bottle happened when the
> ambient temperature was in the high sixties. When it does this, you
> wind up having to put in about a a quart or so of coolant once it
> cools.
>
> He took the car back to the dealer, who promptly took it in to look at
> it, but that day was cooler (about 55 degrees ambient), and it did not
> boil upon shutoff. The dealer pressure tested the system and found no
> leaks.
>
> I'm thinking radiator pressure cap, as the temperatures when running
> seem fine. The bottom line is that he is concerned about the long term
> effect of this behavior, as if the water is habitually boiling around
> the combustion chamber (E.G. area around the exhaust valves) upon
> shutoff, it's probably not a good thing for the metal around this
> locally overheated area.

It could be due to a lot of things, but for $15 for a new cap, that
is where I would start.
.

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