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My car has been overheating at idle after a long period of time - perhaps 20-30 minutes in traffic. If I drive, it immediately starts to cool down again. I don't drive often so it hasn't been an issue but I would like to get it working properly.

I took it into a mechanic to get checked out because diagnosing the problem is beyond my technical ability (I do some light car work and generally know how things work) and they said that they were absolutely sure it was the head gasket, and quoted me a bill of $2700 (which seemed high, but that's not really relevant to the question).

I'm mostly wondering - are the symptoms consistent with a head gasket? I know one of my fans has been knocking when I turn it off and occasionally does not blow hot air so I thought maybe it might be related to that, but the mechanic was very adamant it was a head gasket issue.

To my (limited) understanding, a head gasket issue should become more pronounced when the car speeds up, not when it is idling.

Am I totally wrong on this? What would make the mechanic so sure it was the head gasket (he didn't provide any reasoning behind his thought process).

EDIT:

It was a blown radiator fuse (and to a lesser extent some leaky radiator hoses)

First mechanic was very clearly a liar.

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    Which car is this? Make/model/year? Are you losing coolant? – Zaid Jan 13 '17 at 3:08
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    I put together a rough guide for overheating problems here. Try to add as much information as possible by editing it into your question. Thanks! – Zaid Jan 13 '17 at 4:21
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    2005 Ford Mustang. As far as I know I am not losing coolant (at least not at any appreciable rate). I would have to check more consistently. – Andrew Alexander Jan 13 '17 at 7:44
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    I'm still.womdering why this isn't related to the fan? If the fans aren't working right, this is exactly what I'd expect ... fine while driving (plenty of air flow) and overheating when the car is stopped (no air flow). In no way does the symptoms scream head gasket to me. – Pᴀᴜʟsᴛᴇʀ2 Jan 13 '17 at 11:12
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    ..and occasionally does not blow hot air seems like a useful detail – elrobis Jan 13 '17 at 15:34
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It would be good to ask the mechanic to explain to you how they are sure that it is the head gasket – head gasket problems often share symptoms with other problems.

One way that a head gasket failure could cause overheating would be if the gasket failed between a combustion chamber and the cooling system. Combustion gasses could leak into the water jacket and displace coolant leading to the overheating. This is fairly easy to diagnose definitively by testing the coolant for the presence of carbon monoxide.

A simple thing you could do would be to top up the coolant and monitor it for loss. Coolant going out via the tail pipe (leaking into a cylinder) may be noticeable as steam (white "smoke") or as a sweet smell in the exhaust. Leaking to the outside will leave residue, or you could add some fluorescent dye to the coolant to make it more visible.

That said, I'm wondering about the mechanic's diagnosis, overheating at idle could also be caused by reduced coolant circulation (perhaps due to a water pump with a badly eroded impeller) or by a lack of airflow over the radiator at idle which could be caused by a fan failure or by something blocking the radiator. But I suppose it would also be possible that while idling a normal reduction in coolant flow could allow a small head gasket leak to display coolant that would be replaced, or mitigated, when the engine speed caused more vigorous circulation to resume. A check for combustion gasses in the coolant would confirm, or not, this theory.

  • Turned out to be a blown radiator fuse - so definitely an airflow issue. Got a second opinion today. – Andrew Alexander Jan 20 '17 at 22:01
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There are so many things that could explain slow overheating

A compromised head gasket is just one of them. Other (less involved) reasons include:

  • a radiator fan that doesn't turn on when it should
  • low coolant flow (e.g. failing water pump)
  • debris inside or in front of the radiator

Without knowing the specifics of your vehicle, you should see loss of coolant if the head gasket is bad, and even then loss of coolant could be due to other reasons. That information is key to assist you with further diagnosis.

Also, arm yourself with data before sinking money into a fix like this. I think a compression/leak-down test on all cylinders would be useful to help you identify if a bad head gasket is to blame here.

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    Water pump missing a few fins also. – Robert S. Barnes Jan 13 '17 at 11:58
  • The other guy answered a minute before you so I have to accept his answer, but I'll give you an upvote as well. Turned out to be a blown radiator fuse and some leaky hoses. – Andrew Alexander Jan 20 '17 at 22:00
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Mine did this. After checking everything basic and replacing things/bleeding air out .. it ended up being the water pump. It wasnt out but was old and not pushing enough water through at idle causing pressure to build up at idle.

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