99 civic EX - AC on and the heat gauge goes up

Hi there,
Wondering if anyone can confirm my diagnosis of my sister's car, 99 civic EX 175000kms automatic.
Driving with AC on, the heat gauge will suddenly start to go up. Usually
sits around 1/2, it goes up to about 3/4. The AC stops working. They turn off the AC, heat comes down.
Just not entirely sure why the AC has anything to do with engine temp since the condenser and rad are separate? Belts are good, but here is a weird one:
She checked the radiator level once it cooled off, and the fins were showing, it looked as though the rad was low. There was still coolant, and it wasn't overheated according to the gauge, so she proceeded to the local autoparts store to get some premix antifreeze. Once she got there, bought the antifreeze and came outside to fill up the rad, it was full!
My diagnosis is as follows:
1. Bad thermostat - probably needs to be replaced anyways, the car has 175000kms on it.
2. Bad fan switch on rad - if the fan doesn't kick in, and the AC is creating more heat when its on, it could contribute to overall engine temp increase.
3. Plugged rad fins - we live in Manitoba, lotsa bugs! so maybe the rad fins need to be carefully cleaned out to allow for better heat transfer efficiency.
Any other ideas? The timing belt was done recently, but the water pump was left alone. I did suggest they replace it, but they didn't I guess it could be the water pump too.
Any input would be great!
t
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"loewent via CarKB.com" wrote:

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The problem is in the glove compartment. There's an unread owner's manual in there. Once she fills the rad to the top, and the reservoir to the MAX mark, she will have to do it again the next two times the engine is cool.
There's so much air in the system that the fan switch is sitting high and dry when the thermostat opens. Without any coolant in the reservoir, the system can't displace the air it expels when the engine is HOT. Hopefully she hasn't cracked the HEAD.
'Curly'
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Thanks Curly,
Wouldn't the real problem be 'where is the antifreeze going'? If this is indeed due to low coolant or air bubbles, what would cause that?
t
'Curly Q. Links' wrote:

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could these symptoms be indicative of a failed head gasket? It was replaced a few months ago when the timing belt snapped and we had to replace some valves.
loewent wrote:

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"loewent via CarKB.com" wrote:

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It could be indicative that nobody told her to check / fill the reservoir to MAX whenever there's been any work done on the cooling system. It's in the owner's manual where they describe changing the coolant..
She's been boiling off a bit more coolant each time she drove it since the work was done, with nothing to replace it.
Head warpage / cracking is quite possible since a red hot head gets quite a jolt when any coolant splashes around inside it.
Your water pump is supposed to 'weep' a bit too. That's (another reason) why it's important for owners to check the coolant level in the reservoir from time to time.
'Curly'
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