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99 STS
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Discussion Starter #1
I have 1999 Seville STS with 140,000 miles. Very well maintained. Zero codes. I have to add a gallon of coolant about every 6 to 8 months. No visual signs of leakage so I assume it's going onto the street. It doesn't ever run hot. I get message "check coolant level" about every 6 to 8 months. The heater doesn't get very hot which makes me think they're related. Any suggestions to fix my problem would be appreciated.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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Really slow leak ............ that surge tank should be about half full, cold. Coolant expands as it heats, so it needs that airspace to work against.

Rent a cooling system pressure test kit from a large parts store. COLD, pressurize the system to 16 psi. Let it sit for a half hour or so, check the pressure (did it drop ?) and start looking for weeps with a good drop light.

If you had heater core problems you would most likely have a coolant smell in the cabin and damp carpeting up near the console/hump front areas.
 

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99 STS
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93 Posts
Discussion Starter #3
I dont have a coolant smell in the cabin and damp carpeting up near the console/hump front areas. I'll rent a cooling system pressure test kit and go from there. I'll probably flush the system and replace the thermostat to see if that helps the heater. Thanks Submariner!
 

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2010 DTS
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87,303 Posts
It might be evaporating before it drips. Radiator side tanks are known to crack and seep coolant as well as the surge tank. Also the heater pipes that run along the rear head (and impossible to see).

P.S.
Dried Dex leaves a chalky white telltale residue.
 

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99 STS
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Discussion Starter #5
I found the leak using the pressure test kit! It was a 3 inch heater hose. Thanks Submariner and Ranger!
 
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