Hi, I own a 98 S90 with just over 209k miles. I'm trying to figure out what could be causing my low coolant light to come on. Even after topping off with coolant, after a short time, the engine temp goes near all the way in the red. Never quite getting into the red. But, when at a standstill, the temp stays at normal. I've looked under the car while it's running and can't see any signs of a leak, even after sitting overnight, no leaking signs. Someone please help

Check the upper hose neck on the radiator (driver's side). They sometimes crack and 'bleed' coolant before they pop off catastrophically (this is a known weak point, so be advised). I've also lost coolant thru a faulty reservoir cap on our 960 (the coolant just seemed to disappear) and at the rear of the engine thru one of the heater hoses at the firewall where the rubber hose connects to the metal hose. Also check the heater control valve at the driver's side firewall.

Does the coolant in your overflow tank have any atf floating on top of it?

DEWFPO
--
1998 S90 083,228 and 1995 964 154,100

If your low coolant light is coming on (and you talk about "topping up") it sounds like coolant is disappearing somewhere, but no evidence of an external leak? A cooling system pressure test could narrow things down.

If you can obtain an infra-red temperature sensor (or find a helpful repair shop, and safely get the car to it) this will confirm whether your car's gauge is accurate and you do have an overheating situation.

An internal coolant leak and excessive engine temperature when under load, could indicate a leaking head gasket. Try to get a cylinder compression test, or a "leakdown" test. Most shops have the equipment to do this.
--
Bob: son's XC70, dtr's '94-940, my 81GL, 83-DL, 89-745(V8) and 98-S90. Also 77-MGB and some old motorcycles.