I just topped up my coolant in the Rad and the reservoir two days ago and now today i was low to the point of not having heat and the motor was having trouble keeping cool. Addedalmost 3 litres to fill up again! This is my DD and it has been giving me more grief than my old sunbird. there is no visible leaks anywhere, and the motor runs fine. there is no smoke or smell coming from the exhaust and the heater core lines are the rubber lines (the proper lines, not Aluminum). The heater core is fine (no leaks).The water pump seems to be functioning properly. another not is there doesn't seem to be any in the oil apn either. i pulled the dipstick right after a 30 mile drive and there was no milkshake consistency or smell of antifreeze. the oil looks as though it was just changed. i just bought it two months ago and the last guy said he brought it to the shop for maintenence before he sold it to me. (oil, fluids, etc.) any ideas? it is a 99 LT model with 255k KMS on it. I just replacedthe transmission on it ($1600 later), i don't need any more on this thing. Please help me out ASAP!!!![:'(][:@]

 

Try renting a pressure tester, pump it up to about 14# NO MORE,and see if it holds, it should say right there for a long while. If not look around try to see if you could find the leak intake manifold,(espically front and back) water pump, look under the truck for any drips and trace them down. I had the same problem and turned out to be the intake gasket under the a/c compressor ever so slight. I'm tearing mine down today. You could be losing it internally too. But, I'm willing to bet that's your problem too. Try that pressure test first. Good Luck.

 

could be bad intake manifold or head gasket. but if there is no visible liquid after your drive, id be leaning over towards head gasket. it sounds like its slowly burning, long drives will burn it faster. sounds like you drive this truck a lot, but it just a thought.