ok I figured I may want to buy a replacement for if and when I retire the Bonnie. Looking at a 2000 with 54k on it owned by an old guy who died his son is selling it. Looks good undercarriage and runs good and everything. Here is the but ... the rad was half full and the heater was blowing cold air. We topped it off with almost a gallon of Dexcool and it blew good and hot. ? is where did the coolant go ? The oil needs changing BUT its not overfull or foamy looking. No signs of leaks anywhere on the motor. Whacha think ? I think I can buy the car for 5500.00 but im a little concerned ??

I would be too. Check the oil and see if it looks milky.

Better yet..tell us which 2000 it is.

Supercharged or NA?

not foamy, non supercharged. Cant see any leaks ?

Could very well be a leaking UIM that has failed. It takes a bit of coolant to fill the lower manifold before it gets into the cylinders.

After it was filled...did you let it run a good long time and hear the fans kick on and off a couple times?

nobut I will when I go back. it seems as if it leaked more than a half a gallon it would be in the base by now,no ? Would it still be worth 5500.00 or could it have damaged the motor ? Should I do a pressure test if so how much pressure ?

They can leak and burn an amazing amount before it shows up.

Pressure test if you can to 16lbs... let that sit for 20 or so minutes..
Let it run and hear the fans cycle a few times.. (you know..start it..and pop trunk...look under..waste as much time as you can.)

Running like that frequently give the car time to warm up and all the hidden things start to show up.

Also, check the radiator pressure cap. Mine was leaking, as was a friend'* mother'* '99 Olds 88. Check around the coolant elbows also.

If it was driven a lot of short distances, even at those miles, both the upper and lowers could be leaking. The Olds 88 had the upper done around 40k. The shop didn't do the lowers at the same time. About 9 k more, and the lower started leaking. It left a small pool between the #1 and #3 injectors. As the owner was an older gentleman, he could have had a similar driving style as what the 90 year old owner of the Olds has.

if i do a pressure test would it show if a problem ? Is it still worth 5500.00 if UIM shot ? Could it be a headgasket ? Again seems if that much coolant was gone it would show i the oil ? If it gets good and warm would it show and if so where ? i dont see any signs on the garage floor ??

This is what Kelley Blue Book shows for a base SE with no other option packages.

Excellent $6,540
Good $6,035
Fair $5,430

The "Fair" includes mechanical issues.

A pressure test should tell you if there are leaks somewhere. As Boosty pointed out, these cars can loose a lot of coolant and have it burned up before it shows in the oil. Over what period of time did it loose that coolant? Also, the coolant can drip onto warmer areas of the engine, and evaporate without ever hitting the garage floor.

As an added test, while the car is running, smell the exhaust and see if it has a sweet smell to it. If in doubt, take it to a mechanic and have them do a good check over of the vehicle.