I have had a coolant leak for some time. I replaced the UIM and LIM gaskets a little while back, it appears the UIM may have a small leak, but I'm not entirely convinced.

Recently, I'll fill the overflow tank to the mark when the engine is cool. It will stay at that level for at least a few weeks, and then eventually I'll look again and the tank will be empty. However, the system always seems to be full when the radiator cap is removed. If the UIM were leaking, wouldn't this level drop after the tank emptied? Is this more likely a tank or cap leak?

If your not finding any puddles on the floor, and no coolant in the oil, and your not smelling a sweat smell in the exhaust, then your most likely not leaking coolant. I say leave the tank empty, but watch the radiator. If it'* not dropping, then you may have a leak with the tank.

For a little while there was a puddle that would sit on the LIM to the left of the thermostat which appeared to be coming down from the UIM, but I haven't seen that in quite some time. Is there some kind of stop leak product for coolant that may help this issue?

I've let the tank sit empty for at least a few weeks and the coolant level in the radiator was fine the last I checked.

Do not put stop leak in the radiator. Your asking for trouble.

Have you tried having someone start the car in the morning when it hasn't been run for a while as you look around the engine bay for any leaks? If it is leaking out of the coolant reservior you should be able to see the leak or the liquid running out, you should also make sure there are no holes in the hose running to the reservoir. The coolant has to be going somewhere or getting burned off.

I have not, as I said the reservoir seems to stay filled for quite some time. It has to be a slow leak wherever it'* going. I'm moving to Texas tomorrow though so my coolant is becoming evermore important Fortunately the car will be riding the thousand miles instead of driving it.

I'll inspect the hose going from the reservoir to the radiator. Maybe just replace it. I know a lot of hoses have been nearing their time lately.

Danthurs, you're opposed to even some of the lighter duty stop leak? I was looking into Bar'* as I hadn't heard much negative about it.

I do not like anything in my cooling system other then coolant. I've seen what happens inside a engine with that stuff. Gums up the water pump, gums up the passages, gums up the radiator core.

I used a stop-leak in my radiator, and while it didn't seem to gum up anything, my radiator developed a worse leak in a different spot less than two months later. Look at it this way: if you've got a gunshot wound, you can either buy a box of Band-Aids and patch the symptoms, or go to the hospital and cure the root cause.

In my case, the plastic tanks on the left and right sides of the radiator cracked. The typical tank crack will result in a slow, slooww, ssslllooooowww leak. At best, coolant will gently dribble down the side of the tank. On mine, it looked like the radiator cap was leaking, until I took a close look and saw the source about a half-inch below there.

My advice is, if you find a cracked tank, grab another radiator from the junkyard and swap them. It'* actually a pretty easy self-repair. All you need are hand tools and a two-gallon bucket.

There are also aftermarket all-aluminum radiators*which are immune to cracking tanks and are X% to Y% more efficient at transferring heat, but they're three to four times more expensive than a used OEM part.

You could be only be leaking coolant when the temperature gets high and the pressure builds. Look for water stains in the engine compartment.