Checking a coolant leak

Here’s the situation. I an loosing coolant in the reservoir as in it is completely empty in 2-3 days but I am not loosing coolant out of the radiator. I’ll run it for 3 weeks, check and it’s still full. The obvious answer is a leaky reservoir bottle or where it attached to the system. What’s the best way to check/fix it?

I would think a leak would be pretty obvious. Are you getting coolant on the ground? Can you smell any when you’re driving? Does it gradually disappear over the three days, or does it just seem to disappear in a single event?

Sometimes a defective radiator cap can cause the coolant to just puke out of the cap instead of going into the overflow reservoir like it’s supposed to but that should also be pretty obvious. Still, a radiator cap is cheap, super easy to change and has been known to cure all sorts of weird coolant system problems so may be worth a try.

Are you sure you’re not losing coolant in the radiator? It may be hard to ascertain the fluid level in there just by peering into the cap. If you let the thing go for, say, a week and then top the reservoir off does it take the same amount of coolant as it would if you only went three days?

No obvious leaks. I check the radiator by topping it off and I get a few ounces in before overflowing. At the rate I’m losing coolant out of the reservoir, the radiator would be bone dry in a week if the leak were there. Radiator cap has worked fine for years; can they wear out?

Yeah, definitely. The cap isn’t just a lid, but is also a valve that regulates the pressure in the cooling system and controls flow to and from the reservoir. Assuming you’ve got a car where the cap costs $10 (instead of one of the fancy ones where it’s integrated with the reservoir and expensive) I’d say it’s worth a try. Take a close look at the place where it screws in and the lines to the reservoir while you’re at it.

I almost hate to say it, but check the oil too. If you see something white and foamy on the dipstick instead of dark and slippery, then that’s where your coolant is going - because you have a failed head gasket and a blown engine.

Voice of bitter experience here.

I can’t think of a reason why a leaky head gasket would just eat the coolant in the reservoir and then stop, though. But, yeah, if it’s not leaking (which it sounds like it may not be) that’s pretty much the only other option.

I wouldn’t panic just yet, but a little detective work may be in order. The coolant is going somewhere, the trick is finding out where.
It could be a radiator cap that is no longer holding enough pressure, this would cause coolant to escape primarily as steam so you won’t see puddles. A new cap in relatively inexpensive and an easy fix.
You may have a pin-hole in the radiator (or any of the hoses). That would allow steam to escape but not leave a puddle. When your car is fully warmed up (on a nice cool morning) pop the hood and look for steam.
The head gasket may be leaking, check the oil for milky residue. Oil mixed with coolant will look like a milkshake. Depending upon the where the gasket is compromised, the coolant may be leaking into the combustion chamber. With a fully warm engine, check to see if there is white steam coming from the exhaust. That is typical sign of coolant being burned.

Good luck and report back with any findings.

Replaced the radiator cap, topped off the reservior and drove it 15 miles to work. Get there open it up and no lost fluid. Come out 3 hours later and there’s a puddle under the radiator. FUCK! How far does AAA tow? Open up the hood and still the same level of fluid as when I started (comparing cold to cold). Open up the cap and radiator is full to the brim.

Drive it to the store by my house. Same story but puddle much smaller.

Drove it home and havn’t had a chance to check cold (no pressure) levels or pubbles due to darkness. I’m willing to say that the problem is solved if I knew what those GD puddles were from.

Did you drive with the AC on?

A mechanic can do a pressure test and spot the leak pretty quickly.

I think I’m going to have to do that. Big puddle under the reservoir where the lower radiator hose is as well. My best guess is that with the old (16 psi and probably worn) cap there was not as much pressure in the system. With the new 18 psi cap the leak is worse.

Echoing what MannyL was getting at with his question: Are you certain that the puddle is coolant and not just water (i.e., condensation from the a/c system)?

The OP could taste it. Just a tiny drop of the fluid on the tip of the tongue as the stuff is poison. If it’s sweet tasting, good chance of being coolant. However I haven’t tasted any of the newer stuff like Dex-cool.

  1. Never had a condensation problem before the new cap
  2. My reservoir is now a couple inches lower
  1. Never had a condensation problem before the new cap
  2. My reservoir is now a couple inches lower
  3. 4 dead cats under the truck

Finally have a diagnosis. Bad thermostat gasket.

I would stick with the new cap and see if the reservoir rides a little higher than before. I see a lot of cars where the reservoir blows out the water while driving but the radiator stays full. I quit filling mine and it quit going down.

Except the new cap didn’t change anything after 3 months