Coolant leak?

[nhbilly]

I have a 1998 Honda Civic Ex - 131,000miles that always seem to be low on coolant. I don't have a leak nor do I smell or see coolant in or around my car. Every time I check the coolant it is always a little low so I top it off. Is the coolant system suppose to be pressurized even after it has cooled down?

I'm out of ideas.

Thanks
nhb

[attgig]

take it to a shop. they'll put some chemical in it, so that it'll show where the leak is under UV light. that'll pinpoint the leak.

[Daedalus]

Keep looking for the leak . There are only 3 places coolant can go in a short amount of time; leak out of the system, leak into the combustion chamber, or leak into the oil. It can also evaporate from the overflow bottle (unless system is closed), but that takes a while. Because of the pressure some leaks only show up when the engine is fully warmed up. It can spit a few drops on the freeway and then stop before you get to where you're going. Check for bubbles in the radiator and see if the oil looks brown. If either of these, you may have a bad head gasket.
Does your overflow bottle have a pressure cap on it, or is it vented?
Have you had any work done recently where they would have had to drain the coolant? Sometimes shops don't bleed the system well enough, and it takes a little while for all the air to work itself out.

[nhbilly]

Keep looking for the leak . There are only 3 places coolant can go in a short amount of time; leak out of the system, leak into the combustion chamber, or leak into the oil. It can also evaporate from the overflow bottle (unless system is closed), but that takes a while. Because of the pressure some leaks only show up when the engine is fully warmed up. It can spit a few drops on the freeway and then stop before you get to where you're going. Check for bubbles in the radiator and see if the oil looks brown. If either of these, you may have a bad head gasket.
Does your overflow bottle have a pressure cap on it, or is it vented?
Have you had any work done recently where they would have had to drain the coolant? Sometimes shops don't bleed the system well enough, and it takes a little while for all the air to work itself out.

Oil is dark black, coolant is clean and green.
Overflow tank is vented.

I did change the radiator back in the summer of 2008 before I drove it to Tx straight 22 hours. No problems at all.
The coolant system is pressurized when hot but should it stay that way when cooled, but then again my system is vented.

[Daedalus]

It should not stay pressurized, but how much pressure are we talking about? If the ambient temperature is warmer after it "cools down" than it was before the drive then it'll have a little pressure buildup...same thing if you don't drive the car at all but the day warms up. The radiator cap holds system pressure up to about 15psi, then releases coolant if needed to stay at 15psi. When the coolant cools down and contracts enough, it'll create low pressure in the system, and the cap will allow coolant to be sucked back into the system from the overflow bottle. The cap can go bad, but I don't see how that would cause the residual pressure you're seeing. A bad cap will 1) not hold system pressure up to 15psi, 2) not allow coolant to get sucked back in, resulting in a vacuum and low coolant levels in the radiator [EDIT: YOUR SYSTEM IS LOW IN THE OVERFLOW BOTTLE, AND NOT THE RADIATOR, RIGHT???], or 3) not vent pressure when it's supposed to, resulting in high system pressures and possibly a rupture somewhere (thus a leak).

A shop can do a UV test as suggested, and they would probably do a pressure check, where they pump the system up to 15psi and see if it holds. If not, they start looking for a puddle somewhere. Make sure the carpet in the passenger footwell is dry; a bad heater core will often leak there. You can also do this yourself if you can borrow or rent the tool. It's easy to do on your own. Hardest part is getting under the car to look for the leak. Just make sure you don't overpressurize it. The pressure cap might be labeled with its pressure rating.

[nhbilly]

It should not stay pressurized, but how much pressure are we talking about? If the ambient temperature is warmer after it "cools down" than it was before the drive then it'll have a little pressure buildup...same thing if you don't drive the car at all but the day warms up. The radiator cap holds system pressure up to about 15psi, then releases coolant if needed to stay at 15psi. When the coolant cools down and contracts enough, it'll create low pressure in the system, and the cap will allow coolant to be sucked back into the system from the overflow bottle. The cap can go bad, but I don't see how that would cause the residual pressure you're seeing. A bad cap will 1) not hold system pressure up to 15psi, 2) not allow coolant to get sucked back in, resulting in a vacuum and low coolant levels in the radiator [EDIT: YOUR SYSTEM IS LOW IN THE OVERFLOW BOTTLE, AND NOT THE RADIATOR, RIGHT???], or 3) not vent pressure when it's supposed to, resulting in high system pressures and possibly a rupture somewhere (thus a leak).

A shop can do a UV test as suggested, and they would probably do a pressure check, where they pump the system up to 15psi and see if it holds. If not, they start looking for a puddle somewhere. Make sure the carpet in the passenger footwell is dry; a bad heater core will often leak there. You can also do this yourself if you can borrow or rent the tool. It's easy to do on your own. Hardest part is getting under the car to look for the leak. Just make sure you don't overpressurize it. The pressure cap might be labeled with its pressure rating.

Radiator was low until I topped it off last night.

[renovation]

you can have a shop run a pressure test on it .to see if they find a leak . or they can install a dye in the system to find it.

[Daedalus]

Radiator was low until I topped it off last night.

Then are you certain the pressure you're observing isn't actually a vacuum, with air rushing in when you remove the cap? It's OK if there's a little vacuum in the system as the caps aren't perfect. From now on monitor the overflow bottle level. It might vary a little bit bewteen readings so don't add coolant unless it actually hits the "min" line. If it turns out the cap is preventing coolant from being sucked back into the radiator replace the cap.

[nhbilly]

Then are you certain the pressure you're observing isn't actually a vacuum, with air rushing in when you remove the cap? It's OK if there's a little vacuum in the system as the caps aren't perfect. From now on monitor the overflow bottle level. It might vary a little bit bewteen readings so don't add coolant unless it actually hits the "min" line. If it turns out the cap is preventing coolant from being sucked back into the radiator replace the cap.

I replaced the cap. The rubber seal on the old cap seems to be worn out and crumbling.