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Coolant level drops in reservoir with no visible sign of leak

I have a '98 Honda Civic with 230K miles on it. Last month, I suddenly saw few drops of coolant on my garage floor. I opened the hood and saw coolant splashed over the engine. I opened the coolant reservoir and it was empty. Surprisingly, the car never overheated and neither did the temp needle go up. I took the car to the mechanic (Honda shop) and they said that the radiator is leaking. I replaced the radiator and after driving for couple of weeks, the same issue re-occurred. The coolant ran low in the reservoir and there was coolant splash over the engine. I took the car back to the shop and they said that the hose was not properly installed. They fixed it and said there is no sign of leak. I have been driving for a week now and check the coolant routinely. I have noticed that the coolant level has again dropped significantly. But, after the last fix, I do not see any visible sign of leak. There is no evidence of coolant anywhere under the hood. I am not sure if there is something else going on now. Any suggestions? Is there something else I should check? Please advise.

Coolant Could Be Leaking “Internally” Within The Engine And Being Turned To Steam Where It Will Exit The Exhaust Tailpipe.

A bad head gasket is a suspect, for example, but investigation will need to occur. I’d have the cooling system pressurized and tested for holding pressure. Should pressure drop and not hold and no external coolant leaks are found then the leak is internal.


There was probably air trapped in the cooling system after the repair. And it just took a while for all the air to purge out.

Keep an eye on the coolant level and if it doesn’t drop that’s all it was.


Tester - Thank you. I will continue to watch the coolant level to see if there is any additional drop.

Common sense answer - Thank you. I will check the oil to see if I notice any change in color or texture.

There’s a lot of other possibilities then a head gasket. Head gasket is well down the list of possible causes. The leak may small enough that it just evaporates when it hits a hot engine. Do what tester said…then if there’s still a problem take it back. Could be as simple as a loose radiator hose.

MikeInNH -Thank you. Will continue to monitor the coolant level for the next few days.

'96 Honda Civic? When was the last time you got your timing belt replaced? Did you replace the water pump too?

If this water pump is the original, it’s way overdue for a replacement. A water pump can leak through the shaft seal only when the engine is running, and to varying amounts (leak rates). If the level should continue dropping, keep the pump in mind as a candidate.

I have a 98 Civic Si coupe. 245,000Km. Just want to say I enjoy these discussions immensely. Great to see on-topic, respectful and knowledgeable posts.

I drive a 1998 Civic DX with 227,000 miles on the odometer, and I’ve gone through almost exactly the same thing, only I didn’t have coolant all over my engine. I am willing to bet you need a new radiator cap.

I noticed the smell of coolant coming from under the hood a couple years ago, so I got the cooling system pressure tested and had the radiator replaced when it was found to be the problem. Earlier this year, I noticed the coolant overflow container was overfilled when the engine was hot, and the container was underfilled when it was cold. Putting on a new radiator cap solved the problem.

If replacing the radiator cap doesn’t solve the problem, you might consider getting the cooling system pressure tested again (assuming your mechanic did that before he installed a new radiator), but I’d be willing to wager money all you need is a new radiator cap.

I would suspect a head gasket leak causing the trouble also. You can have the coolant checked to see if there are exhaust gases in it. That would prove there is a head gasket leak.

I echo what Whitey said . . . I just had this issue a month or so ago, do the pressure test first, look for leaks. A new cap is in order anyway (they don’t come with the new radiator!). My Civic is a '95 with about 230,000 miles. After I looked for weeks, I pressure tested the system and found a small split on the heater core hose (head to core) and now it’s fine. Couldn’t find the drips 'cause it dripped onto the exhaust and evaporated. Good luck! Rocketman

Follow the good advice given above. Use this sequence:

  1. replace radiator cap (when engine is COLD).

  2. If no resolution, pressure test your system

  3. If you don’t find a leak, check the coolant for presence of combustion gases (which would indicate possible head gasket or other engine issues).

If you want to investigate this yourself, it’s not difficult or technical.

Step 1 only requires a stop at an auto parts store, and replacing the cap after the engine has cooled down several hours later. …simple. Very easy to do it yourself, and the cap is pocket change cheap.

For step 2, find an auto parts store which has a tool lending program. Autozone is one which does. Borrow a pressure tester, and follow the simple instructions precisely. If the gauge drops, you have a leak…use a strong flashlight to search hoses, water pump area, etc for leaks.

Step 3: Autozone also lends out the tester for this…it’s a bit more complicated, but not much.

I wonder if you have some problem with the hose leading from the radiator to the overflow reservoir. You wrote that the shop already checked that…but remember they made one mistake, they may have made another, maybe pinched the hose and created a leak. Check it.

I’m going to second rocketman and Whitey on this. Check those heater hoses, especially the one under the distributor. If it has oil on it and is swelled up, it needs to be replaced, as well as the O-ring for the distributor. Both simple jobs. Then a new radiator cap.

I like the water pump shaft seal. These rarely show a leak unless you do an overnite cold pressure test.