I recently replaced my radiator with a new Koyo and a coolant overflow tank after I was losing more and more coolant than usual. Clearly, something was wrong since I would see leaks under the hood. Everything seems fine for short drives, but after slightly longer drives (possibly AC turned on), I'm noticing coolant splattering a good amount under the hood after and the overflow tank getting lower and lower. I hope the photos give a better understanding.

Is this a poor radiator cap? Bad hose(s)? Skinny tubing between radiator cap and overflow tank?


This is an '05 with 140k miles. Thanks!

 

​There appears to be some underspray below the top hose on DS opposite of the cap side. I don't see how you get spray on that side unless it's leaking from the hose. Spray patterns also appears to be consistent with this since the cap side of the upper radiator hose has little spray.

Do you still have the stock radiator cap? Have you tried replacing with the OEM one to see if that resolves the issue?

The OEM cap is rated to 1.1 bar. The Koyo cap is 1.3 bar I believe. Could be the hose is getting old and not providing as tight a seal as it used to and with the new, higher pressure, some coolant is getting by.

Could be a bad cap as well and allowing more than 1.3 bar thus the leak. You can rent/borrow a coolant system testing tool from Autozone/O'Reilly and test your system/cap.

 

Thanks. I'm afraid to use the stock radiator cap as I remember reading it wasn't compatible with the Koyo radiator. I'll take your advice and try renting the pressure test tool. I hope those shops have compatible ones with the S2000.

 

Yes the stock cap wont fit the Koyo rad.
Did you make sure the big upper coolant hose wasn't damaged when replacing the radiator?
Looks like that is the source of most of the leak, at least from these pictures.

 

Koyo caps have been know to fail. I would recommend a JDM cap from MUGEN/Spoon instead. I have a Spoon thermostat/fan switch/radiator cap(aftermarket) bundle I sell.

 

Any idea how hot this system is getting? It's obvious there's a leak somewhere. Cap gasket is always the first suspect.

Not sure why the 1.3bar radiator cap is being used on this aftermarket radiator. A coolant mixture of 50% water and 50% ethylene glycol has a boiling point of 223°F. A cooling system even with 15psi (1.03bar) cap will add 45 degrees for a final boiling point of 268 degrees. The OE cap is 1.1bar (16psi). Can't see that the higher (max) pressure radiation it will hurt though.
-- Chuck

 

I took zze86's advice and rented a cooling system pressure tester from O'Reillys. Before testing it, I thoroughly cleaned all the components to easily identify where a leak could be coming from. Since the Koyo Radiator / Cap specifies 1.3bar, I pumped it to about 17psi. The pressure holds at 17psi and only drops about 1psi every 2-3 min. I'm not sure if that's normal or not. The only fresh fluid I noticed was a tiny bit under the top hose on the radiator / cap side.

Nothing seems to be wrong with the hose after inspecting it from the surface, so I simply adjusted the clamp where the fluid showed by simply bringing it closer towards where it meets the radiator.



One thing to note is that the fluid always seems to get depleted from the overflow tank, but when I remove the radiator cap, coolant is filled close to the brim. I wonder if I can drive on it for a little and see if it's something occurring from the components from the radiator cap and to the overflow tank. Is it dangerous to keep driving on an empty overflow tank? At what point will it introduce air into the system if I don't keep filling with coolant?

I did notice a small tear at the edge of the tubing where the overflow connects to the radiator, but I don't know if it's in the open part of the line where fluid can escape or not (see pic below).

 

Okay after wiping everything clean and going for a short drive, I no longer see the splattering on the too of the engine bay. It seems be originating lower around the drain plug and bottom hose.

I remember the bottom hose being a pain to detach from the old radiator and requiring a lot of prying and twisting. I hope it's not damaged and just more or needing to be secured better with the clamp. Similarly, I hope the drain plug wasn't damaged somehow.

I'll dig around there in the morning to see what it could be. In the meantime, any advice would be appreciated.

 

Temp sensor is a known area for leaks. I used Teflon tape when I swapped rads just in case, as I hear that temp sensor tends to leak if not tightened properly.

 

The pre-06 cars use a fan switch in the lower section of the radiator. (DBW cars have a temperature sensor in the same place and the ECU controls the fans, not the switch.) Torque is 17lb-ft with a new O-ring behind the switch. If a new O-ring wasn't fitted it's a suspect. Honda Service Manual is very conservative regarding gaskets and O-rings and invariably specifies "replace" when they're removed. We can usually reuse these parts but best practice is to follow the manual.

-- Chuck