mx5cartalk.com

Hello chaps. For some time my SE with > 130K on the clock has started to loose coolant very slowly. I have to top up the overflow tank every couple of weeks especially in the height of summer. There are no telltale signs of leak anywhere including the garage floor. The car has never suffered an overheating issue as far as I know. Would you suggest the radiator cap is the culprit here?

this started happening to my NB. Couldn't find a leak anywhere

Luckily it was due for its timing belt change, found the water pump to be leaking very slowly, with the coolant drying up before it got a chance to drip out the cam covers

I did the timing belt in Dec '12 with OEM parts including the waterpump. I have done a similar job in my previous NA and knew what to do to make sure the gasket didn't get pinged and to use the proper sealant. There wasn't any leak for nearly a year after the belt job but you never know what could have happened now. I hate to tear into the front end of the engine again given the extra plumbing I have to deal with in the SE.

Any coolant stains near the coolant tank? It might be the radiator cap getting too old. The spring is not working properly, then causing the pressure not correct and when the water/coolant get boiled, it run back to the cold area which is the coolant tank. There a a small hole on the coolant box and coolant will split out from there.

The coolant tank is clean as a whistle but I will start from the simplest and cheapest and just change the radiator cap which is about time for a new one at this mileage anyway. The thermostat is in working order with no undue delay for the water temp to reach to the normal range which is just a smidgen below the middle of the temp gauge.

Check at the rear of the engine head as there is another gasket and housing. I have had a slow leak from there and the coolant settles down on the top of the gear box bell housing.
Also check your heater hoses

Also take a look around your heater core.

Or maybe the drain plug under the radiator just didn't close properly

Caps are cheap. Just get one and see.

Okibi wrote:Also take a look around your heater core.

ie. for dampness around the footwell

Best bet: Take it to a radiator joint. Get them to hook up a pressure tester to the system. It will show where any leak is, instantly. It takes about 10 minutes to do, so will cost you bugger all.

Or... buy a pressure tester. They're not horribly expensive, and will be with you for the rest of your life. You use them to test the system, and the cap.

I peered down the firewall at the back of the engine and found a stain free gearbox. Checked the passenger side carpet - couldn't feel any dampness or smelled sweet coolant smell. I'm going to check the drain plug for tightness tomorrow. I'm always paranoid about over tightening or worse by cross threading that plug which will give me no ends of strife.

I'm not sure if it's my imagination but somehow I can smell coolant when the engine is hot. Scratch head.

I know for certain that the coolant has not disappeared into the engine as there is no steam out of the tail pipe and no funny looking foamy oil on the dipstick. There is no reason for the head to give way since the car has never overheated.

I took it to a radiator joint today where the guy told me I had to leave it there overnight for him to pressure check when the engine is cold. I also mentioned to him that the radiator top tank is a little brownish and maybe about time for a replacement. I was quoted around $300 fitted with my own supply of coolant. What do you guys think? Is there a better radiator alternative for a street use SE which has a bigger radiator than the others?

Try another radiator place. The engine only needs to cool down a bit so the radiator cap can be taken off for the test without making a big mess.

If you can smell coolant, then it is leaking externally - could still be the heater core. If the top tank of the radiator has gone brown, then it is probably about to fail and could have hairline cracks (at this stage) causing the leak.

It's pretty easy to remove the stock radiator. Get a new top tank fitted by a radiator place - should be under $200. The engine can easily be flushed with the radiator out and then all that's needed is to replace the radiator, fill the system with fresh coolant and bleed it.

There will be plenty of DIYs on the internet.

Not worth the cost of getting a top tank replaced (the bottom tank was made out of the same stuff at the same time), if you're at that point just upgrade your radiator.

if the cap is stuff. its not hard to see if it is. if you are unable to the the visable signs and are questioning it. then it's not probably it.

as above, pressure test will find it.