Radiator Cap Leak

[twincamrider]

I have a Moroso radiator and high pressure cap fitted together with an expansion tank. The rad cap is pooring out water when hot. I have examined the cap and it apears to be OK with nothing obviously faulty. The rad neck is all in order too. Should I just try replacing the cap or has anyone got any other ideas? Many thanks.

[dingocooke]

sounds like the cap is wrong for the rad neck and the rubber seal is not sealing out; to check the rubber washer is actually touching the base of the neck; dry both items, then put a drop of wd40 or light oil on you finger tip and smear it on the bottom of the rad filler neck, then screw down the cap and remove; youll soon see if its touching or not.

[twincamrider]

Originally Posted by dingocooke

sounds like the cap is wrong for the rad neck and the rubber seal is not sealing out; to check the rubber washer is actually touching the base of the neck; dry both items, then put a drop of wd40 or light oil on you finger tip and smear it on the bottom of the rad filler neck, then screw down the cap and remove; youll soon see if its touching or not.

Thanks. This is the kind of cap that seals to the top of the neck not down at the base. The rad cap came with the rad, both are new so I think they are a match and they apear to be OK.

[dingocooke]

so the neck has no overflow tube in the side of the neck?

[Clive]

Hi Mark,
Is the expansion tank full when the engine is cold? If so it could be excess coolant escaping as it expands when hot.
The expansion tank is what it says, to take up the excess coolant as it expands. It is not a header tank that requires to be filled with coolant when cold.
Once your cap has poured out water when hot, allow the engine to cool and check the levels. There should only be a small quantity of coolant in it. If so run the engine up to temp again and keep an eye on the temp gauge and re-check.

[twincamrider]

Originally Posted by dingocooke

so the neck has no overflow tube in the side of the neck?

The overflow tube is connnected to the expansion tank.

[twincamrider]

Originally Posted by Clive

Hi Mark,
Is the expansion tank full when the engine is cold? If so it could be excess coolant escaping as it expands when hot.
The expansion tank is what it says, to take up the excess coolant as it expands. It is not a header tank that requires to be filled with coolant when cold.
Once your cap has poured out water when hot, allow the engine to cool and check the levels. There should only be a small quantity of coolant in it. If so run the engine up to temp again and keep an eye on the temp gauge and re-check.

The level in the expansion tank is fine (about 1/3rd full) and the engine temp gauge indicates the proper temp. The only issue is the cap leaking when hot?

[dingocooke]

Originally Posted by twincamrider

The overflow tube is connnected to the expansion tank.

OK so the water is 'pouring' out of the rad cap, where from just around the cap? Im assuming you have a blanking cap on the rad (as you say it seals on the top of the rad neck) and a pressure cap on the expansion tank; is the pressure cap blowing out water from the overflow pipe on the expansion cap or is it just coming out the rad blank cap?

[twincamrider]

Originally Posted by dingocooke

OK so the water is 'pouring' out of the rad cap, where from just around the cap? Im assuming you have a blanking cap on the rad (as you say it seals on the top of the rad neck) and a pressure cap on the expansion tank; is the pressure cap blowing out water from the overflow pipe on the expansion cap or is it just coming out the rad blank cap?

Pressure cap on the expansion tank is fine and not blowing out any water or from the overflow pipe. The problem is only the blanking rad cap pouring out water when hot?

[dingocooke]

Originally Posted by twincamrider

The level in the expansion tank is fine (about 1/3rd full) and the engine temp gauge indicates the proper temp. The only issue is the cap leaking when hot?

Sounds to me like the blanking cap on the rad neck is either u/s or not sealing properly as normally a leak from pressure would first be at the expansion tank neck.

I would investigate the fit/seal of the blanking cap and try another one.