Leaking Small Amount From Radiator Cap - What To Do?

Hi Folks,
My '07 Highlander was in for an oil change a few weeks ago and my trusted mechanic said he noticed there was minor weeping from the radiator cap, so he changed it. The one on it was the original as far as I know, or was changed 7 years ago when I had the timing belt done. I just had the Highlander in for a timing belt change service this week and I noticed another cap on the invoice again. He said, it was weeping so he changed it a second time. It still seems to be weeping. There is no lowering of the coolant noticeable. What do you suspect is the issue? Vehicle runs great. Temp in the normal range. No scent of cooling either. My worry is that the small staining is a warning of a bigger problem about to happen. Any suggestions much appreciated.

Do you have the old cap? And if so, how does the large gasket at the top of the cap look? Is it worn in any way?

If you remove the cap, does the top of the radiator opening, where the top of the cap seals to, look smooth with no pitting or cracks?

Are there any hairline cracks in the radiator at or near the opening where the cap seals?

Is it possible the leakage is coming from where the radiator hose clamps onto the radiator near the cap? It doesn’t look like it from the picture that you posted.


There are attachments on coolant pressure testers to check cap integrity. They also can be used to pressure test the cooling system. To put yourself at ease you might go have your system checked by a qualified tech.


O’Reilly’s and others may let you use a pressure tester overnight for free. Actually, you buy it and get a refund when you bring it back. Like a rental, for free.

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Rinse off the stains and see if they return. Examine the surface around the top of the radiator opening for nicks or damage.

A pressure tester can be used to test the pressure seal but not your leaking secondary seal.

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At 14 years it might be a crack in the radiator.


Looks like a replacement radiator to me, the foam seal on the front edge of the radiator is very clean.

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If it is coming from the cap, it should be removed, cleaned and the seal should be lubed with lithium grease and the cap should be reinstalled. There might be dirt involved. Clean both sides of the seal. Do this with cold engine. The hose looks like it may be interfering with the fit of the cap.

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Petroleum based grease should not be used on cooling system parts, use silicone lubricant or a specific rubber lubricant.

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True, but lithium isn’t petroleum. Anti freeze is also a lubricant and can be used instead.

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No but lithium grease IS a petroleum product. See the 1st page of this lithium grease MDS



Thank you all for your quick and helpful replies. This morning I cleaned the area better than it had been. I used a dry paper towel and then I wet the paper towel with a dab of coolant to carefully wipe down the areas. I inspected the radiator top and cap area closely with a magnifying glass and with my finger. No cracks I can detect. The cap surface and the cap inside area clean and smooth now. The precipitate from the coolant is pretty abrasive and granular when it is left behind. I made it as clean as possible. Now I am doing a few cycles of the engine from cold to hot and over again and looking for any weeping. So far none. Maybe the residue was leftover from previous work or cap replacements? Hopefully. I will let the group know.

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I hope it’s just the cap or something small.

But being a 2007, and the fact that multiple radiator caps don’t seem to fix the problem… I’d be prepared to have the radiator replaced sometime soon. It’s the next logical step.

For what it’s worth, though… it’ll probably be the only/last radiator you’ll have to put on the car.

Good luck.

Let me throw my two cents in. Before you go for all the bigger ticket items check the hose/s, definitely if they’re original. That top hose looks a bit swollen and that spring clamp should be replaced with a band type clamp that can be tightened. All the coolant stains seem to be on the hose side of the cap. That hose can be split up underneath where you can’t see it or the rubber is rotten just plain had it’s day. Have it taken off and have a look. It may very well still be the radiator. I sincerely doubt it’s the cap, besides it should go back to the reservoir. The radiator may be cracked at the fitting $$$$, or maybe it’s just the hose and a clamp $.

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The cap seat on the radiator might not be flat enough. If the problem returns, you could test this by painting the radiator cap seal with a thin layer of colorant and then press a flat, rigid panel onto the cap seal. If the color transfer isn’t complete, then the cap seal may not be flat. I’m thinking about ink and an aluminum panel, or something similar.

The lubricating part of grease is oil, often petroleum. Lithium refers to the type of thickening agent mixed with the oil, often a soap.