Is there supposed to be a gasket under the radiator cap?

I had been losing a little coolant - just enough to make the low coolant light go on - but only after a hot run and sitting in traffic. Couldn't find any leak and I was wondering if a lot of fluid was bypassing the radiator cap and into the reservoir and out onto the street while driving. I decided to replace the OEM cap with a Stant model that was supposed to be OEM equivalent.

The first Stant cap broke apart after about a month and I was still losing coolant.

I bought a second Stant cap and I've now noticed this one is leaking straight out from the cap, past the threads, down the outside of the reservoir, dripping on to the block, and vaporizing.

Wondering if there should be some kind of gasket under the cap? The mating surface on the top of the reservoir looks fine. Or some vaseline on the reservoir threads?

Thanks,

Marc

 

Hi Marc,

First off, I would purchase a new OEM cap (not a generic). If you're still losing coolant with the new cap, then have the system pressure tested for leaks.

Also, search the forum regarding bleeding the air from the cooling system. When I first got my car I kept getting the low coolant warning and it turned out to be air in the system.


Cheers,

Stu

 

There is a o ring in the cap. The OEM cap is not expensive so get one.

 

There is a washer and an O-Ring, I just purchased a new OEM Reservoir Cap from NAPA a month or so ago. I paid $15.74 including tax and received the part the next morning. My fans had begun to run more often and without driving very far, the core of the cap had frozen, and it was difficult to install and remove, and it was allowing air into the system since it was no longer sealing although it was completely 'bottomed out'. I bled the air out of the hoses by squeezing them from the bottom up with the cap off of the tank, adding coolant as the level dropped until I observed no air escaping the tank. I did this two more times before all the air was expelled from the system, checking after reaching operating temperature then cooling down and repeating the process.

 

Thanks all!

I'm having difficulty figuring out which is the OEM cap but I think the Stant caps I have are the OEM ones. @03XKR, the only radiator cap that NAPA sells has a big S in the middle which is the Stant - is that what you have?

Looking at these it's the O-rings that do all the sealing. So I cleaned the neck of the reservoir and lubed the O-rings with Magic Lube. We'll see if that holds it... if not then I might need to replace the reservoir :-(

 

im out here in calif, purchased a new cap from oreily's auto parts store, fits allot of cars, look identical to oem that needed replacement, no leaks and it was about $25.00, you just go in there and tell them you want a cap for a jag, they have allot of parts off the shelf!

 

That is the same one NAPA sold to me, I asked about the 'S' when I went to pick it up and was told that it was a factory authorized equipment replacement part. The new cap screwed on and off without effort, my original took Channel Locks to unscrew, with each turn, out of the blue one day, and took about twenty turns to seat, even though it was obvious it was not sealing at the base and had been leaking at that point due to the residue build up at the base of the Reservoir filler neck. The air entering the system was not helping and my fans were not operating correctly. Check the hose clamp connections on the tank and replace with standard washer clamps if they appear to be leaking, the system must maintain pressure to operate correctly, high flow-low volume I think, also check the seam on the tank with a small mirror as well, as they have been known to split open.

There definitely should be a round, flat, rubber gasket at the top of the 'core', under the cap top, along with a rubber O-Ring lower down on the 'core'. The O-Ring alone will not be able to seal the tank properly; you can get a rubber gasket at the auto parts store, or if you still have the broken one, take the rubber gasket off of that cap. It will never seal properly without the gasket and the O-Ring, and will continue to allow air into the system, which will cause problems on it's own, like burning up the water pump with a pocket of air cavitating at the impeller.

 

I too had a cap that needed channel locks to unscrew. But at the time I hadn't noticed how the "worked". Once I realized that the cap sealed with an O-Ring, it followed that the cap shouldn't need to screwed on all that tightly. I also found that all the lubes I tried to make the cap unscrew more easily actually were making it HARDER to unscrew. They were sorta gluing the cap in place.
So now I keep my threads clean and dry (even wipe off any coolant that gets on them) and I only screw the cap down until it bottoms, and then just a touch more for security (but not with the intent of sealing). I may see the remnants of a drop or two on my reservoir from time to time, but I suspect the tank itself may weep just a bit. And the cap comes off easy as pie when I need to add the 2 friggin' ounces it takes to extinguish the Low Coolant warning.

 

I guess I got lucky
Oreily cap looks identical and works 100%

I changed coolant after that no issues
The system when properly worked by purged air on minutes
I Joe's every squeezed the hose with cap off till no more air while running car
400 miles later ok
Heater was on full at all Times to fully purge