I have had 5 failed coolant expansion tank caps since I last replaced the coolant tank.

• The cap that came with the new OE tank.
• The cap that I had with the old tank
• 2 new Motorad caps from Autozone
• 1 new Gates cap

While the first four caps resulted in falling coolant levels and a rapidly filling coolant tank in the passenger front wing (which I have emptied twice), the Gates cap could not achieve a seal and allowed coolant to leak past it. I even tried changing the cap O-rings for the ones in the other caps and that also did not work.

It seems I will be buying another new cap, this time OE from SNG-Barratt even if it takes a while for it to get here. Has anyone had this happen before?

 

I am on my fourth coolant reservoir cap in slightly over nine years of car ownership. I sourced my current cap from walmart.com in June 2020, part no. MJA4440BA, total cost $10.74. Thus far it continues to function properly. I have tried a relatively expensive Jaguar OEM cap as well as a couple of less costly aftermarket caps. They all appear to be the same and have typically lasted me 2 to 2.5 years....

 

I wish they were lasting that, but these 5 caps have all been bought within the past 3 months and have all failed immediately.

 

Check for burrs or abnormalities in the two sealing areas around the top of the reservoir...

 

I'd say the same, at this point the reservoir itself is suspect. FWIW, my cap broke apart a few weeks back (the plastic top just sheared off the actual plug). I actually had kind of a hard time fixing this. I ended up pulling the tank off the hoses and had to work the plug off with hand tools, even breaking a pair of hose pliers in the process. This was supposed to be a 5 min job replacing a cap!

It is clear to me that the inside bore of that tank neck has to be smooth so the 2 o-rings on the cap properly seal. Then there is the spring-mounted mechanism with its own o-ring. I'd say double check all these surfaces. Going through so many caps in such a short time seems to point to a root cause somewhere else. Another possibility is that that neck is somehow no longer round (?). Maybe a caliper would give you more clues.

Best of luck,, keep us posted.

 

I'd say you're looking in the wrong place. Get a coolant pressure tester adapter & 'rent' a pressure tester from your local auto parts store (Autozone or O'Reilly's in my area) and test the system. It sounds to me like you're getting too much pressure somewhere in the system. Maybe a head gasket, maybe a clogged radiator, maybe a over enthusiastic water pump. But it sounds to me like the problem isn't the pressure cap but something's creating too much pressure (above 15 psi) and pushing out at the weakest point (the pressure relief cap).

It could also be a hairline crack in the top of the expansion tank (in the threads where the pressure cap screws on). This will be tough to locate but with a pressure tester you should be able to find it.

Happy hunting.

 

I checked the reservoir for burs or other abnormalities and it all seems fine.



I have actually pressure tested the system multiple times. I have found no leaks, pressure holds steady at 15 psi all day long. The expansion tank is only 2 months old and is OEM, water pump is also less than a year old and is an OEM part. My radiator as well is about a year old Niessens unit and is not leaking, and I doubt it is clogged. Out of all that, I would only suspect the over enthusiastic water pump... or perhaps there is air in the system?

Anyways, I went back to using one of my older caps. Until I wait for my new OEM cap to arrive. The coolant stopped leaking through the cap and went back to filling the recovery bottle in the passenger fender. I hope to God it’s not the head gasket. I have never overheated this engine and there are none of the other tell tale signs if a blown head gasket.

 

GD,

I feel your pain! Only had my cat for 2.5 years, and I'm already on my 3rd cap. First made the mistake of ordering a cheap one on-line, and that caused all sorts of problems. Now my current one is a OEM unit from British Parts, UK. Let's see how long that lasts.

 

Glad to hear that the system holds pressure. Seems to rule out a head gasket. One other thought is maybe the thermostat isn't opening. But I would think that would show up on the temp gauge. Hard to believe that so many pressure caps is the problem.

My OEM cap is in bars or kPa (I can't remember which). I bought an aftermarket cap when trying to sort out a coolant leak that was marked in lbs (psi). I assume that they were both the same pressure, just different measuring systems, but ... maybe not. If the replacement caps you're buying are in lbs maybe they're not the same rating. Just an idea.