2006 XK8 with 121,850 miles....

As part of my weekly routine fluids and pressures checkup this morning, I attempted to unscrew the reservoir cap to check the coolant level. As I was unscrewing it, the top portion of the cap broke off in my hands. This left the lower portion that juts down into the reservoir stuck in position. There is not enough of the lower portion protruding up above the filler neck for me to be able to grab it with even the smallest needlenose pliers. I can spin this lower portion in place by expanding the needlenose pliers tips against the remaining inside circumference of the cap, but it refuses to lift up and out of the filler neck....

Has anyone else experienced this issue and if so, were you finally able to remove the lower portion of the cap from the filler neck without destroying the reservoir?

 

that happened with my original cap - ended up using a set of jeweler's screw drivers to gently pry the lower piece up enough that I could pull it out of the tank with a pair of pliers. Bought an aftermarket Stant cap that did the same thing after about 2 months

 

Happened to me a few weeks back. Ended up disconnecting the entire tank from all the hoses and "worked" the remains of the cap on the bench. Left tons of small pieces, a flying spring, broken hose pliers and a lot of aggravation. Lucky to even have saved the tank as it was a constant struggle not to scar the inner sealing surface of the tank neck. What a pain. In retrospect, an other option would have been to use some strong (construction?) adhesive and try to temporarily re-glue the cap top back in place, just for a one-time removal. Best of luck, hope your cap comes out easier. Mine was not even spinning...

 

I tried prying it up with several sizes of flat-bladed screwdrivers. No success, and I quickly saw that if I kept doing that I was going to damage the reservoir's filler neck cap threads. Wayne (cjd777) suggested drilling a couple of small holes into the bottom of the remains of the cap and inserting my needlenose pliers tips into them in an effort to work the bottom portion loose. Have not attempted that yet but I will....

Expecting to ruin the reservoir during my attempts to pull this cap bottom out of it, I went ahead and ordered a new reservoir with cap from rockauto.com this morning for about $60. I paid an extra dollar to ship it via FedEx Ground instead of via USPS. It has already shipped and is supposed to arrive here on Tuesday....

This vehicle is currently on its second reservoir and its fourth cap. The reasons why we all despise the plastics in our engine bays keep piling up....

 

Update:

I grabbed a couple of the tiniest flat-bladed screwdrivers I could find in my collection and went after that cap bottom again. Forcing the tip of the smallest one down into the miniscule gap between the cap bottom and the filler neck and working my way 360 degrees around it while pulling the screwdriver inward at all times finally deformed the cap bottom sides enough to where I could gradually work the cap bottom upwards. Once I had it approximately a half-inch above the filler neck, I grabbed it between my thumb and forefinger and pulled it straight up and out of there. Directly into the trash can it went as I called it every cussword in the book....

My removal efforts indeed left multiple small scratches on the inner rim edge of the filler neck so it may or may not continue to properly seal with a new cap. I kept an OEM cap from February 2015 which may still function properly so I cleaned it up and screwed it into place. I will do a test-drive this evening and then check it tonight and again tomorrow morning to see if any leaks are visible. The good news is that I should have a new reservoir and cap by end-of-day Tuesday if I need them....

 

What on earth is going on with these things?
I'm really glad now that I kept the original 1998 (but still functional) one I changed out last year in the box the new one came in.

 

I am thinking aftermarket ones are junk and a bad batch of OEM ones.

 

Keep in mind that the OEM caps will also fail, perhaps just not as quickly as the aftermarket ones. There are rubber gaskets inside all of these caps, OEM as well as aftermarket, that eventually break down and quit sealing properly. My guess is when this rubber gasket fails, so does the cap....

 

Update:

The September 2017 aftermarket coolant reservoir and February 2015 OEM cap are performing as designed after right at 100 miles in 3 separate drive cycles since last Sunday afternoon. No coolant leaks visible anywhere and the cold coolant level in the reservoir is staying where it belongs. My new aftermarket reservoir with cap arrived via FedEx yesterday so when I eventually need a replacement, it will be safely boxed up on my garage shelves....

 

Add me to the list of separating caps!!
Just a thought but what would happen if after I unscrew the top part of the cap and then gently drive the vehicle would the pressure start to force the inner part of the cap out of the reservoir enough to get a grip on it?

 

I doubt it. Once the cap head separates and breaks off, the lower section of the cap is not sealed. Your coolant will heat up and gush out if you attempt to run the engine. You need to pry that section out as the rest of us have done....

 

I put the cap top back on and so far everything seems normal. No sign of a leak. Does that mean the cap is sealed and venting normally?

 

If your cap indeed separated into two pieces, I would replace it immediately. You do not want it to fly apart as you are cruising down the highway at 75 mph....

 

I have a theory ...

"In theory, theory and practice are the same. But in practice, they're different." Yogi Berra. Philosopher & statesman.

I just replaced the expansion tank on my car and it came with a new pressure cap. When I installed the new tank I was tempted to use the original pressure cap but after reading this thread and a few others regarding pressure cap separations I decided to install the new one instead. Didn't want the old one to break off in the new tank.

Anyway ... When I threaded the new cap into place it felt strange. It felt loose and fragile. So I removed it and put the original cap back in. Since then I've gone back and forth between the two caps and I think I learned something regarding these caps.

Here's my theory ... The new cap that came with the new expansion tank is a two piece design. The center core spins independently from the threaded part of the cap. The old cap is a single piece design. The central core does NOT spin independent of the threaded part of the cap. They are a single molded piece. I believe the pressure caps that are having the problem are the two piece designs. It is very possible that when the O-rings on the central core become fused to the tank that when you unscrew the top the force required to break the seal between the O-ring and the tank is greater than the force required to separate the core from the threaded cap section. I think that's what's happening to the caps that are failing.

Since my new expansion tank came with a 1 year warranty I've decided to use the new two piece cap and see if Yogi's theory holds up. But I'll only perform my experiment for a year, after that I'm going back to the OEM single piece design. I don't believe it will separate when removing it.

I also purchased a replacement cap when diagnosing my original coolant leak in the car. I believe it was a single piece design because I don't remember it being loose or fragile when I installed it. It's worth inspecting any replacement caps to determine if they are one or two piece before using them. I think the solution to the pressure cap problems is to ONLY use the single piece designs.

Hope this helps.

 

Like you, I have an aftermarket coolant reservoir (installed in September 2017). I went back to a 2015 OEM cap about a month ago when the aftermarket cap came apart. So far so good....

 

In my case the one that separated was the single piece design.

 

They are ALL subject to premature failure. I now consider these caps to be a regular replacement item. If you get two to three years out of them, that seems to be typical these days....

 

Recently had to replace the expansion tank, as it was leaking coolant into the cavity where the level sensor sits, contaminating the sensor into giving a permanent "Low Coolant" reading on the dash display. I bought the Eurospare tank, as it was half the price of the OEM ($60 versus $130) and have had good luck with this brand of parts in the past. The tank was substantial, but the O rings on the cap did not seal tightly enough, resulting in coolant loss. Fortunately, I had a relatively new OEM cap on the old tank, which solved the problem with the new tank. Always a living and learning process. I have only used two-piece caps in the past. The only long-lasting ones were the OEMs. Though they run twice the price of the many off-brands now available ($20 versus $10, on line), they are well worth the difference in less aggravation. The Uro Part variety was the least durable, in my opinion. There is a problem with the design of them all in that the independent threaded cap has to force the pressure tower piece into the filler neck (if you apply a small screw driver in the groove above the lower, larger O ring of an old cap, you can snap out the pressure tower and see how it is supposed to work). Well-fitting O rings squeeze out the coolant that would normally lubricate the process, resulting in the binding, wearing and non-sealing of the rings. My attempt to remedy the binding is to put a light coat of silicone grease on the O rings whenever the tower does not seat easily and try to not check the tank every other day! Not easy to do when you are trying to preserve a near 20-year-old car and losing coolant. Wonder if anyone has figured out replacement O rings (?)

 

That silicone grease on the cap's sealing O-ring is a good idea. I also try to minimize unscrewing the cap. For awhile I was checking it every day. Now, I check it about twice per week. Once I am comfortable that all is well (at least for the time being), I will go back to checking it once per week....

 

Well, so much for Yogi's theory. I'm also disappointed to learn that the single piece designs also fail such that half of the cap breaks off and gets stuck in the opening of the tank. Guess I'll have to "invest" in a new pressure cap every 30 months or so. Bummer.