Took my '72 Supreme out to run some errands today, and while sitting at a light, I noticed a slight wisp of smoke from under the right side of the hood. Checked my gauges and everything was fine. When I stopped to fill up, I popped the hood and was surprised to see the cap for the coolant reservoir had popped off. I popped it back on and went on about my merry way. Got to my next destination (about a mile from the gas station) and checked it again...it had popped off again, so I put it back on (again). Left where I was and drove home (prob about 5 miles) and checked it when I got home and it was still on.

As I said, no overheating, and even in the 90+ deg heat and stop and go traffic, it never got above 190.

Now, mind you, the cap does a crack (already ordered another one), so could that be why it's popping off? Maybe the heat expansion from the coolant going back into the reservoir is causing just enough of a pressure buildup to cause the cap to pop off due to it being cracked and not snapping down tightly?

I dunno...just a theory...

 

I think it’s the cracked cap. Container is vented so even if that was clogged the crack vents it. A little heat, a tiny bit of pressure and….POP!

 

It may be just the under hood heat itself causing the cap to distort and dislodge itself because of the crack.

 

So it seems the crack is the issue...how can I prevent the same thing happening with my new cap? I've only had this one for about a year.

 

Not accusing but be very careful snugging plastic caps. I have the hardish plastic cap on my version with the wire cage bracket and I have to stop myself any time I put the cap back on. “just…stop…as soon as…it…seats…DO…NOT…tighten… it….” is what I have to keep saying to myself.

In the meantime some electrical tape, plastic staples or epoxy may give it back some grip.

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