Radiator Overflow Cap Not Threading Easily


Hi I have a quick question … this has never happened before but the last time I checked my coolant level everything was great, the overflow cap was stuck on pretty hard but I was able to get it off, no coolant loss since the previous check, but when I screwed the cap back on it was a little more resistant than normal.

Later that day there was a small puddle of coolant under the engine so I popped the hood and checked the cap for tightness. It looked like some coolant had leaked out and the overflow was a little wet so I checked the cap for tightness and tried screwing it some more and it threaded smooth as butter 4-5 turns and then to tightness as it has done in the past.

I checked the coolant the next day and has lost a little as expected and the cap again was being a little ornery. What’s the best way to deal with this situation where the cap isn’t threading as well as it should? Any ideas what’s happening?


If it’s the pressure cap, replace it.



Agreed, replace the cap… You could have cross threaded the cap a little when you put it on, so if you replace the cap and it is still hard to screw on you may have to replace the coolant overflow tank as the threads could be damaged on it… But like already said, replace the cap and go from there…


Does the same problem occurs when set on defrost?
In as much, as I understand, the rear heat works correctly, the problem is not with coolant flow, but with the airflow through the front heater core.

Oops, wrong thread.

Cap threads or tank threads, I changed my cap yesterday. Smooth.

Thanks guys. I had the expansion tank replaced 2 years ago and the cap was troublesome since the beginning, sometimes it would go on crooked and was very finicky. Recently I’ve been able to get it on straight consistently I guess it’s just a matter of getting used to it. Hoping I didn’t damage the tank threads…

I’m guessing the fact the cap went on smooth after getting wet isn’t terribly relevant. My mechanic said the plastic threads could have gotten dry and thus that could be causing the problem.

Also I’m guessing I need to request expansion tank make and model to get the right cap for it…

The coolant reservoir cap can be very difficult to screw on, I usually wet the O-rings with coolant, it then screws on with ease.

@Nevada_545 That cap looks identical to mine thx. The o-rings are those rubber guys on bottom right? The o-rings press against the tank interior right?

Yes, when dry the two O-rings can create excessive drag against the reservoir opening.

Hopefully I didn’t compromise the o-rings lol

If lubricating the o-rings per above doesn’t do the trick, or it still leaks, suggest to purchase new cap. Plastic deforms over time with heat, and some part of the cap may simply have the wrong dimensions to ever fit properly. I had a problematic radiator cap on my truck, did several experiments to try to get it to work, but the only solution was to replace the cap with a new one, which worked immediately, and perfectly.

Just gave it a go today, wet o-rings from distilled water did the job the cap screwed on easy