On my 2000 XJ, Ive installed a new CSF all metal 3 row radiator that took the place of the old CSF 3 row. I used the same cap since its still new and it spun on TOO easily on the new radiator. I put the cap on the old radiator to check and it was much tighter, the way it should be. The cap is a Murray from oreilly.

Has anyone with a CSF radiator for an XJ had a loose fitting radiator cap problem? Maybe i need a different brand of cap?

 

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I'd just go to an auto parts store and try a new cap. Go out to your rig and try and put on the new one to see if it'll make a difference before you buy it (most auto parts stores should let you do this)

 

Any time your changing parts, reusing hardware (in this case the rad cap) can be iffy... Things like suspension should always get new u bolts, nuts, etc. The new rad didn't have a cap? Huh, well either way, like a said, as often as you can, replace hardware!