Failed emissions due to bad fuel cap
 

My XKR failed an emissions test recently due to the fuel cap, it couldn't hold pressure according to the guy administering the test.

It passed last year, any chance his machine was wrong or is it common for a fuel cap to fail like that? Just seems odd for a fuel cap to go bad, I thought a code might appear if it did.

I didn't want to try another tester and spend another $15 when I could order a fuel cap for around $25.

Anyone else experience this, what did you do? Best place to order new fuel cap?

independent Jag shop loaned me one of theirs. I also pointed out how the cap was magnetic and the testing place had something that was not.
BUT I suspect what really helped was the guys doing the testing recognized the car and me for round 2 and let it slide as the car was came up great for everything else

These cars usually are pretty sensitive detecting a leak on their own, but it could take just some debris on the seal between the cap and the tester unit to cause a fail.

My S Type failed last time, they test the cap separately on a special rig.

No codes showing, a new cap and it sailed through.

Mine fails every year. I believe its the testing equipment but can't prove it. Last year I bought a new cap,took it to the tester (I didn't install it) and they had trouble getting it to pass.
I keep it for inspections only.

Same problem here. I replaced my solenoid purge valve (getting the P0442 code) and purchased a new cap. I didn't put the new cap on and handed the new cap to the technician. When I got my paperwork back, it said "fuel cap N/A" as opposed to fail.

I think their equipment is way to sensitive and throws out gas caps as a catch all.

Utrider, I always get the NA on my 07 X Type. I don't think they test it because its attached. But they pass the new cap on the XK8. I think they used one they had in the shop.

Not sure how it would work on these cars, but I had a friend run his Ford van on a near-empty tank on a couple of gallons of methanol, just for the test (it passed).

It wouldn't do your engine any favors if you drove around on pure methanol, but it may burn clean enough to get you by the test.

My 2005 also failed the cap test, purchased a new cap from the dealer - $37 - which passed. Step price, but the two recommended online sites for our caps were sold out, so I assume this is a very common problem. It will be interesting to see how it does in two years (the emissions test frequency here in Colorado). My suspicion with the cap is the retainer which has a ring around the cap so the cap can rotate within the retainer, perhaps that is not providing adequate seal on the test equipment which of course has to be generic.

seems like a little grease or vaseline around the rim of the cap right before testing would cure the problem - maybe.