Ive read over and over these threads about the finicky Evap system but mine is strange.

When I fill my tank I can drive the vehicle about 80 to 100 miles noticing my fuel gauge to almost 3/4 full when I throw the famous "Check Gas Cap" code later followed by a MIL (Check Engine) light.

P0455 comes up each time with my scanner. Clearing the code allows me to drive the vehicle for the remaining fuel in the tank without ever throwing a code again.

Change fuel cap, inspected hoses from tank and canisters and purge valve. All are solid with no brittle pieces or damages to the fittings.

I haven't attempted the TPS yet, but wondered if anyone else notices this and what the resolution may be?

It is very consistent when filling the tank and using about a quarter tank before the P0455 comes up every time.

Thanks,

Doc
2001 XK8 with 64k on it.

 

The test only occurs when the tank is between 15% and 85% full on every other start. If above 85% then the test is inhibited and you will never see the message and MIL.

If the light is on and you clear it you won't see the message until the 2nd time you start the car and only when the fuel tank is in that range.

It feels random, but there really is a method to the madness.

I had a similar problem and it turned out to be the canister close valve (CCV). If you are able, or know someone who can hook the car up to a diagnostic system and actuate all the valves to isolate the different segments, it will really speed up the diagnosis.

It's possible to do all of that manually by applying a voltage to the valves but most of them are not in easy to reach places.

 

This explains a lot. The used-car dealer where I bought mine insisted on filling the gasoline tank before I took possession. It took them thirty minutes to do so (resetting codes I presume) .... I was getting really frustrated, as I had a longish drive home. Then the second time after that, obviously the tank was lower, and I got the code. Clever huh.

 

Pretty much the same thing happened to me.

Picked up the car in Dallas, started driving and I was a good 200 miles away before stopping and starting the car again, at which point I got the gas cap message and the MIL.

The used car salesman cliches are well earned.

Here is some more info about the onboard EVAP tests. The conditions where it will run are pretty specific, so it's easy to see how you could not meet one of the conditions, say if you were on a road trip and the car never really cooled off between starts.

EVAP Testing: Working Behind the Scenes | Search Autoparts

 

Thanks for the insight....

Ive considered by passing the tank pressure sensor with a couple of resistors in a voltage divider to present 2.5 volts to the ECM but uncertain if it will work.

Every article Ive read claims purge valve replacement and canister leaks, but I don't want to throw random parts in it.

Otherwise I need to break down and try to find someone who has a diagnostic scanner for Jags.

Not many out there.

 

Same here. It dumbfounds me that these P0445 issues are so common with the XK8 and yet I've owned two dozen other cars and driven hundreds of thousands of miles without it ever once occurring on any car before.

 

FINALLY got my P0455 issue resolved this afternoon !

Canister Close Valve was removed and tested on the bench and found working just fine. The attached hose, however was not.

Hope this helps someone in the future !!

 

Good, thanks for the feedback, really helps others.

 

Same issue on my 99 XJ8, but I keep getting P0453 code. Did you ever try the resistors at the FTPS connector to drop voltage to ECM down to 2.5 volts? If so, did it turn off the CEL? Thanks

 

This thread is ten years old so these owners may be long gone by now. But look at the hose photo in post no. 7. It may be worth checking the condition of your hoses as well....