Please help- P0455 Evap Emission

[Rayyy]

2004 5.3L Vortec 4x4

So, I’m driving along, and get a warning ding and a message on the cluster that says “Tighten fuel cap”

Once, I pull over… I check the fuel cap, it’s tight.

I’ve driven 1/2 the tank, so if it were loose, I probably would have received this message sooner, no?

Once, I start the engine back up… I see the Check Engine Light is on. Of course, it’s the P0455 code.

Is there anything else that could have possibly caused this?

Is it possible my fuel cap is leaking pressure?

If I replace it with a new one… Can I drive around with the Check Engine Light until it turns off? (I’d prefer not resetting my O2 monitors).

Or do I have to clear it regardless?

Thanks in advance for any insights.

 

[swathdiver]

Could be a number of things. If you do not have a proper scan tool for diagnostics and testing, replace the gas cap with a genuine OE one.

If that fails to solve the problem, you'll need a smoke machine to effectively look for leaks. If that doesn't turn up anything, we use the Tech-2 or similar to create a vacuum in the system and see if that works or not. Could be the vent, purge or fuel pressure sensor.

P0455 is for a large leak, like a broken or disconnected hose, bad gas cap, etc. I had one trouble me for years, turned out to be rotted lines, they were rubber and replacements are a type of plastic.

 

[MassHoe04]

I have been choosing to ignore mine for the last few months. Until recently, too cold to bother with!
Low priority; and has not interfered with operation one bit.

In my case, I know for a fact it has nothing to do with my gas cap. We just had a recent 2-day wave of 90+ F temps. I ran my tank down to within 4 gallons of empty. When I stopped for gas and took the cap off Friday, I had a rush of air blow out the filler tube for what seemed to be over a minute in duration! Tank must have had incredible pressure!!! The cap was holding it all in tight, though.

I will be looking at the vent valve first and inspecting the tubes first, then the charcoal canister.

Eh, when I have nothing to do and feel motivated, I guess...

Until then, I continue to ignore.

 

[Fless]

The evap test will only run with the fuel tank between 1/8 and 7/8 full, on a cold start with the intake air temp (IAT) within about 10° or so of the engine coolant temp (ECT), so it appears that you're meeting that criteria. Just note that the evap test won't run outside of those conditions, and your check engine light won't go out until it's reset with a scanner or the native evap test is successful.

These are simple evap systems so getting a competent scanner on it, and possibly a smoke machine, to diagnose it will save time, effort, and parts costs.

If I had to take a wild a$$ guess I'd say put a new GM OE purge valve on the intake and see if that one's intermittent or stuck open. As well, the canister vent valve at the rear could be intermittent or open. As @swathdiver noted it could be one of several causes.

 

[MassHoe04]

On mine last fall, I replaced the purge valve. That solved a CEL for high flow during non-purge cycle. Must have been getting stuck in open position or not functioning at all.
Purge-related codes have not come up since.

Several times the CEL has cleared itself after the P0455. It must have been after successful test cycle, so many drive cycles/miles, or some combination. The light did go off a couple different times. Of course, it comes back on again at some point when it fails a test cycle.

 

[swathdiver]

Mine would always fail and set the light right before we were about to refuel. The test ran on mine while the engine was off, it would usually happen while out to dinner or visiting relatives.

 

[MassHoe04]

CEL seems to come on approx. 3-5 miles after cold start on 1/8 (or less) most often.

 

[nonickatall]

When the engine is turned on, the vent valve closes and the purge valve opens. The engine vakuum is applied to the EVAP system via the purge valve. There is a pressure sensor which checks if a vakuum in the tank sytem exists. If the vent valve not close airtight, or a line is damaged, or the fuel cap is open, the test fails and gives an error code.

The messages says "check fuel cap" because at a new car this is the most likely reason for the error.

Because the vent valve sits in front of the rear axle near the gas tank, it is very likely that this vent valve is not good anymore.

You than can have two issues. If the vent valve stuck open, you receive the message "fuel cap open". If your vent valve stuck closed, than you can't fill your tank, because the air must evacuate thougt the fill tube and disturbes the filling. This can also be caused by coal particles from the coal filter or container in the vent tubes of the system.

If you receive the message "fuel cap open" it is also possible that your fuel cap is not good anymore. It is as well possible that one of the plastic lines of the EVAP system is damaged, but not very likely.

I would first disassemble the vent valve and apply 12v and blow through the valve. It then must be airtight. And check if there are 12v on the plug, when engine is on.


And take a close look to your fuel cap.