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The disappearing Evap gross leak code.


Bgunner

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So the GF's Chevy Trailblazer came to the house with the check engine light on and running rough so I through the scanner on it and get just a gross evap leak code but nothing saying a skip but it was an obvious issue. Ok a head scratcher but I changed out the boot and spring, mopped up the shit ton of water out of the plug hole just to still have a skip on cylinder 1. I moved the plug to cylinder 2 and got a skip on cylinder 2 recognized. Changed the plug out and no more skip but the Evap code got wiped and has not returned.

Has anyone seen this one before since I haven't? Gone through 3/4 of a tank of fuel but still no evap code triggered.

I find it odd that it disappeared after the skip was fixed.
 


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fastpakr

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Maybe she'd recently left the gas cap open and it just hadn't cleared yet? (Wild guess)
 

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Never let it get get under 3/4 or above 1/4 of a tank and it will never run an EVAP test.
 

Dirtman

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Never let it get get under 3/4 or above 1/4 of a tank and it will never run an EVAP test.
Uhhhhhh you shouldn't be drinking on a work day...
 

Bgunner

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Maybe she'd recently left the gas cap open and it just hadn't cleared yet? (Wild guess)
I did check that also but it was on properly.

Due to it having a leak In the evap system before its had a new gas cap, new filler neck and the purge valve replaced at the beginning of the year by a garage soit would pass inspection.
 

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I did check that also but it was on properly.
Right - I'm suggesting that (maybe) it had previously been improperly seated and set the code, and the physical issue was resolved by the time you reset the codes.
 

Dirtman

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I'm with drunky over there, that the problem may very well still exist but the system has yet to actually perform the necessary test. The system needs to be at a very specific set of circumstances for the EVAP test to complete. So if the codes were cleared when you fixed the misfire, it may take quite a while for the EVAP code to return.

Since it was a gross leak code, in the meantime I would inspect the lines to and from the purge valve, the line to the vent valve, the filler neck and neck vent, the gas cap, and all the fittings on the tank.
 

Bgunner

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In that case time will tell if if comes back. I'm hoping it stays off because the next step is smoking they system to find the leak. It will hit the garage for that one.
 

Bgunner

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What are the criteria for the self test to test the Evap system?

From sno we can tell it needs to have fuel between 1/4 and 3/4 of a tank but what other criteria need to be met to trigger the test?
 

Dirtman

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Needs to be something like above 40 degrees, below 90, engine needs to have been off overnight or 8 hours from the last run cycle, then cruising at a certain maintained speed 50-60mph? with no heavy load on the engine for a certain amount of time with the correct amount of fuel.

Like I said, finicky test...

Forscan can force the test manually, you won't physically meet the test parameters but it will show tank pressure loss when you manually pull vacuum via the computer.

I guess you could also just plug the vent line with something, disconnect the purge line and use a hand vacuum pump to see if it holds vacuum. If you pull vacuum on the purge line and it doesn't build vacuum you know you have a leak.

Edit - forgot you're talking about a chevy... NO FORSCAN FOR YOU!!!
 
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