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EVAP system. Very large leak detected..came back

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saneeshd

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Discussion starter · #1 ·
Van has been showing intermittent "Check Fuel Cap" message for few days. Checked the O-Ring on fuel cap and seems fine. Foxwell scanner showed codes for large leak and fine leak. Anyway i cleared it and drove again. Now the engine light is blinking 5 times and scanner showed EVAP system not ready.

Scanned it again and it showed very large leak detected. Progressed from large leak to very large leak. Seems the pressure build up by the Leak detection pump is leaking out fast. Gas cap seems to be fine. So with the help of my son, i started checking the issue. With a smoke machine, it is lot easier, but since i don't have a smoke machine, i did it with the hand held vacuum pump i have.

1) Removed the EVAP valve and the middle plastic pipe close to the canister.
2) I closed the pipe with my finger while my son was using the vacuum pump on rubber host after EVAP valve. Success. No leak. So the system is leak free from after EVAP valve to canister
3) Now checked the EVAP valve it self. Electronically, it is working. But when i tried to use the vacuum on each side, from one side, it is not holding vacuum and leaking pretty fast. Indicating that the valve is leaking internally.
4) Was about to buy the new EVAP valve, then thought of cleaning it. Used brake cleaner on each side, electronically activated the valve and blew air through it. Both ways. Did it 2 times.
5) Now it passed my vacuum lest. Holding vacuum from both sides.

Since the Foxwell scanner i have can't initiate the EVAP test forcefully, i have to wait the ECU to run and see if it works or not. If fails again, i will replace the valve.

Just wants to update the forum with my observations.
 
Discussion starter · #3 ·
Well, it came back. This time, only "Check fuel cap" message. Scanned and no code for EVAP leak. I really wish it had codes like fine leak, small leak etc. The VW i had, had it like this.

Removed the valve and checked. Electrically it is working fine and controlling the flow properly. But when it tried to do the vacuum test, from the direction of the engine, it holds vacuum. But when i try to do the vacuum from the canister side, it does not hold vacuum above a certain level and holds it when i cover the other side, indicating a leak.

Did any one tested the valve like this? Vacuum test from both directions? For me, since it is leaking the vacuum while applying from the canister side, seems causing the fine leak which shows this error message. Since the engine vacuum pulls the fuel vapors from the canister, does the leak on other side makes a problem? Just confused. May be i need to try pressurizing it instead of vacuum? Like the way the leak detection pump runs?

I really wish the Foxwell have the feature to run the Leak detection pump manually and test it. The VAG-COM s/w i had for the VW and DIS for BMW have this feature. It was easy to find it and clear code immediately.
 
#4 ·
This kind of intermittent EVAP problem turns out to be the large seal on the fuel pump where it seals to the tank with the big lock ring. It was temperature sensitive for me, so work on something, cools down, you think its fixed, but comes back later, rinse repeat. I smoked it, but it is so tucked under the floor it is hard to see the smoke. Pull middle seats and carpet and check if ring is tight...
 
#6 · (Edited)
Mine was the fuel cap. Seal looked good but I replaced it with a new one anyways A few week later the check fuel cap came back on. Cleaned the fuel filler neck really good with alcohol and the seal on the original gas cap an reinstalled it. Check engine light went out after a few stop & goes and never came back.
 
#7 ·
If the scan tool is not showing any code something weird is going on. Check Fuel Cap means a temporary DTC for an EVAP leak has been set. I got a check fuel cap on my Ody back in April and my scan tool clearly showed a P0456 DTC pending (but not a permanent code).

And FWIW, that check fuel cap went away on its own when I let the car sit for a couple days. Hasn't come on since.
 
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#9 ·
Check if you can smell any fuel near the fuel cap.
 
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Discussion starter · #12 ·
Skykowski said:
Following. I have the exact same problem. "Check fuel cap" comes up constantly, but no codes (pending or otherwise) to speak of.

There's a vacuum test the Foxwell scanner can do, but I'm not sure how to interpret it.
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Years back, I had to tackle this on the Passat i had. Very minor leak detected error code. Not the purge or vent valves. The way i fixed it is, i removed the hose from the leak detection pump and pressurized the system with a bicycle pump. Then listened for the sound. Fortunately, i was able to hear it and that car had an additional vent line near to the fuel neck. The O-Ring on that valve was bad. Less than a $ in part and easy to replace, but finding it was the challenging part. I even tried a shop and they could not figure it out. The good thing was, the VAG-COM software was able to run the EVAP test and clear the code immediately. It will run the pump just how the system run it.

Busy with work and all. Have to delay checking it for few days. Will check it and update. May have to take it to the shop to do the smoke test. The Purge valve, since holding vacuum from engine side, is fine.
 
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saneeshd
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