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Old 09-18-2013, 11:47 AM   #1
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High Idle After Fueling

I have a 2000 MK1 1.6 Zetec, and something strange is happening. After I refuel my rev's then sit around 3k and takes about 5 seconds before it drops to about 1.5k and sits there, its also pinging as well when pulling off in first gear during this time. After a few days it returns to normal.

Something to note is that every time I undo the fuel cap there is a rush of air, not sure if its in or out though.

Has anyone come across this or have any ideas what could be causing it?

Thanks!


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Old 09-18-2013, 04:43 PM   #2
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I thought it could be a fuel tank breather hose but then i thought it would be better when the cap had been removed as the pressure would be normalised and get worse over time rather than the other way around?
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Old 09-18-2013, 06:04 PM   #3
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It could be the gas cap vent. The cap is to allow air into the tank and keep fuel vapors from escaping. When full, a small decrease in the fuel volume in the tank will lead to large megative pressure. The cap is still venting (barely) or pressure is balanced in the EVAP cycle.
Because fuel is regulated so precisely for combustion, that negative pressure may be reducing the volume/pressure to the injectors setting up a lean-burn detonation.
(That's just my dart at your strange symptoms)
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Old 09-18-2013, 06:20 PM   #4
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Are u stoppin filling when pump shuts off or do u keep shoving fuel in ?
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Old 09-19-2013, 12:55 AM   #5
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Quote:
Originally Posted by spiller View Post
It could be the gas cap vent. The cap is to allow air into the tank and keep fuel vapors from escaping. When full, a small decrease in the fuel volume in the tank will lead to large megative pressure. The cap is still venting (barely) or pressure is balanced in the EVAP cycle.
Because fuel is regulated so precisely for combustion, that negative pressure may be reducing the volume/pressure to the injectors setting up a lean-burn detonation.
(That's just my dart at your strange symptoms)
Would a first step be to replace the gas cap in this case?





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Are u stoppin filling when pump shuts off or do u keep shoving fuel in ?
I only ever fill to about half so it doesn't shut off at all.
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Old 09-19-2013, 09:31 AM   #6
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Would a first step be to replace the gas cap in this case?
I only ever fill to about half so it doesn't shut off at all.
Then not likely, but the little things are still nice to eliminate. My scenario required a fill-till-FULL.
The only other thing that strikes me is an EVAP/Purge control valve issue or line leak between Purge Control Valve and intake. A MIL/CheckEngine may not be lighting for lack of consistent failure or your driving habits prevent the EVAP test by OBDII to run test.
Check the EVAP hoses for deteriation, especiall between purge control valave and intake. A bad Purge Valve should cause a CheckEngine, but a bad hose after the valve may not. (But why would it settle down later...?)

(EDIT) Wait, that's not right either. EVAP should be bled to air-box before throttle body...

Last edited by spiller; 09-19-2013 at 09:41 AM. Reason: Changed mind on validity of answer
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Old 09-19-2013, 10:35 AM   #7
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Would checking EVAP hoses require dropping the tank?
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Old 09-19-2013, 03:01 PM   #8
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No. The EVAP hoses in the engine bay. A failure of any EVAP hoses before the Purge valve should cause a code.
Go ahead and replace the fuel cap. Its really the only thing that changes that should matter after you add fuel.
Most anything I can think of that could increase the rev/idle speed wouldn't be allayed over driving after fill-up....
When you fill up, do you keep the radio playing? Are the keys in the Run position?
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Old 09-19-2013, 03:07 PM   #9
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You might want to research how "Evaporative Emissions" systems work to vent the fuel tank. That will be helpful in your diagnostics and understanding what the parts of the system are and what they do. You could have a clogged system, kinked system, or bad purge valve. Start with the cheapest repair first if you're guessing!
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Old 09-19-2013, 10:24 PM   #10
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No CEL? I've only had the gas cap go bad once, and it threw a code for Large Evap Leak (can't remember the P number offhand). And if you're only ever filling halfway, it seems even less likely.
That said, a new cap will only set you back $15-20.

When you say it returns to normal after a few days, you mean RPMs of around 700-800 at idle? Or it just stops hanging at 3000?
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