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Super Moderator
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Discussion Starter #1
This could really be a broad range for all Malibus but since I seem to have a few emissions issues with mine, I thought I'd bring it up here for discussion.

I've been reading articles saying that you should NOT top off your gas tank when filling, that once it clicks off then you should stop otherwise liquid fuel could be absorbed into the EVAP system, damaging it.

So today when I filled up, once it clicked off, I stopped filling it and I'm going to continue doing it to see how it works out or if others have other opinions on it. Some people swear it doesn't hurt anything at all, and others say that the pump shuts off for a reason. What are your opinions/facts?
 

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Administrator
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16,621 Posts
Well, it's obvious that the pump shuts off for a reason, but the reason is that it reached its preset threshold and shut off, not necessarily that the car's tank is properly filled.

On my Gen7 I can usually get another dollar into the tank. At $2.50 to $3.00 a gallon, that's another 1/3 to 1/2 a gallon.

I've never experienced an issue on MY CARS, but that doesn't mean that others can do what I do and not have an issue.
 

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Certified Technician
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695 Posts
good way to kill a vent solenoid or wreck a charcoal canister
 

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Super Moderator
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Discussion Starter #5
So I took a closer look in the owners manual, and this is taken directly from page 9-40 for 2011.

"Do not top off or overfill the tank and
wait a few seconds before removing
the nozzle. Clean fuel from painted
surfaces as soon as possible. See
Exterior Care on page 10‑90."

So GM says, don't do it. It doesn't give an actual reason but if it's in the manual, then there has to be a reason for it. The last two fills I've done, I made sure to not top off and I'm waiting to see if any real benefits come from it but I used to top it off quite a bit, sometimes as much as .50 to .75 after it clicked off the first time.
 

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Yes, it's a fact that it can damage the system. Even on small equipment w/emissions this can happen. For example, o a new Vermeer chipper we have at work, under the fuel cap there is a plastic fill ring that says "do not overfill"... for the same reason that fuel can get into the charcoal canister. Though, with a modern fuel pump, I think it would be hard to overfill the Malibu to a point where damage would occur.
 

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Premium Member
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837 Posts
Overfill issue is fuel backing up the vent lines to the evap.valves and charcoal canister. They are meant for vapor not liquid. Don't forget that nice cold gas coming up from the in ground tank at the station is going to expand quite a bit when it warms up on those nice days.
 

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Super Moderator
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Discussion Starter #8
Overfill issue is fuel backing up the vent lines to the evap.valves and charcoal canister. They are meant for vapor not liquid. Don't forget that nice cold gas coming up from the in ground tank at the station is going to expand quite a bit when it warms up on those nice days.
And here, those nice warm days are starting to show themselves, which is kinda nice! Now I know I could probably look this up elsewhere but for the sake of having relevant information in this thread, what is the point of the evap valves and charcoal canister and what can happen if they are damaged with liquid?
 

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Premium Member
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837 Posts
EPA emissions required equipment. Canister stores fuel fumes to be bled off and burned in set cruise conditions. Liquid in the system will trigger evap. system codes for no / low flow, lack of purge, etc. .
We've seen questions for purge valve codes and replacement on here for example.
Top of filler neck is way above all the system component's so even though there may be check valves in the system to prevent liquid getting into vapor lines constant top off needlessly stresses the system.
That small amount of extra gas forced into the tank is not worth a couple more miles between fill ups vs replacing parts and labor to fix CEL's .
 
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