Random misfires (P030x) in 2001 Prius

Hello. My daughter's beloved 2001 Prius (~300k miles) started running rough this spring, and giving P0301/P0302/P0303/P0304 codes (she has a code reader). She is in Boston; I'm not, so Dad couldn't help. The mechanic that worked on it recommended new plugs & coils. Did that; seems to have solved the problem (in the March 2021 timeframe). Then the problem came back, My guess was water in the gas. When I finally got to look at the vehicle (in June), when starting, it ran rough, and the misfire code came back. Clear the code, restart, and the misfire code came back from a DIFFERENT cylinder. After a couple minutes the car ran reasonably well (only a Boston city driving test). My hypothesis is that corrosion at the fuel fill port (see picture #1) allowed enough water to infuse into the ethanol-laden fuel, and then settle near the bottom of the tank, to cause the problem. So when I looked at the fuel fill port, it was massively corroded. See Picture #1. In the Picture #2, note the rust embedded into the rubber seal around the fuel fill cap, which may prevent a good seal. My solution is to carefully sand down the fill port to bare metal, then coat the lips of the fuel fill port with finger nail polish to smoothe and protect.

When sanding, I blocked the fuel fill hole with a paper towel, and frequently sucked out the scrapings with a vacuum cleaner. No sense allowing the debris to make its way into the fuel again. Then I cleaned the metal with a paper towel and isopropyl alcohol to get the really fine stuff cleaned off.

Also gently scrape off the rust from the seal material of the fuel fill cap.

Been driving the car for 2 weeks now, 3 fuel fills, and no problems.

 

If someone has a better sealant for preventing corrosion of the fuel fill port than finger nail polish (I used a really bright pink color to look for places where it wears or corrodes, but the color makes a fuel fill an optically challenging event), I'm open to suggestions.

 

Chap,

THANK YOU! for the pointer to another potential water input source.
The collective memory of this set of threads is astounding.
Do you happen to remember the part number for the pipe?
This is a 2001 Prius, and has lived most of its recent life in the salty states, so
this is something I will want to follow up on.
Other than this recent mis-hap, the car regularly gets 40+ MPG... not too many other battle scars after 250k miles.

 

Following up: I've removed the fuel fill pipe in anticipation of the new fuel fill pipe arriving by UPS momentarily. It was AMAZING how much rust formed at the top of the fuel fill pipe under the plastic adapter... The job looks intimidating, but is easier than it looks. Remove the passenger rear tire after placing the vehicle on a jackstand and all the bolts are relatively accessible. Liberal use of Liquid Wrench or its equivalent may be required. Tools: ratchet with 10mm socket and extender bar required, plus vise grips for the crimp connectors (at metal to rubber hose connection points). A 14mm socket for one bracket bolt. Goggles might be helpful to keep debris (which you WILL knock loose) from getting into your eyes.

FWIW: The fuel fill neck part is about US$199 (list), but may be available for less with online searching.

Toyota Prius Fuel Filler Neck Part Number: 77201-47051 PIPE SUB-ASSY, FUEL TANK INLET

(I hope that link isn't stale)...Alternate part is Spectra Premium FN886 (Rock Auto)