2004 Toyota Prius body rust / corrosion (rocker panels, front fenders)

[MetroMPG]

NOTE: I split this discussion from my Prius build/ecomodding thread because it's something specific that other owners will be searching for more and more as these cars age.

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My 2004 Prius is rusting out in several places.

It's worst in the rocker panels, but the front fenders have also started to go. This thread will show where and hopefully why the corrosion has started,so owners of newer cars (or older cars in less rust-prone parts of the world) can take steps to prevent it.

Eastern Ontario & west Quebec are perfect laboratories for discovering automotive design flaws that cause rust by trapping moisture, winter road salt (the killer) and dirt.

Here's what the rocker panels look like when I got the car





It was easy to poke that screwdriver through the side of the car.

I have an idea about the cause/path of this rust, and hope to offer some good preventive info to owners of newer cars (or cars that live in less salty winter environments) that haven't begun to rust... yet.

[Speeed3]

How do you think the rust started?
It looks like it started from inside. Rusted trough wheel well got dirt in there? Some clip from bottom plastic paneling fell off and allowed dirt in there? Just by itself? Some other way?

Maybe you (mostly meaning other Prius owners) can put rust preventing oil into there from rear spat attachment hole.

[Vman455]

On the 3rd gen, that whole area is covered by a plastic rocker panel.



I wonder if this was one reason Toyota made that change. Is rust in that area common?

[MetroMPG]

Originally Posted by Speeed3

How do you think the rust started?
It looks like it started from inside. Rusted trough wheel well got dirt in there?

It's definitely rusting from the inside out. The rust is a lot more widespread than the photo shows because once the paint has blistered, the metal beneath it is all corroded.

I haven't opened up the rocker panel yet to confirm, but I believe the problem has to do with this "shelf" at the bottom of the rear wheel arch (ahead of the rear tire) that holds dirt, salt & moisture:



You can't see it well in the pic, but where the yellow arrow points, the tip of the screwdriver that I stabbed into the car is visible at a small hole there. Which means water, salt & dirt are getting in the rocker cavity there.

Also, all the factory undercoating has flaked off that shelf, exposing bare metal.

Maybe you (mostly meaning other Prius owners) can put rust preventing oil into there from rear spat attachment hole.

Exactly! I would highly recommend drilling the rocker panel where the rear doors close (above the door seal) to spray something for rust prevention inside the cavity.

And when I cut the existing rust out of the rocker panel to repair it later this summer or early fall, I should be able to confirm the source.

[MetroMPG]

Originally Posted by Vman455

On the 3rd gen, that whole area is covered by a plastic rocker panel.

The problem with those plastic panels is if there's still a design flaw in the car, you won't know that the car is rusting out behind the plastic until it falls off! I've seen lots of cars with plastic rockers that hide serious corrosion.

Is rust in that area common?

Yup. Now that I have the Prius, I'm paying attention to them, and have seen the rust bubbles on the rockers of several earlier 2nd gen cars around here.

[MetroMPG]

I'll post a pic later, but there's also rust starting in the front fenders as well, behind the tire.

It's in the location of the holes in the wheel arch where the plastic inner fender liner & mud guards attach to the panel. On the passenger side of my car, the corrosion has crept around the corner from the tabs out onto the side of the fender.

UPDATE: some pics...

[Speeed3]

Originally Posted by MetroMPG

Exactly! I would highly recommend drilling the rocker panel where the rear doors close (above the door seal) to spray something for rust prevention inside the cavity.

And when I cut the existing rust out of the rocker panel to repair it later this summer or early fall, I should be able to confirm the source.

Maybe you don’t need to drill a hole. (From parts diagram) It looks like rear tire spat is attached to plastic plug with one screw. So maybe you can remove the tire spat and then pull the plastic peace of and then use its hole to put oil in there. Or maybe you could use the attachment holes for rear door sill panel.

Prius rocker panels have inner structure. Only pictures that I have found:
http://techno-fandom.org/~hobbit/pix/hf07/156aby-ch.jpg
http://priuschat.com/data/attachment...5_23.31.29.jpg

Looking forward for your pictures.

EDIT: On the other hand is it even that beneficial to protect the inside of the rocker? If that hole was rusted and worn into the wheel well wouldn’t just stopping the hole from appearing stop the rust from appearing? Or is the hole in there from the factory? Or did that hole start from inside?

[MetroMPG]

Thanks for those picture links of the rocker structure! We'll have more when I cut out my rust.

I suspect the hole on the "shelf" in the wheel well was caused by rust, and then salty water & dirt got inside from there. So if you could prevent the hole, you prevent the rocker rust.

I would still rust-proof the inside of the rocker. It's normal procedure (commercial aftermarket) here to drill access holes to spray rust inhibitor inside hard to access areas. Then you put greased plastic plugs in the holes.

I'm posting pics of the front fender rust in a minute. I'll add them to the previous post, above.

[Speeed3]

Originally Posted by MetroMPG

I'll post a pic later, but there's also rust starting in the front fenders as well, behind the tire.

It's in the location of the holes in the wheel arch where the plastic inner fender liner & mud guards attach to the panel. On the passenger side of my car, the corrosion has crept around the corner from the tabs out onto the side of the fender.

UPDATE: some pics...

Front fender rust might actually be caused by attaching the front mud guard. Those J-nuts will wear (or even rip while attaching) paint off.

We use lots off road salt here. But for some reason Priuses don’t have front mud guards (which off coarse causes the paint to wear from rocker and then it rusts from outside). But since front inner fender liner attaches with plastic clips there’s no rust in there.

[Gasoline Fumes]

Originally Posted by MetroMPG

The problem with those plastic panels is if there's still a design flaw in the car, you won't know that the car is rusting out behind the plastic until it falls off! I've seen lots of cars with plastic rockers that hide serious corrosion.

My outer rockers are totally gone. I can see the ground behind the plastic panels.