Confused: what could cause new front lower shield to pull off?

The culprit is probably air pressure. I had a similar problem with my 2016 Prius. There was an issue in the front grille that allowed air to flow into the engine compartment so when driving at high speed, the pressure would build up and cause pannels to separate.

JeffD

 

Based solely on what you stated...drive a few miles and recheck your work...is a good practice.

 

That's weird. Going from memory, there's a row of bolts at the rear going into weld nuts, that also secure the leading edge of the main, engine bay under panel. Ahead of that is a row of screws going into nylon blocks, and in the middle of that row is a single, push-in style fastener, an oddball, an extra-long one with a loop in the jaw zone.

 

While the design that holds those belly covers in place is solid, once the covers fail and needs to be replaced it usually means the fastening mechanisms have failed too. I've done lots of these for a cab company as well as for friends and I almost never waste my time with the original fasteners and instead use drywall screws to connect all the plastic covers to each other. Not only does it make the installation job faster, but it also will ensure the old fasteners don't fail.

 

Did you properly tuck the front/leading edge of the lower cover under the bumper cover when it was installed?

If the cover's front edge was properly tucked under the bumper cover, the only way for the cover to separate in the front is to somehow grab onto the cover and really yank down on it. Hard.

I suspect the front of the cover was mounted on top of the bumper cover and not under it, that might allow wind and air pressure to catch the front edge of the cover and rip the cover away and hang from the bolts at the rear of the cover as you described.

SM-S936U ?

 

You may have scooped up a bunch of mud that remained in the engine compartment side of the shield. The additional weight, bouncing around during your travels, differential air pressure, combined if it wasn't tucked properly, could've caused the failure. Those clip-nut fasteners that looks like a 'wood screw', really doesn't hold that much weight vs the same fastener with a machined screw nut. Those don't usually fail, unless the plastic piece it's clipped to is also gone (broken)....

YMMV...

 

The steel washers car a lot harder than the soft thin polyethylene plastic sheet that makes up the cover.

Any pull on the plastic undercover makes the edges of the steel washers acting like steel punches cutting through the plastic.

Mu6u answer was to use 1/4 inch neoprene backed large diameter one inch stainless steel fender washers that I purchased on Amazon with the rubber cushion facing and spreading over a wide surface area of the thin plastic undercover.

The covers are still perfectly held to my 13 year old 2012 Prius v and will be there for many years to come.

 

That frontmost under cover reliably snags if you roll the front wheels into contact with typical height concrete wheel stops. And then when backing off, its trailing edge catches badly. It’s noisy when this happens though, unmistakable.

 

Especially if those chocks has a rebar protruding on top of them. Had to replace the scoop on my old Jetta twice.

 

The 7mm holes that the plastic fasteners go in are the exact size that will fit 6mm rivet nuts without drilling.

In the case of the oil access door, I replaced the oil change access door's plastic fasteners with rivet nuts, M6x1.0x20 hex head bolts and the large neoprene lined fender washers that I mentioned in my earlier post.

This way, the bottom cover will always stay in place.

 

I had trouble with my lower deflector plastic rubbing on....well pretty much everything...driveway curbs, curb stops etc. I put the Prius Off road lift on my car and the difference was a massive improvement. I won't own another prius with out one. No MPG loss. No scraping and slightly easier to get in and out of. It is only a 40mm body lift but speaking in ratios.. It makes it more like my old subaru. Not quite the 7" clearance it had. but I think I'm at 5" on my Gen 2 now. Maybe look into that as an upgrade as well. Georgina is correct on Rivnuts. They are wonderful to add. I used them to add Brush guard anchor points to the rear tail lights of my old Range Rover.

 

Beefed-up fasteners won’t prevent the too-low and snag-prone plastic from being shredded by the concrete wheel stops. Like I do for several Toyota foibles, I just adapt, in this case by stopping at least a foot short of the wheel stops. It’s a pain but what can you do.

 

This is the best and most straightforward answer...

 

On Gen2, I used these mounts for the lower plastic shields a few years ago. They are still there and holding the plastic.

 

Once all the panels are screwed together with as many screws as possible from your random screw drawer it will be super durable. Don't wast time or money on this. No one is ever going to see what it looks like under there. Just make it as secure as possible and call it done!

 

I guessed a bit at your frame number:

https://www.amayama.com/en/genuine-catalogs/epc/toyota-usa/prius/ZVW30L/13501/body/5201

probably the same for all US 2011, but check by verifying the frame number, on drivers door frame decal.