Are the plastic underbody panels necessary?

Hahaha, I laugh because this last weeks episode of CarTalk had this exact same question posed to it. The caller ripped off all the plastic under carriage parts in a desert. Their answer for the most part was: if they were extremely important, they wouldn’t be made of plastic!

However, they do play a role in keeping excessive water, rocks, dirt, salt (in snowy regions) away from any belts (in general, not this car), engine oil pan, transmission, exhaust, etc. so without them, you do run the risk of damaging more expensive parts than $550 in plastic.

So, IMO it’s probably worth the effort to find a junked prius first to strip parts (not sure if they use the same parts as the Lexus ct, but maybe include that car in your search) Other than that, you have to weigh your own thoughts on the car in general. Do you want this to last another 9 years? Is the car still running strong? Have you done other maintenance to prevent head gasket issues?


-Spiral

 

I hear ya on the oil flap. So annoying. But mine hasn’t ripped off yet, so I just curse it instead.

I’m guessing the bulk of the replacement cost is in the big section. If that part is less than $400 I’d probably get it myself, I wouldn’t worry about the others. If over $400 I’d take my chances for a couple years. Like I said, it helps minimize Rock and dirt damage, but I’ve also seen plenty of other cars that never had those in the first place that faired well in their life.


-Spiral

 

The oil flap's 'hinges' are just thin connections in the plastic that inevitably get bent until they break. After that happened, I just riveted a couple extension pieces to that edge of the flap, made round holes in them and in the under cover near where the flap 'hinged', and now reattach the flap just using a couple more of the round push clips there.

A nice bonus is now the front doesn't have to be raised as high to change oil, as the flap doesn't have to fold down and clear the driveway ... it can just be unclipped and removed.

 

I have a 2015...it's a salvage title due to a front end collision. The plastic junk under mine was not replaced and was missing when I bought it. I've driven it here in Central NC now for 2 years and 25k miles in all kinds of weather. I have had zero problems with the car in general including anything due to this missing plastic. I have averaged 52 mpg over that time so minimal effect if any on aerodynamics. I'm sure there is some amount of "protection" under there from that plastic, I'm thinking possibly more from small road debris and less from the elements (water) . If the road is wet, water will get up inside there anyway. I've worked on and tinkered with cars all my life and up until recent years....none of mine had any kind of protection under there. Oil pans were exposed, etc.....

I just didn't think it was needed so I left it off. Added bonus is that I don't have to bother with it when I change my oil.
So this is just a perspective from someone who is missing the cover rather than conjecture about what might happen if you don't replace it.

 

They're not "necessary". But if you elect to restore them, this page has most of what you need, maybe not the fastener part numbers though:

Splash Shields for 2010 Toyota Prius | McGeorge Toyota Parts

Reusing the picture:



Item 1 and 2 are up above, in the engine bay, covering the gap between radiator and front wall, so probably not involved.

Item 3 and 4 are inside the wheel wells, closing off the bottom. Again, they may be intact.

Item 10 is sheet metal (aluminum I think?), may or may not be missing, due to wind or whatever, since panels ahead being missing.

Item 8 is the same story as 10.

Item 6 is the main engine bay underpanel. The expression "oil flap" does not appear in the Toyota Repair Manual; the oil change instruction says to remove the entire panel. That said, it certainly is inviting, to just pry that piece open. The downside, it's thin/brittle plastic, a poor hinge, and over time cracks, eventually breaks off. That compromises it, and the pieces behind it.

Also, by only opening that flap, at oil changes, that means the rest of the fasteners on the panel are never touched, and get seized up with accumulated grit, so that when that panel is (rarely) removed, they tend to break. Pros in a rush will break them readily, and then ignore a few broken fasteners, the panel starts dangling, and eventually someone rips it off.

Item 12 is prone to damage from curbs, and even more so from free-standing concrete wheel stops. It's low enough to brush them, gets abraded. And with the wheel stops, it can ride comletely over them, and then snag when backing off.

Also, there's some confusing pictures of that item: the majority are somewhat different than what's pictured above. Some of the parts diagrams show two versions.

Some tips for preserving the integrity of the underpanels (if you ever get there):

1. Remove the entire engine underpanel, every time bottom access to the engine bay is needed. Carefully pry out the plastic fasteners, and before reinstalling, wash them out in hot/soapy water, clear out any grit. Note too, there are a couple near the back that are slightly larger, have grey caps. (see attached for parts numbers of the fasteners)

2. When parking perpendicularly with a curb or wheel stop at the front, stop short enough that the front bumper doesn't go over the concrete. Also, periodically check that front item (12). It can be removed without jacking up the car, and repaired, with something like 2-part epoxy and fiberglas mesh, for example.

I think the main thing that destroys these panels, is "professional" maintenance. The panels and fasteners are finicky, take time and care to preserve.

 

None of my cars (9 total) prior to 2005 had plastic under covers. All of my cars after 2008 (5 including my son's car) had some kind of under body plastic covers. As it turns out all 5 newer cars are hybrid and all older cars were gasser, but I suspect even regular ICE cars now have those under body covers. Not really knowing what the functions, for my first hybrid, I did spend some ~$300 to replace one of the cover that got shredded when I had to drive over 100 miles on snowy highway. I probably didn't need it, but I didn't know at that time. But knowing what I know now, if the car is 9 years old and you only want it to last a few more years, if I was you, I would not spend $550 to replace it, unless you can find much cheaper used or aftermarket parts and DIY install them. One caution is that many of cheap aftermarket body parts have very poor fit. Even with OEM parts, if the car has been in accident, then the fit maybe off and you may not be able to put all the fasteners in. However, being made of soft plastic and knowing they are not cosmetic parts, I would imagine that you can easily force them to "fit".

 

Yeah our son's Mazda CX-5 has extensive underpanels.

My 06 Civic Hybrid (Canada market) had the engine underpanel, but not the two long panels running down the sides. I sprung for those, think they were maybe $70 apiece?

 

No one needs air bags until there's an accident. Having a protected undercarriage is not needed until debris gets bounced around in your engine bay. I also wouldn't want layers upon layers of dirt, salt, debris on vital components like your oil pan, cvt etc.

 

I did substantially the same, except before the stupid hinge broke, and I used screws. That makes oil changes much less cumbersome.
There are less expensive aftermarket panels available.

 

I replaced panels 6&12 in the diagram with aftermarket panels. They are 1/3 the cost and function the same. They are look the same, but some of the mounting holes were a bit different. Once they are installed, no one will notice. The oil filter flap was torn off number 6 when I got it. And I took out number 12 back in January in 8 inches of snow. There was a branch in the road that I couldn’t see. I’d do it again if I needed too, but to each their own.

 

Anyone try to fabricate an alternative under-panel out of aluminum, fiberglass, etc.? I am just curious if a smoother surface would give better aerodynamics.

 

I plan on doing this out of coroplast after the next political election. I am just going to use an old sign.

 

The panels are there mostly for aerodynamics. Think about it - Toyota wouldn't have used them just for looks and added weight. Yes, the oil change flap is flimsy as hell, no argument there. I actually had to replace the main underbody panel when I had my 2010. I believe it cost around $250 for the OEM panel. There are aftermarket panels out there for less money, but I can't attest to their quality, not that the OEM panel quality is anything to write home about.

 

I've kept our underpanels pretty much pristine, but I guess it's not for everybody. If you're getting professional service mostly, never raise the car yourself, it will get beat up.

I've done all the oil changes (17~18 now, twice yearly) and three transaxle fluid changes, and each time I remove the entire panel. I've had it off a few other times, for access, putting in an OCC, and cleaning the EGR.

It takes 10 minutes tops. I take care removing the plastic fasteners, and rinse them out with hot/soapy water before reinstalling. With the bolts I put a drop of oil, every second or third time.

I'm losing track but I believe I've replaced one plastic fastener, have a couple spares still. I got the OEM fasteners.

The front most underpanel has taken more abuse, till I wised up and stopped driving right up to wheel stops. It's too dang low, gets chewed up. But if you stay on top of it, repair any damage (I've used epoxy and mesh), it'll last.

 

I just did oil change on my son's Honda CR-Z. He purchased it used last year, and this was the first oil change since the purchase. Taking off the plastic engine cover was not too hard but I destroyed 1 rivet, and a few other clips were already missing. Looked up the parts number online, only to find out that they are something like $9 a piece. I went with aftermarket ones. 120 pc for $12. LOL Why are those OEM parts so expensive!

 

Most cars these days come with extensive paneling underneath, made out of plastic because it's cheap (if you want to see some of them, search for the Chicago Auto Show threads on ecomodder.com, where I've posted dozens of pictures over the last several years). Their purpose is not protection--cars got along just fine for decades without them; rather, the panels are there to reduce aerodynamic drag. OP, you can leave them off without hurting anything except your mileage.