Project Car Diaries: Testing Many Methods for Keeping Mice out of My BMW

My BMW E46 3 Series has been pretty reliable. Unlike most of these cars in 2024, mine's been well maintained and kept stock since new. However, mice have consistently had a hankering for it. Mice have chewed through fuel lines (twice), made nests on the intake manifold, eaten some under-hood heat shielding, and ripped up wiring near the thermostat. All separate incidents. Meanwhile, other cars in the same driveway (typically a Honda, a Mitsubishi, and an old International Scout) have been largely ignored by neighborhood animals.

So for this installment of Project Car Diaries, I'll run through my personal "BMW ///Mouse plan" and then discuss few more popular rodent deterrents I read about and heard about from friends and neighbors.

I'm saying "repulsion" not "prevention" because the only way to truly keep mice away from something is to physically block them from it. Otherwise, the best you can hope for is to gross them out or scare them off—which won't work on 100% of mice.

Car's in a hermetically sealed bubble? Perfect, you never need to worry about mice.

Car's in a garage? Make sure your insulation is super-tight (mice can fit through micro holes), and keep a tidy space (no food lying around, no rag piles that could become nests). Do old-school snap traps for a while if you already have garage mice that you're looking to evict with extreme prejudice.

Car's stored outside? This is where the battle against mice gets a lot tougher. And unfortunately, this is the parking situation many of us have to deal with. For various reasons, I currently have to park my E46 outdoors quite often. Hence, my noble quest to fend off the beasts—like a mighty knight defending a castle from a dragon! Yeah ... just like that.

Mice like to go where their friends have already been, so the first step of any mouse-reduction campaign has to start with purging the rodent evidence. You should not take this lightly—mouse poop can be extremely toxic, especially as it's turned into dust while you're scrubbing. No joke! Read up on that for a bit before you decide whether or not you're willing to deal with it.

I made a precision vacuum attachment with a standard tube step-down adapter, sealed with some heavy-duty tape I had lying around. That's key to getting deep in the engine bay, which of course, is where rodents like to reside.

Then I cleaned the engine gently and slowly—primarily using Simple Green Motorsports and Tub'o Towels. The "Motorsports" version of Simple Green specifically is really good on sensitive surfaces and stuff that discolors easily, so I feel much safer using it with old plastic than more caustic cleaners. The same goes for the Tub'o Towels—it's a pretty gentle detergent, plus the wipe-on application style keeps any cleaning mess very well contained.

I also replaced the under-hood engine heat shield with this FCP Euro part (if you grab it, get four new replacement screws too so you don't have to be precious with the old ones).

The interior cleaning was a standard one. Make sure you change your cabin filter if you've had a mouse problem—they love to use those as toilets. Luckily, cabin filters are typically super easy. I changed my Honda's in an auto parts store parking lot a few weeks ago.

Mice hate the way peppermint smells. This folktale is substantiated by experts and some established pest removal pros—several companies make peppermint-scented products specifically pitched as mice-repellant to be applied on cars.

I tried this one—it came out milkier than I expected and it looked like it might even stain black plastic, so I applied carefully and tactically.

Believe it or not, this is an OEM Honda part. It's comically expensive, but largely out of curiosity I felt compelled to buy some. The tape is impregnated with chili pepper, which mice tend to find unappetizing.

The idea is that you wrap wiring and critical lines in it, and if a mouse takes a nib, they get grossed out and give up.

Mice can pierce through pretty much any readily available mesh or screens if they want to, but I heard that they hate the taste of copper and will typically not gnaw through the material. With this in mind, I got a spool of what's effectively copper chainmail. I ran it across the top of the intake manifold, securing it with the anti-mouse tape.

I know they could get around it—and I had to be careful where I placed the copper because the material can be conductive and I really don't want it transferring a dangerous amount of heat around the engine bay. I will keep an eye on it.

I can understand why a mouse would take up residence in my BMW. They see "Michelin" on the tires and must think they're walking into a Michelin-star restaurant. Jokes aside, it's far more likely that mice will climb up wheels and struts than jump off the asphalt up into the engine.

So I soaked the heck out of each wheel, strut, control arm, and everything adjacent with the same peppermint spray I used on the engine bay. I don't give a dang if there's plastic discoloration down there—and this is the main conduit for mouse incursion. So I went heavy with the stuff at all four corners.

Have you ever seen the show Supernatural? They made about 1,000 seasons so I bet you have at some point, but if not, one recurring situation is that the heroes make circles of salt to prevent demons from breaking a barrier.

I pretty much applied the same principle, laying down a line of this driveway mouse-repellant stuff (supposedly pet and environmentally friendly) in orbit around the parked car.

One catch with this (if it works at all) is that a heavy rain would undoubtedly carry it off or dilute it, so you can't expect it to last all that long.

After a few weeks of being occasionally driven and parked with all those countermeasures in place, I have observed very minimal mouse activity post-treatment. I did see one little oak seed which could have been carried in by a mouse, but it's possible that got pulled in by wind while driving.

So far, I think my countermeasures have helped. I try to park indoors, where no mice enter at all, but I'm feeling pretty good about the peppermint spray in particular and will probably spurt it around all my vehicles with some regularity.

I asked friends and contacts for mouse-prevention tips and got a deluge—like I said, I'm hardly the first person to have this problem. But some are debunkable with causal research.

Mouse traps work. A good ol' Victor-style snapping trap with a little dab of peanut butter terminated all the mice residing inside my garage and house in short order. However, I did not want to put a trap inside an engine bay and I'm not yet willing to set up traps outdoors. Inside a house is one thing, but outdoors is the realm of the animal. My garage is in a very rural location, and it's basically a mouse spawning point. Setting up traps in my driveway feels like instigating an unwinnable war. Do I want to be slaying legions of innocent mice near our house forever? Not really.

I'm allergic to cats, so getting a mouse-hunting cat was out of the question. But a barn cat is a great idea if you have a space that needs a rodent population reduction.

A few people told me that mice hate the scent of Irish Spring soap. That may be because it's a little minty. Regardless, I believe soap is made from animal fat which, in my opinion, might even attract mice.

Rumor has it that mice don't like dryer sheets. This may be true, but I didn't want to put any in my engine bay. If your mouse problem is in your car's cabin, this might be worth trying.

Quite a few people told me I should try an ultrasonic mouse repellant (little devices exist that emit noise mice supposedly find disagreeable). However, rodent experts I found online universally conclude that these are not effective in a meaningful way. It seems mice just aren't that bothered by annoying noises. At least, not ones inaudible to humans.

I would save your money and battery voltage and skip that one.

A strange question with a simple answer: Why do mice keep trying to set up shop in my 2003 BMW while ignoring the 1998 Mitsubishi it's parked next to? It's because the Bimmer has some components made with semi-organic (like, soy-based) materials.

In other words, the mice in my neighborhood simply prefer Bavarian food to Japanese. If you haven't had a mouse problem yet and want to know how worried you should be—Google around and see how common rodent problems are with your particular make and model. If you dig deep enough, you should find info on whether or not you've got many semi-organic materials under your hood that rodents might be attracted to. So far, it seems to me that old American cars and almost all European cars are the favored flavors of Rodentia.

I'm sincerely hoping that my feud with the mice living near my New York garage has reached a stalemate. As long as they don't come to the "restaurant" under the metal of my cars, they can scamper around the property to their heart's content for all I care. At least, unless they get into Bramble's dog run—then it's her domain and all bets are off.

Are there any other methods I might have missed, that you might want to share with the class, or suggest I try next if I find another acorn stash under my hood after all this?