You pop the hood to check the oil and find a pile of sticks, mesquite pods, and chewed-up cholla packed against the engine block. A few days later the check-engine light comes on. Swift Pest hears some version of this story every winter in Tucson, and the culprit is almost always a pack rat.
Why do pack rats move into car engines in winter?
Pack rats move into engines because a parked car offers warmth, shelter, and a dark place to build a nest, all of which become more valuable when Tucson nights drop into the 30s and 40s. An engine stays warm for hours after a drive, and the space around it is protected from wind, coyotes, and owls.
The pack rat most Tucson homeowners deal with is the white-throated woodrat, a native desert rodent about the size of a small squirrel, with a furry tail and large ears. Pack rats build nests called middens out of whatever they can carry: sticks, cactus pieces, seed pods, foil, even keys. An engine bay is just another good place to start one.
Pack rats also chew constantly to keep their front teeth from overgrowing. Wiring, rubber hoses, and plastic insulation are soft and easy to gnaw. Some newer vehicles use plant-based materials in their wire coatings, and many mechanics believe that makes the wiring more appealing to rodents, though manufacturers dispute the connection.
How can you tell a pack rat has been in your engine?
The clearest sign is nest material under the hood: sticks, leaves, cactus segments, or shredded insulation tucked into corners near the battery, air filter box, or firewall. Droppings and a strong urine smell are close behind.
Other warning signs include:
- A check-engine light or warning lights that appear for no obvious reason
- A burning smell when the engine warms up, often from nest material sitting on hot parts
- A musty smell through the vents, which can mean a nest in the cabin air filter housing
- Gnaw marks on hoses, wire looms, or the foam under the hood
- Small footprints in the dust on top of the engine
Cars that sit for days or weeks are the most likely targets. A second vehicle, a project truck, or a car left behind while its owner travels gives a pack rat plenty of undisturbed time to settle in.
What should you do if you find a nest under the hood?
Have the car checked by a mechanic before you drive it any distance, especially if the warning lights are on. Chewed fuel lines and brake wiring are safety problems, not just repair bills. Repair costs vary widely, and replacing a full wiring harness can be expensive. Comprehensive auto insurance sometimes covers rodent damage, so it is worth calling your insurer.
When you clear out the nest yourself, wear gloves and a mask. Rodent droppings and urine can carry disease, and the CDC advises spraying droppings with a disinfectant or a bleach solution before wiping them up, rather than sweeping or vacuuming. Look closely for kissing bugs, which often live in pack rat nests in Southern Arizona.
How do you keep pack rats out of your car?
The most effective step is removing the reasons pack rats are near the car in the first place. A pack rat that lives in a midden at the edge of your driveway will find the engine. One that lives half a mile away probably won’t.
Start with the yard. Clear brush piles, stacked wood, and dense prickly pear or cholla within about 30 feet of where you park. Pick up fallen fruit and mesquite pods. Park in a closed garage if you can, and check that the garage door seals tightly at the bottom.
On the car itself, drive it every few days, and pop the hood once a week to look for fresh debris. Some automakers sell rodent deterrent tape treated with capsaicin, the compound that makes chili peppers hot, to wrap vulnerable wiring.
Ultrasonic repellers, peppermint oil, and dryer sheets are popular, but there is little evidence they keep pack rats away for long. Never place rodent bait in or around the engine, where a poisoned rat can die in a spot that is hard to reach.
How does Swift Pest deal with pack rats around your home?
Swift Pest starts by finding where the pack rats are living, usually a midden in a block wall corner, under a shed, or inside a cactus patch. From there, technicians use exclusion, trapping, and baiting placed safely away from vehicles, kids, and pets.
Pack rats in the engine are a sign of pack rats on the property. Find the nest, remove what draws them, and check under the hood often through the cold months. To stop the problem at the source, contact Swift Pest for a rodent inspection.