Dirt in the trunk

Typically we would approach this as a water leak or a wind leak, but we can call it a dirt leak too. This is almost certainly due to a leak with the rubber weatherstrip around the trunk lid. First look at how the body panels line up where they meet. The front edge of the trunk lid should not be higher than the deck panel in front of it. The gap there and on both sides should be the same width.

Next, open the trunk lid and look for dust on top of the weatherstrip where it should be contacting the bottom of the lid. If dust can settle there, it is not sealing in that spot.

If you have a helper you trust, crawl inside with a flashlight, close the trunk, then have the helper flood the edges of the trunk lid with water. Watch for where it sneaks in. If they suggest dunking the car in a lake to see if water runs in, find a different helper!

Logic would dictate while you are in the trunk, you can look for areas where light is sneaking in, but do not rely on that. The folds of sheet metal usually block most light, so that does not work.

If nothing obvious has shown up so far, you can use a spray can of leak-detecting powder. This is a very fine white powder that you spray onto the underside of the trunk lid or onto the weatherstrip, then close the lid very gently but completely. Open it again, then check if there's any gaps where it did not transfer to the other part. You should see a nice white outline all the way around the lid or the weatherstrip. I never checked to see if you can buy that powder at the auto parts stores. I got mine from the parts department when I worked for a very nice family-owned Chrysler dealership.

The last "special tool" is a dollar bill. If you are not sure if you found a leaking area, place the dollar bill on top of the weatherstrip, close the trunk, then try to pull the bill out. You should feel significant resistance. If there is a gap, the bill will slide out easily and you might even be able to push it back in. There should be too much resistance to slide the bill sideways along the weatherstrip.

Once you find the leak, there are a number of ways to solve it. Few weatherstrips are glued on today. Most slide onto a lip where two pieces of sheet metal are welded together. First look for deformed areas. A common cause of that is running CB and other two-way radio antenna wires between the lid and weatherstrip. Over time the rubber takes that shape, then when the wire is removed, it rarely regains its original shape. To solve that, cut a slit in the weatherstrip close to the channel that holds it in place, then slide in a piece of vacuum hose or some other flexible tube. I have even used electrical wire insulation that I slid off the wire.

If you have a long section that isn't sealing and adjusting the lid will not help, pull the weatherstrip off, slide something inside that channel, then push the weatherstrip back on that way. Electrical wire usually works best, and you can find all different diameters as needed. That tube will limit how far you can push the weatherstrip on. It will be held up higher so it seals better.

If you find chunks have been torn out of the weatherstrip, the only solution is to replace it. You might get lucky and find one in good shape at a salvage yard. That damage is caused by repeatedly sliding cargo across the rubber and tearing it, and by ice forming that sticks it to the lid. To prevent ice from causing that tearing, wax the underside of the lid where the weatherstrip makes contact, and use a rubber maintenance product on the rubber.

Also look at that welded flange the weatherstrip slides onto to see if it has become bent down or is mis-shapen. It needs to be shaped correctly to hold the weatherstrip up against the lid. The sheet metal on some cars is embarrassingly thin and easy to bend.

When you are working with weatherstrip on doors, you will see there are tiny holes in it every few inches. Be sure those are open and not blocked or sealed shut. While driving, air is supposed to flow into them and blow the seal up so it seals better. Those are there to reduce wind noise. You typically will not find those holes in trunk lid weatherstrip, but if you do, be sure they are not blocked.

If you still have not found the leak and you are sure there is something leaking, consider using a smoke machine. These put out a white, non-toxic smoke at two pounds of pressure. We use them to inject that smoke into engine and emissions systems to find tiny, elusive leaks. For this purpose, it is not going to develop any pressure in the trunk to push the smoke out. Instead, set the machine up in the trunk to spew its smoke by itself. Close the trunk lid, then turn the heater fan on to one of the lower speeds. Many cars have a passageway for air to flow from the passenger compartment into the trunk. If yours does not, put the smoke machine in the back seat, then run the hose under the seat cushion into the trunk. It may take a few minutes to show up, but you will see the white smoke sneaking out if there is a leak. This works best inside a garage or shop where there is no wind to blow the smoke away.