The following article gives detailed instructions on performing a BMW E90 trunk leak repair in a 3 series car. Even though we have used a 2013 BMW E92 M3 to perform this repair, this article can be applied to any BMW E9x 3 series vehicle (excluding the touring wagon) produced from 2007-2013, including the 328i, 330i, 335i and M3 sedans, coupes, and convertibles.
A BMW E90 trunk leak caused by one of the tail lights not sealing correctly is a very common issue that effects all 3 series cars produced from 2007-2013.
On a very rainy South Florida day, the sound system in our BMW Repair Guide 2013 BMW E92 M3 went completely silent. We unfortunately diagnosed the problem as a water damaged amplifier due to a trunk leak (see our article Amplifier Water Damage Repair). Approximately an inch of water had accumulated in the left side trunk well where the amplifier is mounted.
After cleaning up the mess, we started a careful visual inspection of the car to see if there were any obvious clues to the source of the trunk leak. There was no visible damage or wear to the trunk lid gasket, and even more perplexing was that none of the interior trunk liner was damp. The trunk leak seemed to be isolated to the left side trunk well behind the tail light.
We decided to do a water hose test to see if we could get the trunk leak to reveal itself. With the trunk lid closed we ran water over the tail light, focusing near the top of the light and trunk lid to simulate rain water run off. Bingo. When we opened the trunk and looked in the trunk well, we could see water pouring in through a small hole behind the tail light.
The repair for this was surprisingly easy. The cause was a worn tail light seal (gasket) that needed replacement. But we also noticed another issue; one of the three threaded studs that mounts the tail light to the body was also leaking. There is a thin synthetic washer that is supposed to seal the mounting studs against the body, but the top one had failed. Unfortunately that synthetic washer is part of the $250 tail light assembly and can’t be purchased separately…not a price were willing to pay for (another) BMW engineering miscue. With a little ingenuity and about $3 spent at the local home improvement store, we were able to come up with a solution using a rubber flat washer that makes the tail light studs water tight again. All of this is discussed in the following repair instructions.
1. Tail light gaskets (required) – Choose your tail light gaskets from one of the following options, depending on the model of your car.
BMW E90 Sedans: There are two versions of tail light gasket depending on the model year and whether they are Life Cycle Impulse (LCI) (updated body design) vehicles. Generally speaking, all BMW E90 sedans sold in the United States with a model year of 2009-2013 are LCI updated vehicles. If you are still not sure which version of the gasket you need (or if you have a non-US delivered vehicle), look at the part numbers on your tail lights and cross reference them in the list below.
BMW E92 Coupes: There was only one style tail light gasket made for all E92 coupes.
BMW E93 Convertibles: There was only one style tail light gasket made for all E92 convertibles.
2. Rubber washers (required) – Unless you are planning on replacing the entire tail light assembly, you will need to replace the stock nylon washers that seal the tail light mounting studs to the body of the car.
3. Tail light bulbs (optional) – Since you will be removing your tail lights to perform this repair, now is a great time to replace your tail light bulbs. They are inexpensive and only take a minute to swap out.
- Left side trunk liner – Remove amplifier cover and tail light cover. Remove first two plastic rivets on black trunk liner trim. Grasp liner and pull out from underneath trim. Fold back neatly exposing amplifier.
- Right side trunk liner – Just one easy step on this side! Turn latch counterclockwise and remove side panel from trunk. ***Remember to protect your battery from water if you are leak testing on this side. Either remove the battery from the vehicle, or cover tightly with a weatherproof tarp.
- Make sure your trunk is closed! Using a garden hose, drench the tail light with water. Pay particular attention to the gap at the top of the tail light and the gutter that runs around the trunk lid and empties behind the tail light.
- Do the water test for about ten seconds, then stop and inspect inside of trunk. If you see water starting to trickle out from behind the tail light into the trunk well, you have a bad tail light seal that will need to be fixed. As you can see in the following image, water was pouring into the left trunk well behind the tail light in our E92 M3.
- Using an 8mm open end wrench, loosen the 3 nuts holding the tail light assembly to the frame. All you have to do is loosen them…they should be able to be spun off by hand.
- Unplug the electrical connector from back of light (it pulls straight out).
- Grasp tail light and remove it from the car.
- Usually a visual inspection of the tail light and the car body will tell you really quick where your leak is. Look at this photo of the tail light mounting holes on the back of our M3…it is very clear where the leak is. The top mounting hole if full of dirt and leaves…a dead giveaway that water is getting trapped and making its way into the trunk.
- A visual inspection of the back of the tail light also shows water damage to the seal. Another concern is the poor quality of the synthetic washer that seals the mounting stud. This will need to be dealt with as well.
- Use a “paint safe” cleaner to remove any old adhesive still stuck to the back of the tail light after removing the old seal. Make sure the car body and mounting studs are also well cleaned – you want to make sure you do not have any existing dirt or debris that may effect getting a water tight seal when reassembled.
- Peel the back off of the new foam seal, and install on tail light as shown in the following images.
- Place a 5/16″ x 3/4″ rubber washer on each of the three mounting studs. This is a BMW Repair Guide suggested modification to cure the issue of the stock nylon washer leaking. We have found the stock nylon washers to be insufficient long term in preventing leaks around the mounting studs. We cured the problem by purchasing these rubber washer from our local home improvement store and installing them on top of the nylon washers:
- Carefully remount the tail light on the car, making sure none of your rubber washers fall off or become pinched. A good trick is to put pressure on the tail light with your hand while you spin the nuts back on the studs – this compresses the washers and makes tightening with your wrench easier. Make sure the nuts are tight (don’t over tighten, you will break the tail light) to complete the water tight seal.
- Reconnect tail light electrical connector
- Perform another BMW E90 trunk leak test with the garden hose to confirm everything was reassembled correctly.
- Reassemble trunk liner.