Repair Time for trunk leak from tail light – 1 Hour
A trunk leak caused by one of the tail lights not sealing correctly is a very common issue in the BMW E90/91/92. Fortunately the diagnoses and repair are simple, saving you an expensive trip to the dealership.
In June of 2018, the sound system in my 2013 BMW E92 M3 went completely silent when I hit a pothole on a rainy day. I unfortunately diagnosed the problem as a water damaged amplifier due to a trunk leak (see my article Amplifier Water Damage Repair). Approximately an inch of water had accumulated in the driver’s side trunk well where the amplifier is mounted.
After cleaning up the mess, I 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 drivers side trunk well behind the tailight.
I decided to do a water hose test to see if I could get the trunk leak to reveal itself. With the trunk lid closed I ran water over the tail light, focusing near the top of the light and trunk lid to simulate rain water run off. Bingo. When I opened the trunk and looked in the trunk well, I 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 I 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 I was willing to pay for BMWs engineering miscue. With a little ingenuity and about $3 spent at Lowe’s, I was 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.
Parts (click on part link for more information)
(1) Left (driver’s side) Tail Light Seal (BMW Part # 63217174405)
(1) Right (passenger side) Tail Light Seal (BMW Part # 63217174406)
(3) 5/16″ x 3/4″ x 1/16″ rubber washers
Tools and Supplies (click on tool/supply link for more information)
(If you have not already removed the trunk side panels and liners to inspect for water damage you will also need the following tools and supplies)
Body trim removal tool
Trunk side liner removal
- Left side (driver’s 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 (passenger 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. 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 10 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 trunk well behind the driver’s side tail light on my E92 M3.
Tail light removal and inspection
- 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 my car…it is very clear where my 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.
Tail light repair
- 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 Lowes 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 overtighten, you will break the tail light) to complete the water tight seal.
- Reconnect tail light electrical connector
- Close the trunk lid and leak test tail light with hose to confirm everything was reassembled correctly…and to give you peace of mind.
- Reassemble trunk liner.
If you have any questions or comments, please feel free to comment on this article or contact us. And congratulations for Doing It Yourself!
The Repair Difficulty Level displays graphically how challenging the repair is, from easy to advanced. Easy repairs usually require very few tools, have short repair times and do not require jacking and supporting the vehicle. As the difficulty level rises, expect the repair to demand more time, specialized tools, and better understanding of mechanics to complete the job.