It is well known in the automotive industry that the first years of any manufacturers' new models will leave the factory with some problems. These problems tend to be the type whose cause is difficult to predict, which makes sense because, otherwise, they would've been taken care of in the design stages or at the factory. BMW has relatively few of these types of problems, but nobody is perfect. For the 1992 model year, BMW introduced a new 3 Series based on an entirely new chassis called the E-36. A new car calls for new problems, and this one is no exception.

An interesting phenomenon is occurring in E-36 BMWs built in June ë94 and before and is causing a variety of customer complaints. What happens is water enters the E-box, which is where the Powertrain Management ECU lives. If enough water enters this area to get into the ECU, then mayhem begins.

The symptom most commonly noted by the customer is that either the "Check Engine" or transmission warning lights comes on. A variety of driveability problems may be associated with water in the E-box as well, or the car may simply no longer run. I have even encountered a water-damaged ECU that caused a draw on the battery that would drain the battery overnight.

So, how does the water get into the E-box? The E-box adjoins the intake plenum cowl for the heater/air conditioning blower fan, which is open to the elements to allow fresh air to be drawn in by the blower. Rain water is expected to enter this plenum, so there are drains at the bottom of it. Under normal circumstances, the drains are adequate to handle any amount of water encountered in nature; however, if the volume of water is too great or the drains are clogged with foreign material, the plenum can fill with water, which then flows into the E-box. If enough water fills the plenum, it may also drain into the passenger side foot well, but it will often go unnoticed because it ends up under the floor mat.

This is a good indicator of the presence of this condition, and the only other way is to open the E-box itself and see if it looks like water has been in there.

The solution for this is to add a third drain hole to the plenum housing and use an improved style of drain hose. There is a Technical Service Bulletin out discussing the repair procedure, which I'll outline here.

Repair Procedure:

To gain access to the area of the firewall where the new drain will be located, you will need to remove the plastic screen covering the plenum, the plastic portion of the plenum mounted on the firewall itself and move the wiring harness out of the way. The cylinder head cover (valve cover) also needs to be removed for clearance. Cover the exposed area of the head with some shop cloths to keep it clean. There is some insulating material on the firewall. Carefully pull it back and move it out of the way. Remove and discard the right side plenum drain hose at this time.

You will notice that there is a circular area stamped in the firewall just above and to the right of the original drain location. That's where the third drain hose will go. Because of space constraints and the need to keep metal shavings out of the engine, BMW suggests we use an Electrician's Conduit Punch (a knockout tool) to make the hole for the new drain. Also, because of limited space, it's better to have two conduit punches (one 1/2" and one 3/4"). Use the 1/2" punch first because it requires only a 10-mm pilot hole to be drilled, which is easier to accurately locate and will produce fewer metal shavings than the 3/4" hole required for the 3/4" punch. Note that the size these punches are named by is not the size of the finished hole, but by the conduit they are intended to be used with (the 3/4" punch makes a 28.2 mm hole).

BMW states that hole saws are not recommended for this procedure. Once the new hole is made, clean up the rough edges with a file and shoot a little primer on the bare metal to seal it. Then cut a matching hole in the firewall insulation. Hopefully by now, the two new style drain hoses you ordered will have arrived, and you can install them pointing 30° to the left of straight down. Reinstallation of the remaining parts is the reverse of removal.

If nothing else, this should serve as an argument for the importance of technical service bulletins in efficient car repair. They serve as terrific sources for information from the manufacturers, especially concerning problems unique to their cars.