Expansion Tank Over Expansion

It’s been some time since I’ve made an actual post, aside from just photos, time to get cracking. You may recall that during my last track-day event at MSR-Houston the M3 Sedan‘s cooling system decided to fail at the least opportune time. If you don’t, here’s a recap:

Note: The part covered in this post is officially called “Expansion Tank” according to Real-OEM and other BMW repair documents I’ve come across. It may also be refered to as a “coolant tank”, “coolant reservoir”, “radiator reservoir”, or other similar terms.

I had driven the car the entire 220 miles from my home to Angleton, TX, only to be welcomed by a “coolant level low” advisory on the OBC as I pulled into the hotel parking lot. Strange, I had just given the car a good inspection and topping off of all fluids the day prior. I pulled into a spot, popped the hood, and was greeted by a fine stream of blue coolant spraying from the bottom of the coolant expansion tank.

Unfortunately I was far from home, it was nearly midnight, and I had a paid for track-day to attend in the morning. Time to channel my inner-MacGyver and jury rig this car, all I need is some chewed bubble gum and a paper clip. At the very least I had to make it road worthy so I could get home, I really didn’t want to pay another ridiculous tow fee. With it being so late at night, my only option was to hit Walmart and look for a means of patching the leak.

Fortunately Wallyworld was just a half-mile down the road, so I topped off the tank with some water and limped the car over. The leak was small, so I wasn’t hugely concerned about overheating. I hit the hardware aisle to see what I could find. My first thought was good old JB Weld, which they had in stock. But hanging right next to it was JB WaterWeld putty… hmm, this might just do the trick. It claimed it would withstand the temps and pressures I’d expect to see, and the putty consistency might make it easier to apply and stay in place. I threw it in the basket, then picked up some sand paper to rough up the application area, then headed back to the hotel parking lot.

Now when I normally travel to track days, I’m either towing the car and have a truck full of tools, or I at the very least pack a “go-bag” with the bare essentials. Guess I wasn’t thinking too clearly, because I only brought along items for bleeding brakes (wonder why) and cleaning windshields. I didn’t even have a screwdriver, d’oh!

Fortunately the night manager at the hotel counter was able to rustle up a flat head from one of the junk drawers, things were looking up. Now I had to go about pulling that coolant tank. There is certainly a proper, by-the-book way of R&R‘ing an expansion tank, then there is a way of doing it in a hotel parking lot with only a head-lamp and rusty screwdriver.

Eventually I wrangled the tank out, spilling whatever fluid remained (please do not tell the EPA.) Once out I could see there was a stress fracture or split about two inches long down one corner of the tank. I used some brake cleaner to clean off the surface, which was slippery from the glycol coolant. I then sanded the surrounding area to provide a good bite for the JB Weld and cleaned again with the brake cleaner.

The JB WaterWeld putty comes in a cylindrical plug, with the consistency of a clay bar. Like all epoxies, it has two parts; with a hardener as the inner filling of the plug. Simply slice off the needed amount, then kneed it until the color is uniform. I then stuck the putty over the crack, quickly flattening it and adding more layers. I attempted to feather out the edges as best as possible, the more surface area for adhesion the better. It was a bit difficult to keep it from sticking to my hands more than the plastic, but I eventually got it down. I had an hour to wait for it to cure to handling hardness, so I took it inside for a little better temps/humidity.

Now in the wee-hours of the morning, I set to re-installing the tank. Everything went back in smoothly, and I filled the tank back up with a gallon of distilled water. I left the car sit overnight to allow the epoxy even more cure time before I ran the engine, which would pressurize the system. A few short restless hours later I was back out in the lot, warming the car and bleeding it. I suppose I was lucky, but I didn’t have too much difficulty getting the air from the system.

I continued to check the coolant level and repair throughout the day, after every track session. Everything held together nicely. I even opened the bleed screw a couple times just to verify there wasn’t any air trapped, but it was fully bled. After a warm-up, four 30 minute sessions, and the better part of the “happy-hour” session it was still going strong. I was amazed to see that the JB Weld held, it has earned a spot in my track “go-bag”.