What’s Clunking on the BMW E46 Project Car?
Your fearless Editor begins to uncover why his project car was so cheap to buy.
Welcome back to another installment of Project BMW E46, one of many automotive fascinations that takes all of my money and makes me question my life choices. To get new readers up to speed, here is the story of how I bought a $1,500 BMW E46. This is my fourth car purchase that cost less than $2,000, for better or worse. After buying the car, I had to get it through a smog inspection so I could register it. I also spiffed up the 19-year old paint, which makes the car (from a distance) look much nicer than it actually is. Afterwards, I did some basic maintenance, because who knows when these were last changed.
That brings us to the current installment, and current predicament with the car. When I bought the car, there was a distinctive clunking sound coming from under the car when shifting. It was especially prevalent in the lower gears when there’s more twisting force being applied to the drive line. If I shifted very, very carefully, it was mostly mitigated, but I knew that something was wrong.
E46’s have a factory defect where the sheetmetal that attaches to the rear subframe is too thin and not sufficiently reinforced. What happens is that through a combination of old age and hard driving, the rear subframe can actually tear away from the sheetmetal. When this happens, things get ugly. It is repairable, but it’s not for the faint of heart. Before total failure occurs, symptoms are present to help diagnose the problem. Specifically, a thunking, or clunking sound from the subframe wiggling around. Or, visible stress cracking in the rear body structure that ultimately lead up to the subframe tearing away from the chassis.
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This isn’t my first rodeo with cheap cars, and my gut instinct was that the car had a shot differential bushing. Before I bought the car, I jacked it up and took a flashlight to the rear suspension and subframe area. Everything seemed in good order, so I took a risk and bought the car.
Well, nothing left to do but start tearing the car apart to diagnose the mystery thunk and clunk. Thus far, the E46 has been pretty easy to work on. It’s not my first BMW, and I doubt it’ll be my last, but all the same, most things are quite straightforward when assembling or disassembling them. Step one, was the get the car up on stands. Step two was the remove the exhaust from the car. On the E46, the driveline is covered by a giant heatshield, which is obscured by the exhaust. It’s all one piece from the headers back and comes off in about 20 minutes.
Exposing the issue, one way or another
With the exhaust and heatshields removed, the entire driveline, from the transmission back was exposed. Although I was sure the issue was differential-related, I still looked at everything else, just in case. The transmission mounts were old and squishy, but not torn or broken. The driveshaft looked good, with the guibo intact and a center support bearing that looked to have been replaced at some point. Additionally, when turning the driveshaft, no thunks, clunks or binding was present. The culprit has to be the differential. Looking at the diff bushings revealed no noticeable defects, though it’s hard to get a good picture when everything is assembled in the car.
So everything looked good. But, I wasn’t deterred, as I had a diagnostic solution to oust the trouble maker. I had my room mate hop in the car, start it up while on jack stands and simulate driving by going through the gears. All it took was one good shift from 1st to 2nd and issue became immediately clear. The rear differential bushing, a common E46 problem, was allowing the diff to move a good four inches and bang into the subframe. That’ll do it.
Diagnostics complete, it was time to wrap things up. That is, of course, after some open header action, a requisite when the exhaust is taken off any car. My plans for this car include a thorough and comprehensive overhaul of everything underneath this E46. Diff bushings are just another thing to add to the list. Stay tuned for the next episode as I make a modification game plan and timeline for the car. Track time is coming soon, so it needs to be ready to rip.
