Project Car Diaries: How My Cheap Diesel BMW Has Slowly Driven Me Insane

The story begins with a lesson on making impulsive purchases. I had a rare free day one Sunday, and Facebook Marketplace turned up a pearler. A 2008 BMW 320d for the suspiciously good price of $3,900 AUD. Nothing else in that price range was newer than 2002. 

I drove straight over and when I saw the car, it was love at first (diesel) rumble. The 320d pulled well, handled well, and save for a non-operational AC and dull paint on the bonnet, it kicked ass. And with only 100,000 miles on the clock! I negotiated to $3,700 and picked it up the next day. I did some research overnight and reckoned I’d come out a champion—similar cars were selling online well in excess of $7,500.

Driving home, I was ecstatic, right up until I noticed the car’s first odd ailment. Cruising at 50 mph on the long drive home, I noticed a little perturbation on the tachometer. My speed was constant, but the RPM would rise and fall by roughly 100 RPM, over a period of a couple of seconds or so. I suspected the electronic accelerator pedal, but the problem remained on cruise control.

I suddenly felt the chill of anxiety—another sensation familiar to anybody who has bought a cheap car for fun. The “what did I just buy” shivers were intense. I figured the price was good because of the busted AC, an expensive fix in the E90 cars. Now, I suspected the issues were deeper. 

Suddenly it seemed like red flags were everywhere I looked. Flags that my rose-colored glasses had hidden when I first looked at the car and found myself enamored. There was grease marker on various parts under the hood. Radio presets from interstate saved on the radio. The fact that the guy I bought the car from was leaving town the next day, and seemed to have altogether too many cars at his house for a regular person. He was also remarkably practiced with filling out the paperwork for a private sale. 

Maybe I’d been had, I thought, but I’d gotten the car cheap enough that it wasn’t an outright bilking. Anyway, I was being fussy. Otherwise, the car was doing great, so I dived into researching the problem. I suspected some kind of fuel flow issue. However, the vast majority of forum posts out there suggested the issue was down to the transmission. Specifically, the cause was commonly attributed to the torque converter clutch, with the RPM variation being caused by the converter repeatedly trying and failing to lock.

Remedies varied, with some suggesting transmission fluid additives like Lucas Stop Slip or Lubegard Instant Shudder Fixx. Others suggested a transmission service, involving fluid replacement and a filter change. The latter sounded doable and more reasonable than squirting perverted aftermarket fluids into my BMW, so I set about sourcing the parts. I spent $200 on a replacement plastic transmission pan with the inbuilt filter and $150 on third-party Penrite fluid that claimed to be compatible with ZF Lifeguard 6 fluid required for the six-speed auto box. 

I’ve done filter changes a million times before, but this was by the worst I’d contended with. I got the BMW jacked up and dumped the original fluid, which looked old but not terrible. I was a fastidious little wrencher, and when I replaced the pan, I even followed ZF’s torque pattern for all 24-ish bolts. I felt like I deserved a pat on the back for that.

Filling was where I began to curse BMW. The transmission had to be filled from a port on the side of the ‘box. I’d bought an electric pump for the purpose, knowing that there would be zero hope of simply pouring the fluid from the bottle into the transmission. Brutally, my pump was too long to fit neatly under the car, so I had to fill a bucket with the fluid and hold it at a 45-degree angle while the pump filled the box. Once I’d gotten as much in as I could, I had to start the car to circulate the fluid throughout the gearbox before adding the rest. 

The transmission is considered “full” when a steady stream of fluid trickles out of the fill plug when it’s at a temperature of 40 degrees C (104 F). I’ll admit I just ran the car for a while and then topped it off, before closing it all up. I’d made a huge mess on the driveway in the process, but I considered the job a good one. A quick blast up the highway suggested the problem was fixed, and I was joyous!

Until the next day. The problem was still present and I couldn’t explain why. Then the transmission started to shift with a great bang from time to time.

A week after my meddling, I got a horrifying “Transmission Fault” message on the dash, with the car sticking in second gear in limp mode. My tender loving care had apparently made things worse. Had I trashed the car over a niggling fault?

I threw up my hands and took the car to a local independent BMW specialist. They indicated that the car was probably still okay since it drove in under its own power. For $500 or so, they drained the box on the suspicion my third-party fluid wasn’t up to snuff. They refilled it with the pricy official ZF fluid for the 6HP19 auto and got the level right. Apparently, doing the job properly involves one mechanic in the driver's seat, shifting gears every few seconds to circulate the fluid, while a second fills and checks the level below. I obviously hadn’t done that.

This service fixed pretty much nothing. The car still shifted hard and threw occasional fault messages that would clear on a restart. The car would often feel okay when cold, but when the car got warm, shifts would keep getting worse and worse as I drove on, as would the RPM fluctuations. I had no faith in the car’s reliability, though I did keep using it on a daily basis to try and diagnose the issue.

I relented and decided to try a tube of Lubegard Instant Shudder Fixx, wondering if some sticky goop would fix my problems. Of course, that stupid side-fill plug on the transmission got me again. I did my best to jack the car up so that the fluid would tip away from the fill port, and I had my very helpful partner run the gears to circulate the fluid. Regardless, as soon as I opened the port, transmission fluid spilled out. I presume all the additives did too. Again, I found no difference. 

I needed a car that I could trust, and I didn’t have one. I was frustrated. I was in despair.

I redoubled my research efforts. I learned the intimate differences between various transmissions used in the E90 BMW. Countless videos. Countless forum posts. Some relevant, most not. I eventually came across the perfect explanation: the transmission valve body seals. 

The main valve body seal is a rectangular seal between the valve body and the rest of the ZF automatic transmission. The plastic-bodied seal is known to crack and otherwise fail, leading to reduced hydraulic pressure in the transmission. This would explain the hard shifts and the failed locking of the torque converter, and it would also explain why the car was so much worse when warmed up. 

Swapping the seal involves dropping the transmission pan, and then the valve body. Then the rectangular seal and four others can be changed. Often, the “mechatronics sleeve” is changed at this time, too—a round seal that stops fluid leaking past the transmission’s electronic connector. I bought the lot for $170 and decided I would be fastidious about capturing the fluid that came out to save spending more money on bespoke European ATF.

I was amazed to find the job easy … at first. I got the valve body disconnected and dropped in just a couple of hours, with nothing fighting me along the way. I swapped out the seals, and started reassembly, popping the valve body back in place with a torque wrench for good measure. I actually smiled.

Then everything got tough. The new mechatronics sleeve, which seals around the electronic connector, was uncooperative. Try as I might, the new rubber seal was larger, and it just wouldn’t seat home. I literally fought it for hours. I had a bad slip with a screwdriver I was using for a lever, and little was working. Eventually, after a rest, one final big push got it in. I threw the transmission pan on, and saw the finish line in sight.

I just had to put the electronics connector back on and the job would be done. 

But the connector wouldn’t go in. I pushed and prodded and pulled, but it wouldn’t budge. Videos barely touched on this problem. For the handful that even showed it, their connector just slid in with moderate pressure on the rotating locking sleeve. Mine wasn’t moving at all.

I wrestled with it for two hours, utterly in denial. I knew I should check the pins inside the connector, but I wasn’t ready to deal with the horror that would imply. Eventually, I couldn’t ignore it any longer. As the clock approached midnight, I slid my phone up and took a shot of the mechatronic plug.

I had smashed the pins when the screwdriver slipped.

Cold, greasy, tired, and beaten, I broke down at the result. This made the valve body utterly useless. I couldn’t see a way forward. It was my lowest point for a long time. Somehow, in trying to fix a minor problem, I had entirely bricked the car. I gave up and went to bed.

Any plans I had for the next day were long ago dismissed. I had to do what I could to fix this. I spent two full hours trying to wrench the brand-new mechatronics sleeve out of the transmission so I could drop the valve body. I entirely destroyed the $50 seal in the process. I dropped the valve body and got my first proper look at the situation. Three pins, bent significantly off-axis, were stopping the connector from mating properly. 

With new needlenose pliers, I gingerly bent them back into position. Thus began the laborious process of reassembling everything again. 

I held my breath as it came time to push the connector home. A gentle press. Nothing. A deep breath, and then a little more pressure, and… snick. It found its home. The relief that washed over me was instantaneous. 

I suddenly had all the energy in the world to finish the job. I wouldn’t be tangling with the stupid fill method this time, either. I’d simply put the same amount of fluid back in that I’d taken out, since the mechanics got the fill level right last time. Plus, I’d pour it in through a funnel hooked up to the cooler lines. Work smarter, not harder!

To my sheer amazement, to this day, the car actually moved when I slotted it into gear. I couldn’t believe I’d saved the day after nearly destroying the valve body connector. Hilariously, though, my problem persisted. The hard shifts were gone, beautifully, but the niggling RPM fluctuation was still there at cruise. 

At this point, my obsessive research had turned up stories of a weird additive that was sometimes added to ZF gearboxes, called LifeGuard 1. No longer being available, some forum users talked about using Lubegard Instant Shudder Fixx instead. This time, I squirted the tube down the cooler lines rather than fussing with the transmission’s side-fill port, and it went straight in. Amazingly, after a little driving, the cruise RPM fluctuations were gone. 

Throughout the process, I had mostly been working on vibes rather than positive diagnoses. I was never even 100% sure the transmission was at fault; I merely read posts and assumed. Then I’d wrenched like a drunk lunatic to try and fix things. It was a miracle the car still worked at all.

My efforts had solved the hard shifting issue and the RPM fluctuation at cruise, but something still wasn’t quite right. The car would show a surging idle, particularly when first started, and was beginning to show an increasing hesitation when accelerating.  Weirdly, though, the car seemed to be very responsive on mornings when the temperature was below 55 F.

I researched harder than I ever had before, and gained access to BMW diagnostic software through a person only known here as Renegade. I started combing through sensor readouts, trying to find something, anything that corresponded with the issues I was seeing. I headed out on the road, with my partner, heretofore nicknamed “Data Fae,” calling out readings while I observed the car’s behavior. 

Eventually, something came to light. The fuel rail pressure seemed to be fluctuating like crazy. I dived down that rabbit hole, and I haven’t come back up for air just yet.

Got a tip on how to deal with stupid BMW problems? Let the author know: lewin@thedrive.com