I Fixed My BMW’s Fuel Delivery Issues But It Continues To Vex Me

One day last year, I decided I was going to buy myself a car. I’d grown tired of my Mercedes E240, which had been fine, if uninspiring [Ed note: even with its angry eyes?]. I spent the day driving a series of terrible Honda HR-Vs, and then something on Facebook Marketplace caught my eye. A 2007 BMW 320D for just $3900. Too good to be true; seldom anything past 2002 was selling for those prices, let alone a BMW in good condition. I had to check it out.

I bought the car the next day, having knocked a further $200 off the price. $3700 all in, and that’s Australian dollars, too. The seller claimed it was so cheap for the lack of working air conditioning, which bothered me not in the winter. I felt myself having secured a bargain, despite one red flag. It kinda looked like the guy that sold me the car was running a car flipping operation out of a house and without a license, by and large forbidden by state laws.  But I wanted my cheap BMW, and it drove great, so I bought it. Who could say no to a 2.0-liter turbodiesel  rear-wheel-drive sedan for that kind of money? Not I.

It was a day later that I first reckoned with its issues. A barely-perceptible RPM flicker at cruise. I Googled and Googled and Google led me astray. I spent the next few months changing transmission fluids and filters and seals until eventually I found that a dose of Lubeguard’s Instant Shudder Fixx solved the problem for less than $30.

Of course, that was after I’d been to hell and back. Using a third-party transmission fluid that was supposed to be compatible with ZF Lifeguard 6 led to hard shifts and transmission warning messages. Getting the right fluid in has largely solved the transmission problems, though it still jams in first or second sometimes under a heavy right foot, with the engine soaring to the rev limiter until it eventually grabs the next gear. I suspect that it’s down to some of the wrong fluid still being in the transmission. ZF recommends multiple fluid changes if the wrong fluid is used, as the transmission and torque converter tends to hold on to a few quarts even with the drain plug out. At the same time, I’m not exactly eager to spend another $250 on more OEM-spec fluid, nor go through the excruciating procedure to get the levels right.

So, transmission problems solved, the car drove great again, right? Well, no. The car started exhibiting an obvious RPM fluctuation at idle, of around 100 rpm. This didn’t seem like a transmission thing, and engine diagnostics suggested it wasn’t either. Hooked up to a laptop with a CAN bus tool, I could see the fuel pressure fluctuating all over the place, in rhythm with the RPM fluctuations. The car was also often hesitant when you pressed the accelerator pedal, often taking a second or so to respond with more power. Given the diesel engine lives and dies by fuel flow, these issues seemed likely to be related.

Genuinely can't figure out if this is a fuel rail pressure sensor issue, fuel delivery issue, or wiring issue.

I am exhausted, though. pic.twitter.com/HN33jcyXJU

— Lewin S. Day (@rainbowdefault) August 21, 2023

My first suspect was the fuel pressure sensor. A replacement was only around $150, so I ordered a third-party model as it shipped the soonest. It seemed I was cursed for not going OEM, though, as the new part seemed to present a different output voltage range to the existing sensor, and the car wouldn’t run properly with it installed. Sourcing what I thought was the right Bosch part number, I found I had the same problem. I simply couldn’t figure this out; it was like there were different versions of these near-identical sensors that had different output ranges. That, or something truly weird was going on, electronically speaking.

I found one other person who reported a similar issue with an N47 engine in a 520D. Instead of displaying the usual 0.5 volts with the fuel rail pressure at zero, his displayed 0.3 volts. So did mine! However, his problems were down to an empty fuel tank caused by bad fuel level sensors. When running normally, his sensor still displayed those voltages, and ran fine. I decided to temporarily rule out the fuel pressure sensor and kept hunting for an answer. In any case, if you know if BMW uses a weird 0.3V-zero diesel pressure sensor, unlike most others? I’d love to be told I’m not a crazy person.

I kept running diagnostics on the car and diving into forums to figure out what was going on. Eventually, a diagnostic procedure suggested the fuel quantity control valve might be at fault. This is a valve controls the quantity of fuel that flows from the low pressure pump to the high pressure pump. Controlling the quantity of fuel reduces load on the pump, according to diagnostics firm Auto Ditex. It also keeps fuel temperature lower, because less fuel is being needlessly pumped to high pressure and then sent back to the tank when not needed by the injectors.

The fuel quantity control valve being faulty made a lot of sense. My research told me that in cold temperatures, below 59F or so, the engine doesn’t use the quantity control valve, simply leaving it in the fully-open position. I’d noticed that the car would often drive much better when starting it on a cold morning, at least until it warmed up a bit.

Investigating further, I decided to unplug the valve and see what happened. This was no mean feat, with the valve buried at the back of the engine under the intake manifold. When I started the car with it disconnected, though, I was amazed. Sure, the car was in limp mode and running at reduced power. And yet—no RPM fluctuation to be seen! Surely, this had to be it.

I didn’t really fancy changing the part myself; it looked far too difficult to reach. I wanted to get the air conditioning fixed, anyway, so I took the car to a mechanic. I asked them to investigate the fluctuation and hesitation issue for me while they were in there.

After three long weeks, they finally rang me to say they’d gotten the air conditioning sorted. Someone had been in there before trying to fix it, they explained, and made a total mess of the wiring. They’d had to reprogram several modules and put everything back as it should have been stock, but they got it done. I wasn’t really surprised by this. Over time, I’d found a few grease pen marks on various components under the hood. I suspected the vehicle might have been repaired over time with bits and pieces, and not always in the best way.

Beyond the air conditioning, the mechanic told me they found an issue with the fuel rail, and swapped it out for one off another engine that was going to be junked. I was busy getting made redundant that week, and just wanted my car back, so I took this explanation as valid. I assumed it was an issue with a pressure relief valve or something, and moved on.

In the weeks that followed, it became obvious that the fuel delivery issue hadn’t been solved. The car was still hesitating, particularly when pulling away from a stop. I vacillated between selling the car off and forgetting about it, but I couldn’t. I needed to know what was going on.

I bought myself a proper Bosch fuel quantity control valve, or at least I thought it did. It appeared to me in every way to be the real deal, but the hologram sticker code didn’t check out on Bosch’s site and it didn’t come with replacement fasteners, either.

In any case, I had to know if it would fix things, and I set out to do the swap. Only, I faltered. Between cables from the ECU, coolant hoses, and the intake manifold, access to the valve was near impossible, even with the cowling panel removed from the engine bay. I tried for half an hour on a hot day, and threw my hands up in frustration. No way would my tools get on to the bolts and free the valve. It was hopeless. Besides, I’d just started a new job (hello, Autopians!) and didn’t have time to be messing around with my cars (ironically).

Months passed and I found myself still stuck in the same position. I’d bought this BMW 9 months ago, intending to replace my Mercedes. And yet, I still owned both, because I could never quite trust it! I need to sell one or the other, and yet I couldn’t bear to sell the BMW without knowing what was wrong with it. I had to change that valve and see what happened.

I bought myself a new Torx socket set and found my smallest socket wrench, and had another crack. Focused, calm, and determined, I freed the first bolt. I could hardly believe it when the second came free, easier than the first. Within an hour, I held the valve in my hands, all thanks to having the right tools for the job.

I couldn’t believe it. This impossible task, achieved! Sure, it’s not that crazy or difficult a job, but for me, this was a big win after some very trying days spent tangling with that transmission.

Inspecting the part showed little obvious problems, but I pressed on regardless. It’s entirely possible that after 100,000 miles, the valve is gunked up or worn, no matter how good the fuel filters are.

And yet, reassembly remained. The first bolt found its thread just fine, but the second, I fumbled. Lost in the bowels of the engine bay, and try as I might, I couldn’t find it for the life of me. An interstate BMW dealership had the bolt for $15, but wouldn’t ship it. A British eBay listing had one too, for just $5, but shipping would push that to $40. Such a pain. I just needed an M5x18 Torx fastener, did it have to be so hard?

I headed to a specialist fastener dealer, and they had something close. An M5x16 with a T25 head, albeit a button head instead of a socket head. “Great!” I said. “I’ll take five.” 2 millimeters shorter (~1/16th inch), but no big deal. There was plenty of thread to do the job. It was all good… until I realized when I got home that these were security Torx bits, with a little button in the middle. The Torx sockets I’d bought wouldn’t work with them.

In the end, I had to go and buy another set of Torx security sockets that finally let me get the thing slotted home. I’m pretty sure I stripped the security fastener as I tightened the valve into place, too, so it was all or nothing. This had to work or the car was pretty much doomed.

To my joy and relief, it did. The car suddenly had its verve back. The nice punch of torque from the turbodiesel was right there when I hit the pedal, no longer coming in late or not at all due to a lack of fuel. The gorgeous handling of the 3 Series never left the car, but now I had the engine back to match.

It’s still not perfect; the transmission is still hanging in gear often enough that I don’t feel confident to drive the car hard. I’d hoped maybe fixing the fuel delivery issue would solve the transmission tripping over itself entirely but it’s not quite so. I’m tempted to try another fluid change to get the last of the supposedly “wrong” fluid out, but I know I’ll be mad if that doesn’t fix it. The other half of me is tempted to sell it and use the proceeds to whack new tires on my Mercedes and fix some worn suspension components.

For now, I continue to flip-flop on the matter and pay for both cars to stay on the road. So, grand Autopians, which way do I go? The older 1998 E240, with its ho-hum V6 but roomier interior, or the turbodiesel 3 Series, with its enjoyable handling and lovely engine, let down by an auto that’s not all there? I can’t decide, but maybe you can help me.

Image credits: Lewin Day