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I bought my BMW 330i ZHP with a clear dash and a clear heart. I test drove it a few miles near the seller’s house, then loaded it on a trailer to take home. When I got home, the first thing I did was unload the car and get a burger at In-N-Out. Suddenly, after I ordered my double-double with no onions, I felt a few stumbles from the normally smooth straight-six and a flash of yellow appeared on my dash: Service engine soon. Ugh.

E46 BMW ZHP dashboard detail showing service engine soon light.
Chris Rosales

It drove normally at speed, had OK power if a little diminished feeling, but idled poorly. It coughed heavily every few seconds and I knew something had gone wrong. It also dipped the idle heavily when the clutch was depressed after any sort of load. I know the typical E46 problems and checked codes to confirm my suspicions of lean codes, which came in the form of a P0171 and P0174, lean codes for both banks. Immediately, I got a smoke tester and checked for vacuum leaks.

At first, it was excellent vibes because the most obvious vacuum leaks were there: the air intake boots and the “F” connector that connected to the intake boot. I fixed those leaks, started the car, still had a stumbling idle. Next step: Use an OBD2 reader to monitor some engine data, specifically fuel trims. Fuel trim data will tell how much fuel the ECU is adding or subtracting from the preset fuel map, expressed by a percentage over the long and short term. A positive percentage is the ECU adding fuel and a negative value is removing fuel. Long-term trims are the better diagnostic tool as short-term trims can vary heavily by design.

BMW F-connector leaking smoke from smoke test.
Chris Rosales

Initially, I saw some positive numbers which corroborated my lean codes, around 20 percent long-term, which is a huge fuel correction. I investigated further and looked at more parameters. I read that the mass airflow sensor (MAF) can soft fail by reading low, so I looked at my grams per second at idle and it was indeed low. I was running around two grams per second where the car should be running at 2.8 grams per second. I had a weak MAF for sure.

Before I took a trip to the parts store, I spent a few more hours making sure there were no vacuum leaks with my smoke tester. I also pulled the ignition coils and inspected them and inspected the spark plugs. The coils were in great shape but the spark plugs looked old with burnt porcelain. Add that to the list. 

Then, I removed the DISA valve on the side of the intake, which is BMWs version of variable-length intake runners. On my first inspection, I couldn’t find anything wrong with it, so I checked it off the list, but I missed a crucial test. More on that later.

Disassembled E46 BMW engine bay for inspection of vacuum system.
Chris Rosales

Finally, I reached behind the intake manifold to verify that the three vacuum ports were still capped off and checked under the intake manifold for any cracked lines from the CCV system. If they were faulty, the smoke test should have shown it, but verifying it doesn’t hurt. Luckily, my CCV looked like it had been replaced recently.

My local auto parts store had two different kinds of MAF in stock and a set of the correct spark plugs. I did some research and read some pretty strong opinions on E46 MAF sensors. Many claim that the VDO unit is the only one that works properly, while some have had luck with higher quality aftermarket units but most seem to switch to VDO after continued issues. There was a Delphi and a VDO near me. In the name of science, I bought the Delphi first for around $250, which is incredibly expensive for a MAF.

Mind you, Delphi is not some low-end eBay parts maker. This is a legitimate tier one parts supplier for a few different automakers. My curiosity got the best of me and I installed the new MAF right in front of the parts store like the sad BMW tragic I am. I even used my official BMW screwdriver from my service kit. 

For all of that clownery, the car ran significantly worse at idle with the Delphi unit. I broke my OBD scanner out in the same parking lot, deepening the circus aesthetics. Now the fuel trims read massively negative and the engine stumbled harder at idle. I left the parking lot for home, mainly so I could stop looking like a stereotype.

Weirdly, it had a painterly application of fuel at the low end which resulted in copious low-end torque but fell flat near the top. Even though the negative fuel trim meant that the car was now running rich, the car still refused attempts to clear the code after a good few days of driving. I couldn’t fathom how the car ran rich for a solid 100 miles and decided that it was still running lean, but I suppose that is the BMW lifestyle. Mercifully, the new plugs solved the biggest idle stumbles, but a consistent roughness remained, especially at cold start.

E46 engine bay.
Chris Rosales

This time, I removed the MAF at home and put the stock unit back in for the trip back to the parts store. I swapped the $250 Delphi for the $385 VDO, which feels like outright robbery. I could have gotten it cheaper online but I preferred to have the parts same-day rather than paying for overnight shipping. The information isn’t clear, but the OEM MAF is said to have a platinum sensing element that degrades over time. The VDO is allegedly the only MAF built with the correct element.

I’ll be damned because the forums were right. The annoyingly rough idle was suddenly gone. Mass airflow read at the exact 2.8 grams per second. There was still some strange running under load and the cold start idle remained rough. Yet, my fuel trims were closer to zero than ever. Instead of my starting 20 percent negative trims, I had worked my way down to ten percent negative. The car would still trigger a check engine light around ten percent fuel trim correction, so I wasn’t quite there yet. I still had a ways to go.

So far I’ve checked off MAF, vacuum leaks, spark plugs, coils, and CCV. All that was left was the fuel pump and the weird E46 fuel filter/fuel pressure regulator combo. There is an easy test for the fuel system, however — all it takes is a $20 tool from Harbor Freight. I picked a fuel pressure test gauge up and did some quick diagnosis.

You’re looking for two things: the specified fuel pressure and how long that specified pressure holds after priming the fuel pump. The pressure gauge comes with a fitting that fits onto a simple Schrader valve on the fuel rail, just make sure to use Teflon tape to prevent leaks. Hop into the car and turn the key to ignition on without starting the car. The gauge should read 50 psi and hold that pressure for 30 minutes. My car came up to 45 psi and quickly bled down to 30 psi in a single minute. This is a smoking gun for a bad pressure regulator. 

E46 BMW fuel pressure testing demonstration.
Chris Rosales

If the pressure held while staying at the low 45 psi, that is a case for a bad fuel pump. Generally, E46 fuel pumps soft fail and cause the car to shut off at quarter of a tank before any other symptoms. The pressure rapidly bleeding off meant that I headed to the parts store once more for a new fuel filter assembly, which I quickly replaced underneath the driver’s door of the car. Initially, the car ran incredibly rich and felt like it had a burble tune, then settled into an improved idle. Finally, my long-term trims trended towards four percent negative. My fuel pressure gauge read a steady 50 psi as specified by BMW.

A noticeable amount of power got restored thanks to this, with the engine singing happier than ever. I drove it around for 200 miles like this but that check engine light remained stubbornly on. With nothing left to check, all I had left was retracing my steps. I spent several nights smoke testing the ocean blue Bavarian with no success, with it running nearly perfectly but still just a touch strange and with an inconsistent rough cold idle. 

There was finally one tired night where I stumbled across an old Pelican parts guide on how to test for a bad DISA valve. As I read, I quickly realized that I had one more thing to test on my seemingly good DISA valve. There is a vacuum port at the bottom of the DISA actuator that can be capped with your thumb while the flap is held closed. I performed the test. I’m sure you can guess what I found.

The flap is supposed to remain mostly closed and hold with your thumb over the vacuum port. My DISA opened slowly, indicating an internal vacuum leak with the actuator. My DISA was toast this entire time.

I got to the parts store 20 minutes before closing. They had a new DISA valve in stock, which I tested with my trusty thumb. It held pressure just fine. I installed the new DISA at home, and suddenly my issues were gone. Not five minutes after installing this and running it did the BMW accept the code clearing request, and suddenly I had a clear dash. My fuel trims were functionally zero. I’d done it. The car pulled smoothly and powerfully, my idle was gorgeously smooth, and my dash was a clear sea of amber with no annoying yellow. 

If you get a lean code back from your E46, I recommend taking up faith. Or, take the lessons I learned here and apply them to your journey. I can say confidently that the end result is worth it, there was a good gut of power left on the table thanks to these failed parts. 

Just make sure you inspect things correctly and don’t swap parts in the parts store parking lot. Get a chai tea and do it at home.

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