Vacuum leaks can be notoriously difficult to diagnose, as they can range from small cracks in a hose or intake pipe, to a clogged crankcase vent system, to an improperly sealed oil filler cap. This is largely the reason I have ignored my E39’s minor leak, because diagnosing it would mean removing the whole air intake system to check everything over. Plus, barring an occasional stumble at idle, the car seemed to run fine. However, last week, when it started tripping the check engine light and logging fuel trim codes, I decided it was time to sort it out.

The repair was simple – I had a small crack in one of the crankcase vent tubes, as well as in the air intake pipe. In the spirit of bangernomics, rather than buying new stuff, I simply used a bit of sealer and taped everything up; with such minor cracks, it really is not necessary to replace things at this point. While I had everything apart, I also gave the throttle body a good polishing.

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On Saturday morning, I got in the car to set out for a drive, but as I pulled out onto the road, I noticed the car had no power. It just wouldn’t go, and when I put my foot down and it completely bogged. Turning around, I parked in the driveway and hooked up my diagnostic tool, which pulled a code for the inlet camshaft position sensor (code P0340, if you’re interested). At this point, I was so frustrated that I slammed the bonnet shut and headed out in the Leon.

When I opened the bonnet this morning again to look for the part I was going to need to buy, I noticed this.

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Yes, that’s right. I forgot to plug the sensor back in when I was reconnecting everything.

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With a mixture of embarrassment and relief, I reconnected the sensor, started the car, let it run for a bit, and then took a short drive up and down the main road. The car runs fine now, and all the codes are cleared.

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There are two things to take away here. First, it is quite something that one little sensor can completely cripple a car. This particular sensor sends readings about camshaft positioning and timing to the ECU, which then uses that data to calibrate everything else, like timing and fuel injection settings, accordingly. Given its role, I am surprised the engine even ran at all. Second, when you do a job like this, double-check that you’ve reconnected everything!

Hey, we all make mistakes.

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