Engine Tapping (2004 R32)

[ARJ0019 0]

This is a 3.2L in a 2004 VW R32. The engine has 130k miles. Long story short, the car absolutely hates rain. It was raining very hard a few weeks ago and I drove through a deep puddle. Car does its usual engine shutoff. Missfires when trying to start it until all the water is gone. This is normal, atleast for this car. However this time it was different. I drove to a gas station, and when I got out I heard a tapping noise.

 

I measured the tapping to be around 280 taps/min at idle. This is a reasonable/expected number, since the car idles at 600rpm, which means the cams are rotating at 300rpm. Taking into account error in measurements, you would think its related to the cams. Additionally, the sound appears to be from the cylinder head on the passenger side (location further confirmed using a stethoscope).

 

I took the car to a Bosch mechanic, asked them to fix the sound and provided lifters since that's what I suspected. $1400 later all they did was put in the lifters and didn't actually check to find the sound. Turns out the lifters were not the issue. They later said they suspected a valve guide, but they never removed the head so I'm suspicious of that diagnosis. There is another reason that I am confused. The sound is not only dependent on rpm, which is what you would expect if the bad part was directly driven by the crank. Instead, the sound is also throttle position dependent (obviously throttle position changes manifold pressure/vaccum). If I'm casually accelerating the sound will be ticking at a reasonably slow pace (barely slow enough to count each tap), when I then take my foot of the accelerator the sound changes to a very fast tap.

 

Usually a load and rpm dependent tap would hint to a worn wrist pin. However, in that case the sound should be less when decelerating, and more when accelerating. For me its the opposite. Additionally, a worn wrist pin sound would normally become quiet when removing ignition from that cylinder, but when I removed the coils (preventing ignition) the sound was unaffected.

 

I removed the accessory belt and the sound continued, so its not any of the accessories. I also used a stethoscope to check the evap valve and the rotary intake valve (variable length runner valve), both were operating as intended and are not the cause of the sound.

 

For the engineers out there, I unfortunately cannot gather any good data on this. I've tried using my phone accelerometers to generate a FFT, but the sound does not cause enough vibration and the sensors only pick up the higher intensity signals. I also tried using the microphone to get a sound spectrum. However, being that it's such a low frequency the fact that it's not a true wave makes the results worthless. If it were a higher frequency it would probably still show up even just being a on/off signal, but not at the low frequency of idle.

 

So whats your opinion? What part in a VR6 could make this noise?