spark knock after TB job
 

90 S4 automatic

I replaced TB and water pump last weekend due to leaky water pump. I now have a persistent spark knock/detonation under acceleration. I rechecked cam gear positioning relative to TDC and it all lines up correctly. Distributor cap and spark plug wires are all snug and correct. Could I be slightly out of time due to moving the cam gear/rotor around a little while installing the belt, still be on the correct teeth but be slightly off on position?

The hall sensor connection behing the passenger side cam gear was unplugged during the job, since the wiring runs through the middle of the timing belt. It is plugged in, but I wonder if maybe some of the wiring is broken since it looked pretty brittle to begin with. If the hall sensor is not working correctly, does it retard the ignition and would this cause the spark knock condition.

Any other thoughts/suggestions?

I really know very little about the later cars as far as specifics of timing; do the distributors have any method built in for adjustment at all? I've worked on other cars that use a Hall effect driven timing that have a "base" timing adjustment, but even if this adjustment was slightly off the electronics would still make proper timing.

As I understand how this system works, the sensor signal is sent advanced of maximum timing, and retarded from there. Seems like a failed sensor would give horribly retarded timing and loss of power, spark knock is normally a product of excessive advance. Not knowing what the sensor mounts to, is there some sort of bracket that may have been moved during the service? Certainly if the mount position of the sensor was somehow off-spec, that could throw the timing off.

Did you position the distributor AND rotor correctly during reassembly? If the rotor is out of range the computer cannot adjust the spark far enough to stop the detonation.

Dennis

I marked them before I removed them, but I'll recheck.

I pulled the plog behind the passenger side cam gear( hall sensor?) and the wires were a little frayed and the largest one was missing pieces of it's protective jacket. I painstakingly worked small strips of tape around that wire for some shielding, thinking it might be picking up interference.

It runs much better now, so either it needed the tape, or I plugged it back in better.