Bad Knock Module?

[nextboat1]

Hello,
I have a Mercruiser 5.7 with a TBV ignition and a mercrcarb. I have been having issues with the boat running seemingly rich at idle, as well as fouling plugs and "sputtering" or backfiring while trying to plane. It will eventually get past this, but still feels very lethargic in the higher rpm range. I first thought fuel, so i drained and replaced. Checked fuel pressure and it looked good. Had the carb rebuilt, same issue. Replaced coil, plugs, wires, cap, rotor, and ignition sensor, no dice. Verified good spark with tester and 12v at coil. Checked compression, looked great. I then tried to check timing and when I grouded the purple/ white wire there was no change in rpm or engine tone like I thought i would immediately notice. So, before going any further, I looked in the manual and it stated that I should have 8 to 10v on that wire and when I checked there was 0. I tried disconnecting the knock sensor, no change. Would it be possible for the knock module itself to be bad causing the timing to retard enough to cause my symptoms?

 

[Fun Times]

It appears you'll want to be checking the purple/white wire that connects to both the ICM and knock module.. Pins C on the Knock module and possibly pin F on the ICM (See page 4B-14) Due to it appears you aren't getting a response when grounding the service purple/white wire. Also you'll want to remove, clean, inspect all the wires, wire pins & connectors at both modules for any damage or wire pin backing out of the connectors....It seems you may be loosing a signal or power to one of those module wires. Try checking them for power, ground and ohms check them the best you can.

Yes one test I believe would be that you should see around 8 - 10 volts at the knock sensor blue wire connector by connecting a multimeter to it.
Another test is you should see 8 - 10 volts at the purple/white service test wire with using an multimeter. Connect red lead of meter to service wire and black to ground with engine running. Then there's another more accurate test you need to do found inside service manual # 24 page 4B-16 for the knock system.
Basically with the meter still connected to the service wire, and using a non powered (standard) test light, rev the engine up to 1500 RPM and with the test light connected to + 12v battery power, rapidly touch the blue knock wire connector pin and watch to see if the meter changes in volts from what you initially seen in the first test of the purple/white service wire.

Let us know what you come up with, good luck.

 

[nextboat1]

Made it back down to the boat today. Checked voltage at the purple/white wire and got 5 volts. I figure that i must have had a bad ground on my volt meter the other day. I then verified that I have 12v at the purple wire on the knock module connector. I disconnected the blue wire from the knock sensor and still had no change. I disconected the plug from the knock module, started the boat, no change other than I had to increase the throttle to keep the boat running.