Here is a little summary of how the knock system works. This is some of the knowledge that trained techs learn.
There are 2 different systems for modifying spark advance based off of knock sensor input for Volvo Penta MEFI 4 engines. The first is the knock system. It is active above idle after a minimum temperature and run time have been met. It can retard timing up to approximately 10 degrees based on input from the knock sensor. The other system is the octane rating system which only operates in the upper 1/3 of the power band. The octane rating system fine tunes the spark advance under high RPM high load conditions. The octane rating system works as follows. The MEFI 4 system has 2 spark tables, one for high octane (approx 93) and one for low octane (approx 87). Once the engine is above a certain temp, a run timer has timed out and the engine is in the upper 1/3 of RPM and load rating for the engine, the octane rating system will fine tune the spark advance to obtain the optimum spark timing for that particular operating condition. The 2 tables are based on RPM and MAP.
As an example, one engine’s high octane table may show 29 degrees of spark advance at 4600 rpm with 3” of vacuum in the intake. The low octane table for the engine’s same operating condition may show 26 degrees of spark advance. Once the octane rating system is active, as the knock sensors show that the cylinders are knocking, the ECM will step timing down from 29 towards 26 degrees to make the knock go away. After the knock is gone, the system will start advancing the timing back towards 29 degrees until the knock sensor just starts to indicate knock. The system will always try to maximize spark advance to where knock just starts to be indicated.
All of this shows up on the scan tool and Diacom under octane rating. 100% means that the value from the high octane table is being used and 0% means that the value from the low octane table is being used. Obviously, ratings between 0 and 100% are using values between the high and low table values.
None of this will set a code. As mentioned earlier, a code 44 is only set when little or no activity is seen on the knock circuit.
This is one of the reasons why an EFI engine can out preform a carb engine. And also why you should fix this system rather then try to disable it.
edit for added info.
And remember that this is a MEFI 4, it’s not a very smart computer. A catalyst engine does this and tailors the fuel mixture to EACH cylinder!
