496 ho knock sensor fault
 

So I have been chasing an issue ever since I cooked an IAC a couple months ago. After replacing the IAC, pig tail, and having whipple repair the pcm I now have fault codes for knk sensors 1 and 2.

This only occurs when the RPM goes over approximately 2k I get the 2 alarm beep and it throws codes for both sensors, this of course limits power to 90% (it can idle all day with no issues). The engine sounds great and I have checked for any lose accessories etc, also replaced spark plugs. The boat still runs great (except when you get up to the 90% power limit 4,600 rpm) Oil pressure is around 50 at idle. I really don't think it is an internal motor issue based on how well it sounds and runs.

Has anyone else ran into this issue and is their something that I should be checking. I'm open to any and all ideas at this point!

Is it throwing a Knock Sensor Hi or Lo code? Merc states acceptable ranges for the sensors are 83,000-104,000 at idle. An open in the circuit is 7,900-12,000. A short is 3,800-5,100. Sadly the 555 FSM does not specify what measurements these are.

It doesn't state high or low just fault

I'm going to step out on a short limb because I have not does as much research on the 555 as I've done with the MEFI-3. That said you could be getting a code 44, which means it cannot detect the knock sensor.

If this is the case then the most likely cause is the fuel your using. When the motor is throttled up the first time after motor is started, the ECM tries to induce a knock by advancing the timing to much. This only happens for a couple ms, and if the knock is sensed, the ECM is happy knowing the sensor is working. The ECM then sets timing where it should be based on RPM, load and programmed curve. IF the knock is not sensed, then the ECM retards timing to reduce power and protect the motor.

So if you should be running 87 octane and your speeding more money on 93, then this can cause the code 44

Problem solved... issue was with the pcm not seeing a knock signal. Put new pcm on and all is well

Ouch!

Asking --- your pcm went into Whipple for repair and that was not discovered by them - what did they fix about your PCM ? - you have been chasing problems all over the engine beforehand and afterwards.

Or did you knock out the PCM with all the voltage mess after the PCM was sent back to you?

The IAC driver and the clock chip were repaired. Still had issues so purchased a new one from whipple and it and the old one both do the same thing throw knock sensor codes for both sensors. Bought another new pcm from a merc dealer and all is well now. Whipple thinks it may be a programming issue however I needed a PCM for this weekend that's why I got another one.

You know if this was a MEFI I would expect to see a code 44 for what was happening, but being a 555 I thought they done away with that code. I know you didn't see it, but just makes me wonder

Just an FYI this issue was only on early generation MEFI 1 units was rectified with a new algorithm on later units. Would not apply to MEFI 2 or newer. I've got the bulletin on hand but am too lazy to look it up. ;)