I changed out the 5.7 motor in my '90 Iroc a month or two ago. I now show the SES light (code 43) about 10 seconds after cranking the engine every time. It doesn't matter whether it just idles or if I am agressive with the throttle.
The car did not have this issue before the engine change. The only difference is it now has a double roller timing chain, roller rockers, and a hydraulic cam instead of the roller cam.
Am I to understand that a bad knock sensor is probably the culprit or is there too much internal engine noise now?

 

Could be a bad knock sensor. It may also be that the wire broke or that the connector fell off the sensor. Note that the knock sensor is grounded through the threads where it mounts to the block. So excess tape can be an issue.

What that ECM does to check the knock sensor is apply 5 volts to it through a resistor. The resistance of the sensor pulls the voltage down to about 2.5 volts. It is this 2.5 volts (+-) that the ECM is looking for. If too high or too low then code 43 is set.

Note that this is how the '90 - '92 TPI ECMs check the sensor. Other set ups with other ECMs will vary.

RBob.

 

I changed the knock sensor and the 43 code still persists. I checked voltage on the wire going into th sensor and show 12v going into the sensor with the ignition on and 14v when running. I grounded the sensor I had removed and plugged the connector into it and still have the 43 code.
I thought it was a lower voltage going into the sensor?

 

It could be the set timing connector under the dash has became disconnected.

 

Yes, should only be 5 volts on the wire when disconnected from the sensor. Sounds like that is the fan switch wire/connector, not the ESC sensor wire.

With the ignition on ground the wire. If the fan turns on, then you'll know what it is really for.

RBob.

 

I was suspicious of the wire after I posted this morning I did another search and read where another member had the same thing happen to them. I went and pulled the wire going to the sensor in the head with the same connector on it and it had something like 4.8v on it.
I swapped the two and my code 43 went away. Go figure. Plugging everything back in after the rebuild the two must've gotten mixed up. Set my static timing again and it runs like it should.
Thanks for the input guys.