OK here is my problem, i have a 1994 z28 all stock. i started having a real slight knocking sound from the engine about 3 weeks ago which has grown louder, i took it to my mechanic the other day and he told me it was spark know, and that it could be a number of things causing it. well i got in it today and drove to the store and then on the way home the service engine light came on, so i checked the codes when i got home and found that the codes were 36 and 42 both seem to be related to the ignition. so my question is why would it take almost three weeks to throw a code for the knock and would the two codes thrown cause spark knock. any help would be appreciated.

 

Possibly some bad fuel? Wouldn't hurt to do an oil an filter change.

 

"spark knock" is unusual in these cars. The stock PCM programming is capable of pulling up to 15* of ignition timing to prevent spark knock. Put a scanner on it and see what the knock sensor is doing, and how the PCM is responding.

It would take some severe timing issues to require more than 15* of timing to be pulled.... but DTC 36 may be indicating a spark timing problem.

DTC 36 means the high resolution cam position signal from the optical sensor in the Opti is not reaching the PCM. Could be a problem with the sensor, or the wiring to the PCM. Check the 2 ends of the short Opti harness from the clip on the passenger side of the intake manifold to the Opti. DTC 42 is for a problem that is most often related to the IC Module on bracket on the front of the driver' side head, but can be related to a faulty spark signal from the PCM on the white wire to the ICM, or the coil, the Opti if the wiring harness for the ICM and the coil. Best thing to do with DTC 42 is get the factory diagnostic logic diagram for your specific year, from the factory manual, and work your way through it.

Losing the high res pulse form the cam position signal should not cause problems.... the PCM just uses the low resolution signal, with a VERY MINOR loss of spark timing accuracy.

 

not sure if i said this in the first post but just put a new knock sensor in on friday with a fresh oil and filter change. also had the opti, plugs and plug wires done last month. I had thought that maybe it was some bad gas but i use 93 all the time so pretty sure its not fuel related, i'm thinking that it has to be a sensor problem.

 

With a scanner, you can see what the PCM is doing in response to the audible knock. That's the best first step.