Hi all.
I just completed doing a total tare down and reassembly on my 86 Iroc Z 305. Before I started the project I had a cooling problem, fans would not kick on until the car got to 230 deg. When this would happen, I would step on the gas and hear the pistons rattle and knock, service engine light came on. a code 42, knock sensor. I did replace everything that had to do with cooling and fan sensors and relays, even replaced the knock sensor. No fix.

So, for other reasons like OIL leaking problems , I tore the motor completely down. No engine mods were done Plane jane stock. Put a new thermostat, 186 deg and a new Temp switch 160 deg. Put the motor back together, fired up at the snap of the key, adjusted timing and drove it for the last few weeks. Fan kicks on at 223 deg, cools it to 210 deg and shuts off.
Drive it around the city and guess what "42 Electronic Spark Timing (EST) or Bypass Fault"

What in the world is going on here.

 

check hei module

 

Take the module to an AutoZone and they can test it for this. And have them print out a troubleshooting guide for this fault if the module tests good. That is a very common problem, sometimes intermittent, and can be hard to find.

 

So I have to take the HEI module out of the destributor and have it tested? Sounds like a good place to start

 

https://www.thirdgen.org/forums/tech...code-42-a.html

This post has the code 42 flow chart for 82 to 86.