Changed Knock Sensor... NOW I am getting warning light

Discussion in 'GM Powertrain' started by moogvo, Aug 7, 2012.

  1. moogvo

    moogvo Epic Member 5+ Years 1000 Posts

    So after wrestling with this truck to get rid of a hesitation, I changed out the knock sensor. I reset the computer and started it up. It started to hunt when I ran the engine speed up to 2000 RPM. I let off the throttle and it idled at 1100RPM and finally set 2 codes. Mind you that there were NO codes prior to changing the knock sensor...

    I got a 35 ([FONT=Verdana, Arial, Helvetica, sans-serif]idle air control sensor circuit fault) and a 42 ([/FONT][FONT=Verdana, Arial, Helvetica, sans-serif]electronic spark timing (EST) circuit grounded). The only thing that was changed was the knock sensor. Nothing else was disturbed. WTF?[/FONT]
  2. dpeter

    dpeter Epic Member 5+ Years 500 Posts

    So how do you do that without disturbing anything?
  3. moogvo

    moogvo Epic Member 5+ Years 1000 Posts

    The knock sensor on this motor is on the passenger's side, threaded into the block right above the coolant temp sensor for the gauge. There is nothing in the way. A 22mm socket and it's out in 30 seconds. I did, however disconnect the brake booster line and forgot to reconnect it. that was my SES light. Put that back on and it runs... worse than it did before I changed the sensor.

    So now, I have rebuilt the TBI unit, replaced the plugs (twice), wires (twice), cap and button, ignition control module, coil, fuel filter, fuel pump, knock sensor, EGR valve (twice) and the EGR solenoid. I have just about spent more on parts to fix this problem than I bought the truck for to begin with!

    Now, in addition to the running problem, the clutch is starting to make noise and the A/C compressor is blowing freon out the front seal. I am really beginning to regret buying this truck! But then again, that seems to be the rule, rather than the exception.
  4. dpeter

    dpeter Epic Member 5+ Years 500 Posts

    OK, my point of reference was the knock sensor under the intake manifold, darn hard to swap that one out without disturbing something. Sounds like a damaged wire harness since both codes refer to circuit faults. I THINK the timing circuit had to be grounded to set the initial timing and then ungrounded for normal opperation. A male and female connector hanging loose along upper firewall out of a larger group of wires. You didn't see them and connect them by chance? Beyond that, I have no more to add except that it will likely be cheaper to fix the truck rather than replace it. Best of luck!
  5. moogvo

    moogvo Epic Member 5+ Years 1000 Posts

    Thanks for the input. My timing connector is behind the dash. I disconnected it a few days ago to check the base timing. It was connected back. My engine doesn't have knock sensors under the manifold. That is on the newer LS engines. I have an old, garden variety antique 4.3 V6. My engine light was caused by the brake booster line being off and creating a whopping vacuum leak. the light is off now but the truck still runs like I have a fire engine hooked to the tow hitch.
  6. aloxdaddy99

    aloxdaddy99 Epic Member 5+ Years 1000 Posts

    I had a similar issue on my ZR2 after I changed the plugs,wires,cap and rotor. It turned out to be the coolant temp sensor wire connection had broken off. I saw the wire hanging but couldn't find where it was supposed to go. So I went to the Merchants and asked for help. The manager came out found the problem and told me what it was so I could do it on my own and save some cash. I know that is not much help but it may be something you could check.

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