On a project car I am working on, using a 98 Fbody 3800 pcm, me and a friend learned something new about turning off the P0300 random cylinder misfire code. I dont know that this applies to ALL 3800s, but it sure explains a few instances that I have ran into in the past. Apparently - at least with this pcm - when you set P0300 to "No error reported" so the code wont trip it also dissables all visable misfires for each individual cylinder on the scanner. We verified this with using HPTuners scanner and using a GM Tech2 scanner. Once setting the code back to its OE setting it would show misfires as it should.... that is IF you have any! It really makes me wonder how many pcms turning off this code with cammed guys is preventing viewable misfire data. A 97 GTP that I built a new engine for last fall had a bad misfire after a few miles and would never show any misfires with my scan tool or with the HPT scanner no matter the condition such as just sitting at idle or driving down the road. It turned out to be a bad spark plug out of the box and all was well after that but I never understood why I couldnt view misfire data until I learned of this. With HPT there is a check box for SES and as a stock file this is checked and I uncheck boxes for codes I turn off as well as set the option to no error reporting. We tried checking the box with leaving no error reporting and there was still no data. When code P0300 sets the car will not go into closed loop and dissables a few things. Even though the code was turned off, wouldnt set in the history or turn the ses light on, closed loop was still dissabled even commanding it with a Tech2. So for those trying to hunt down a misfire that you know is present but not picking up with a scanner then you may want to try turning the code back on to see what it does. I am very curious as to what years/models this really effects.
