There is a possibility the voltage regulator , which is built in on the PCM could be at fault . We must remember that a healthy battery and charging system is necessary for a PCM to work properly and for all sensors to function properly as well . Our best diagnostics tool at this point will be a scan tool . If possible , a hand held one with flight can be used on a test drive to try to trigger the glitch and capture its reading to find the power loss . If this is not possible , a dyno session will have to be done which unfortunately means a diagnostic fee . Let's try to exhaust all checks up to that point before spending that fee . When an engine either stales or misses and rpms drop , naturally , the alternator reduces speed and there is a voltage decrease ( drop ) which is sensed by the PCM . If there is just a driveability problem such as a over rich condition that is causing the engine to stumble , this must be corrected as being the obvious to see if this corrects the problem or at least eliminates one more problem. Let's not jump to the extreme , this may still be within your scope to diagnose before going to the shop . Autozone does read codes for you but I do not know if they will scan for driveability problems . If you feel your PCM may be at fault , start inspecting engine harness wiring that feeds back to the PCM for any shorts that may have caused a fault in the PCM . A PCM does not just wear out , they short due to an electrical problem caused by wiring shorts or a surge that draws more voltage than the circuit normally puts out or reads . We must do,all we can inspection wise and check the simplest things we can that we are capable of before a dyno session is applied . It's terrible to find out you paid a high price for this service and the result was for example a faulty intermittant purge valve on the evap tank . I personally try to avoid that . I hate paying money to someone when I could have found and fixed myself. Realizing your asking for what to diagnose so that you can in fact do it yourself is what we can help with . Ok , to recap , if your satisfied with fuel pressure and spark quality is strong , we must comtinue to go after the computer related items . Simply , if a sensor misses a beat but does not log atroubke code because it does not do this long enough or enough times to be significant , the code never logs . Conversely , if there is a fault in the PCM and it is not reading sensors correctly , this happens too . It will come down to inspection of wiring , vacuum lines and everything visual since there is no trouble code to help us . Yes , a scan tool will speed up things by reading sensor input and output , alternator and other functions but if not available , manual checks will have to be employed . I went through this in December 2004 with my '92 XJ and went as far a my multimeter can go before the display froze in single digits temperatures . I was nearly sure it was the PCM not putting out strong enough voltage to the coil . Turns out I was right , but a Chrysler dealer had to charge me to find it ( indoors of course ) and replace the PCM . If I could have just gotten it started , I could have read coil volts cranking and running . Hope you are not experiencing a problem this extreme .
