I have finally solved my random, intermittent misfires after probably 5 years of on/off frustrating pursuit. I've been at forums, questioned people, went to the dealer, and nothing really was getting at it. It was intermittently misfiring, hesitant or sluggish. (2005 GTP)
Short story: Vacuum leaks.
Detailed story: The main areas seem to be the soft tee at the S/C connection (which I broke when degreasing the engine), the soft elbow at the S/C-snorkel tube, the tube connection at the flexible snorkel connector, the evap. purge valve O-ring (this may be the most hidden leak). I built up connection fittings with electrical tape at the brake vacuum booster connections, and at the flexible snorkel tube connection. The soft tee was replaced the universal hard tee, and the soft elbow was replaced with tubing with a long-sweep. Connections at the MAP sensor and s/c bypass actuator and the s/c bypass solenoid were replaced or checked. The upstream O2 sensor was also replaced, and it gave better readings, but it probably was marginally OK, as was.
Previous checks and/or replacements that never really affected the intermittent misfiring specifically include: spark plugs, spark plug wires, cleaned throttle butterfly (had effects), fuel injectors, replaced ICM and coils (with a used ones), checked for arcing in the dark, checked fuel pressure, looked for questionable grounding connections and re-seating/cleaning ECM, ICM and other connections, carbon cleaning (had effects), attempted catalytic converter cleaning (with 10% lacquer thinner over 90 miles) and numerous "Italian" tune-ups. I pursued other research before replacing the crankshaft sensor, testing cylinder compression and just replacing every sensor or recommended part on the "could be" premise.
The engine runs much tighter and smoother, is more responsive, starts more instantly (not that I noticed anything anyway), pesky misfires have ceased, fuel trims have narrowed (I run an AeroForce Interceptor gauge to monitor readings) and transmission shifts are better. I am convinced this will continue and that it is fixed. This thing runs as about new at 126,000 mi., and I am impressed! The only thing it really needs now is a transmission rebuild, as I beat on it too much when I thought is was a hot S/C car, LOL - now I'm trying to preserve whatever is left.
I usually come around for help on the internet for help, and this time I can post help that is easily discounted or needs EMPHASIS. So thanks to any help I've checked around for here, as there are many quality postings and answers on this forum. I hope this inexpensive maintenance can help someone keep their car running good without the anxiety and cost of the thousand possibilities (that are probably not the issue).
(You may see the same post on another GP forum, LOL.)
2005 GTP
Stock
Tweaking of shift points and fuel mixtures
