I'm posting this information in case you may run across a similar problem.
PROBLEM
1998 Cadillac DeVille with Northstar control system and with intermittent MIL, sometimes flashing. Vehicle was taken to dealership and P0303 code extracted (cylinder 3 misfire). Number 3 fuel injector replaced and all is quiet. MIL turns on, again, sometimes flashing. Vehicle taken to an independent shop, P0308 (cylinder 8 misfire) code. "Motorvac" treatment given, problem goes away. MIL turns on, sometimes flashing. Vehicle taken to dealer, DTC P0308 present. On way to dealer, MIL turns off. Customer states the problem often goes away. Tech performs power balance test and recommends replacing #8 injector. Customer refuses, claims dealer incompetent. Customer pays $150 diagnostic fee and leaves. Since "customer" is a family member, I get to look at the car, next.EXPERIMENT Scan tool (OTC Enhanced Monitor with Pathfinder 99 firmware) showed P0308 in history, both long-term fuel trims were negative (as expected on a hot engine -- although one was more negative than other). Vehicle appeared to be running well when examined. Ignition on cylinder 8 'scoped and compared to "known-good" (I used #6 since this was convenient and PCM was not complaining about this cylinder). Both cylinders were firing at about 10 kV. "Good" is between 10 kV and 15 kV. Decorative top cover was removed , tape from #8 and #6 fuel injectors' wiring was unraveled and current peak and duration measured. Both injectors were drawing 1 ampere, pulse width was about 3.2 mS (scan tool showed 3.3 mS was commanded).
Pathfinder 99 allows individual injectors to be turned off. With engine off, idle air control motor was disabled and engine was started. Selectively turning off injectors showed approximately identical RPM drop (before fuel control kicked in). Engine was turned off. Idle air controller was reconnected and new DTC ("cannot control idle rpm") was cleared. Although the power balance test was not done "by the book," its outcome convinced me that, at the moment of the test, no problems existed in #8 cylinder.
HYPOTHESIS System appeared to be in very good control of fuel delivery, although slightly different long term fuel trim (LTFT) readings hint at a possible air leak (injector O-ring, intake manifold, etc.). Misfire was not present. Initermittent problem is symptomatic of electrical connectivity issue, characteristic of fretting corrosion (due to microvibration inside electrical connector and oxygen ingress causing oxide formation -- do a web search if you're interested in details).
REPAIR #8 fuel injector connector was disconnected and injector's pins and connector clips cleaned with Caig Labs' "Deoxit D5" (a very good contact cleaner/lubricant) and reconnected. Connection reopened, recleaned and final connection made.
FOLLOW-UP Vehicle has been running flawlessly for three weeks. Family member told next time P03xx code present, all injectors will have connections cleaned. Total time spent on job: 2 hours.
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Cordially,
Bohdan Bodnar