Nissan Repair: Suspect timing issue, crank angle sensor, volt ohm meter

Question
Recently I experienced a loose of power in my '98 Altima.  When accelerating it was weak and there was a noticeable diesel engine sound from under the hood.  NOT detonation, more like the normal sound you hear under the hood from a well running diesel.  This went on for a week and then suddenly it went back to normal.  Had good acceleration, and the engine was quiet under the hood.  During this time there were no fumes, no smoke from the exhaust, it idled normally and there were no other issues to speak of.  Just weak acceleration.  Also the distributor was tight and the rotor was properly secured.  I strongly suspect an intermittent  shift in the timing.  Some kind of crank angle sensor issue.  BTW, I am a long time Z car restorer and have built many performance L28 engines.  So I'm no slouch with cars, just don't know the Altima that well.  But in this case the Altima behaved as though the timing had been advanced too much. I did mark the position of the distributor with a cold chisel and remove it and inspect the encoder wheel and all interior components and made sure the screws were tight and the once internal spade connector was tight.  Found no obvious visual faults.  The interior was clean and oil free.  So I am befuddled.  I'm sure the problem will come back eventually and more frequently as often is the case.  Thanks for your support and I look forward to your expert reply!  Frank

Answer
Frank,

I would suspect the knock sensor or the mass airflow sensor since you do not have any codes.  If the check engine light is on send me the codes.  Autozone or Checker will check these for free.  If there is no check engine light get a Haynes Manual and follow the instructions for checking the mass airflow sensor with a digital volt ohm meter.  Lastly, I know this sounds strange but check the converter to see if it has melted and a chunk of it is moving around blocking exhaust flow.  The Altima and Sentra are known for this.