91 Nissan Sentra 1.6 4speed manual question about timing-

MAF sensor? Symptoms sound right. I'm not sure of position of and type of MAF sensor in a '91, but by mid '90s (GA15/16 "DE") they are a "hot wire" type mounted just above the throttle body, and they frequently fail. OEM replacement is not cheap, but you might find someone offering reconditioned units at about 1/2 OEM price.

Not sure if a fouled / malfunctioning MAF sensor would always set a CEL. If there's no reading from it at all, then probably yes, but if the voltage is fluctuating and incorrect as described in the link below, perhaps not. link:

refers to a later model, but still a GA 16 DE engine, which when I look at posts above is what you seem to have (my mistake - I thought the GA 16 "DE" was introduced later than '90/91).Are either of the distributors you have dirty inside from oil leaking from the internal seal on the shaft? (not from the main O-ring seal between the distributor and rocker cover) That's also a common problem once they've got up there in mileage, and contrary to what Nissan told me, relatively easy and cheap to fix yourself.

if the wires on the 91 MAF sensor are totally sealed, you may find it acceptable to insert a (straight) pin into each wire near the connector, to allow access for voltage measurement. if the wires are NOT totally sealed, you may find it acceptable to "back probe" the connector pins with (straight) pins for voltage measurement. respondent "Me" is correct about faulty MAF sensors, the MAF allows the ECM to calculate the injector pulse width. if you ever have the need to see if the ECM is sending a pulse to each injector, you can insert a "noid" light into the specific injector circuit (i once used an analog voltmeter to see the pulse, when diagnosing an engine that would not start).

does your 91 have the TPS (throttle position sensor)? if it does, you can check he TPS using the (straight) pin method, however, i would be surprised if this is your problem.

can you give more details about the "stumbling" problem? does it occur when you suddenly open the butterfly when NOT driving? or does it occur only when you are accelerating (under load) while going through the gears?

this thread assumes you have checked for constant fuel pressure to the injectors (and that the fuel pump is NOT an issue).

would be interested in more details about your Consult device.