1991 Nissan D21 4 cyl. Manual trans.

Truck will crank and start but runs VERY rich. Won’t rev up much above an idle. Changed the MAF sensor but didn’t help. Read the ECU code and the only code was for the air temp sensor. Don’t think that is causing the issue since it has been disconnected for years. Tried to test MAF. Black/white wire reads battery voltage. Black wire reads ground. White wire reads almost battery voltage with switch on. Not sure what it should read. Thanks

You should have KOEO:
Blk/Wt (power) 12v
Wt (sig) 5v
Blk (ground) .100mv

Sig @ idle 1.4
Sig @ WOT 3.9

Common problem is the ground. You said it's good, how did you test it and what voltage reading?

The IAT does have an effect on the MAF. Cold and hot air have different densities and different air flow rates and values.

Thanks for the info. Did some further tests at the MAF sensor with my volt meter. Blk/wh wire had 12.20 volts. Blk wire had 11.98 volts. Wh wire had 0 volts. However when I did an ohm test of the white wire it read open. Not sure what I’m seeing?

Knowing how you're doing the voltage tests will help me understand your data... are you doing it by back probing? unplugging the connector? KOEO?

KOEO. MAF sensor unplugged. Meter grounded at battery for voltage test and ohm test. Meter powered at battery to check ground on Blk wire test.

Thanks for info, helps me a lot.

Most likely have a broken (open) signal wire. Go to the ECM. With KOEO check for 5v on the white sig wire there. If 5v, the sig wire is open between ECM and MAF. The open can be anywhere. Check the connector male and female pins first for good contact and or corrosion. Make sure the female pins are not spread open. If connectors are good you have to find the open in the wire. You can string a new wire as a work around but finding the open is better. Look for contact, stress and heat points, along the harness. Fix that IAT as well you'll get better gas milage.

Did you say you had 12 volts on the black wire?? Was you grounded at the battery negative and probing the black wire? Because if that what you were doing that’s a bad ground.

Notice several people doing this. Use DVOM red lead attached to bat + and black lead attached to sensor ground to test the sensor ground.

Thanks for the responses. At this point the truck won’t start. I’m pretty sure the spark plugs are fouled. I checked the throttle position sensor with the sensor unplugged and the koeo there was 4.6 volts on one wire 5 volts on another and the remaining wire was to ground at the harness plug.. Same setup koeo air temp. sensor harness had 5 volts on one wire and ground to the other wire. Same setup koeo but with the FAM PLUGGED up the Blk/wh wire reads 12.6 volts and the Blk reads 10.5 and the white wire reads 10.5 volts. In one response you said to go to the ecu and check the voltage on the white wire. Question is are there multiple white(signal) wire for each sensor or are they all common to one wire? At this point I don’t have a wiring diagram but I’ll search for one. Another question is if I have signal voltage for the other sensors I’ve checked can the ecu unit be faulty? Thanks again.

The engine might run with the MAF unplugged. I know a 95 will. Not positive if a 91 will.

I found a wiring diagram for the truck. It looks like pin 17 on the ecm is the signal wire for the maf. If there is no voltage there is it a good assumption that the ecm is faulty? Or can there be another issue?

Yes. It would indicate a possible ECM problem. Check other sensor pins such as the TPS at pin 20 for a 5v signal. Don't throw parts at the problem, test first.

Finally had time to work on the truck. Did some further testing at the ECM. With KOEO and all the sensors plugged up heres some voltages I found. Pin 42 is the ground for the ECM and read good to ground. IAT: b/g wire(21) read to ground. YL wire. (26) read 4.9 volts. TPS: P wire(37) read 5.15 volts. W wire (20) read .66 volts. B/G wire (29) read to ground. MAT: B wire (16) read 10.14 volts. W wire (20) read 10.75 volts. B/W wire (48) read to ground.not sure what any of this tells me. If I unplug the MAF sensor pin 20 W wire reads 0 voltage. Any input would be very beneficial.

Good info... noticed you may have two pins confused but lets make sure. Pin 20 white wire goes to the throttle sensor. Pin 17 white wire goes to the MAF. You reported pin 20 having both .66v and 10.75v. I think you mean pin 17 has 10.75v and pin 20 has .66v. Correct. Please check again.

The MAT white wire should have been identified as pin 17 at the ECM It read 10.75 volts when plugged up at the MAT sensor. Unplugged from the sensor it read 0 volts at the ECM.

One other house cleaning issue... Pin 48 is a Black wire. Pin 47 is a B/W wire. All black wires are grounds. All B/W wires battery voltage.
MAF has three wires, B(ground at ECM pin 16), B/W(12v from IGN) and W(signal at ECM pin 17).

You have 10.14v on your B MAF ground wire. You should not have more than 100mv on your ground wires. Anything more is a problem. High voltage on a ground wire means the ground is bad.

I have to say you're all over the place with voltage readings on all the wires tested so far. Go back and read your posts. I mentioned before and it is very important. When doing voltage tests, use one good ground location only for the black lead of your multimeter. Either a body ground or block ground or the battery ground, nothing else. At one point you mentioned having an open on the MAF signal wire, however now you're reporting 10.75v on that wire. Your battery is probably low after all this testing so it around that 10.75v range. It can be open and rubbing on a battery voltage wire. I had an 86 1/2 D21. Great truck but prone to head gasket problems. Also the retaining clips on the sensors connectors broke easily which made for loose connections. Practice meter readings to get consistent and accurate. Testing should start at the sensors first and work your way back along the wires to the ECM. ECM's are pretty hardy so don't condemn quickly. Certain sensors share grounds and power but never a signal. So a shorted TPS can affect the MAF, CMP and CKP sensors.

You're doing fine... it can be frustrating but rewarding when you fix the problem yourself.

Thanks for the input. If I unplug the the MAF sensor there is 0 voltage on the signal wire.so does the sensor feed voltage to the ECM or does the ECM feed voltage to the sensor.

Both :silly: The MAF has a hot wire inside it. The hot wire has resistors attached to it and is powered by the ignition 12v. To make the circuit work you have to have a ground, hence the B ground wire. The ECM keeps the hot wire at pre determined temp as it senses the air rushing past it cooling it off. The ECM sends out 5v on the signal wire to the MAF signal resistor and as the resistor on the hot wire reacts to temp changes, the 5v signal voltage changes. Typically, .5v at idle and up to 4v at WOT. :blink: Hope I did not screw that up too bad :woohoo:

So the voltages and grounds have to be correct for the circuit to work. If the voltages are outside the limits the ECM sets a DTC. I'm not an electrical engineer so I hope I got everything in there.