rough idle, won't stay running, runs fine with MAF disconnected

flounder · #1 02-12-2020, 08:38 PM

93 xlt 2.3l 130kmiles
First KOEO codes: 121= throttle position sensor out of self test range or
throttle position voltage inconsistent with MAF sensor. Double checked for any vacuum leaks and replaced TPS. No change. Test voltages to MAF sensor, check out o.k. but cannot run proper MAF test or KOER codes due to engine won't stay running. Replaced MAF sensor. No change. Engine runs fine with MAF unplugged due to stored baseline data. Cleared codes re test KOEO and
pull these codes: 157= MAF below minimum voltage
158= MAF above max voltage
543= Fuel pump circuit open, battery to PCM
The only thing between the battery and the PCM is the fuse (ok) the inertia switch (ok) and the fuel pump relay, so I replaced the relay. No change.
I would not suspect a an ignition or a fuel pump problem since it runs fine with
with MAF disconnected. My Haynes manual is pretty useless when it comes to individual component tests. My next guess would be testing the PCM or the electrical to the EGR ( diaphragm holds vacuum). Any help with test procedure info. or suggestions would be much appreciated. I usually won't replace a part unless I get a solid diagnostic on the problem , however this one is taking me on quite a ride and I need my truck back.

djfllmn · #2 02-13-2020, 06:09 AM

check the wiring from the MAF back to the PCM

flounder · #3 02-13-2020, 03:44 PM

all voltages and grounds are correct as is the resistance from maf to pcm on all 4 wires.

tomw0 · #4 02-18-2020, 02:38 PM

Do you have VREF to MAF? It uses a fixed/specified voltage to run the heater wires that are monitored... if VREF is wrong, or questionable ground, the current used will be wrong, and reported as out of range, possibly.
tom

flounder · #5 02-18-2020, 11:06 PM

Hey Tom, Thanks for the response. I'm not sure what VREF is. I found a procedure to all 4 wires from the PCM. 2 had 12volts and 2 had ground. Unable to test in running mode since it won't run with the MAF plugged in. Can you guide me to some info. on testing VREF? Thanks

tomw0 · #6 03-12-2020, 10:52 AM

I do not have access to 93 info, but most use a 5V 'VREF', a known voltage, to feed the MAF and other sensors. It is less than 12V, which is 'battery' voltage, which also is not stable due to alternator charging circuit output variance. Give it a lower, stable regulated reference voltage, and measurements can be more accurate.
I do not think you should have 12V on two conductors at the MAF. See if you can find a wiring diagram for the engine controls, and check the expected voltages at the MAF connector.
The MAF is more subject to needing a stable voltage as it uses that voltage and the known cooling effect of ambient air to calculate the airflow coming in to the engine. If the formula V=IR comes to mind, thats the one. R changes with temperature, and with a constant known V, the I will indicate how much cooling is happening, etc etc blah blah.
Look on the web for a circuit diagram, or perhaps on-site. There are other Ranger sites, and at least one has some diagrams, though I don't remember which.
tom