91 Explorer, hard start, rough idle, test data, need help
 

My 1991 Explorer was running fine last week, now extremely hard to start, and keeps stalling.

I read through many, many postings about this issue, and I have some test results, need help to figure out what to do next.

I did the scan tool and here is what it shows, there are no faults in the computer, pending or otherwise:

Boo / Bpp OFF
Can Purge ON
ECT (v) 3.0
ECT F 74
RPM 860
FuelPw1 (ms) 3.1
IAC (%) 54
IAT (v) 3.3
IAT F 62
Loop Status Open
02 Sensor 1 (v) 0.00
Spark 18
ST FTRM1 2
T/P Mode C/T
TPS (v) 1.2
VPWR (v) 14.7
VRef (v) 5.0

I did the TPS sensor voltage readings, they are:

1.07 full closed 4.45 Full Open
Ref Voltage 4.79


I also did the MAF readings, and that seems to stay at .94 no matter what the RPM.

When I unplug the MAF and start the truck, it starts right up, no problem and RPM goes to 1100 steady.

With the MAF plugged in, the idle goes to 1500 for a few seconds, then the truck wants to, or does stall.

Based on what I am reading, the voltage of the TPS should be .8, so it looks like that sensor needs to be replaced.

Also, the MAF looks to not be reading correctly. It thinks the RPM is about 900, no matter what the real RPM is.

Should I replace these two sensors ? Any reason I should not replace them ?

The 02 sensor, the voltage seems to be wrong. Should it be reading 0.00 ?

Fuel pressure and coil and wires and plugs all are fine.

The problem seems to be bad data from the sensors messing up the fuel mixture.

When the MAF is unplugged and the truck goes into its default mixture emergency mode it starts and runs perfect at idle.

Not a good way to drive it, but it seems to rule out everything else except for the sensors.

Any help is welcome.

Thanks

Bill

One of the steps to do a quick check of the MAF sensor is to unplug it and if it corrects the problem ,repl the MAF sensor.
The o2 sensor should be reading about 4.5 supplied by the ECA until it gets into closed loop. It should be supplied with 12 volts.
Before doing any replacing it would be a good idea to confirm that the 5 volt Vref or 12volt supply voltage is present at the senors.
TPS is within range.
ECT of 74F should read approx 1.9 volts

Need to know which engine is in the vehicle.

Check the fuses !

Unplugging the MAF does fix the problem. The 02 sensor is showing zero volts, so I am guessing it is dead and needs to be replaced.

The TPS sensor shows the lower 4.79 volts, not sure if that is OK.

Other sensors show 5.0 volts. This is why I thought the TPS was bad, also the fact that the the TPS was reading higher than 0.8 closed. I was getting 1.2

I also noticed the IAT, which is part of the MAF assembly, that was showing air temp much higher than it actually was outside.

The engine is a 4.0L V-6. It is the only one that they put in the 1991 Explorer. I think the VIN code is an X for that engine.

What do you think ?

Thanks

Bill

I replaced MAF, starts normal now, does not stall, but idle is still a bit rough.

Driving is fine, smooth, does not stall when stopping, just rough idle.

I think I have a misfire, but not sure. Will pulling the spark plug wires one at a time hurt anything ?

I see videos showing this, and it says it can damage the coil packs.

Other than that, any way to tell if there is a misfire ?

My computer in the truck does not have codes for that, and I am not even sure a 1991 would have codes for a misfire.

It is way before OBDII was started.

I don't want to change plugs and wires, just to change parts, if there is no need and the rough idle is coming from something else, like the IAC or the TPS

Thanks for the help

Bill

Quote:
"I think I have a misfire, but not sure. Will pulling the spark plug wires one at a time hurt anything ? I see videos showing this, and it says it can damage the coil packs."

If, theoretically, you were to pull off a single coil pack from a non wasted spark type of HEI ignition, then yes you could easily burn that coil up since it will build voltage and melt the coil internally due to no ground path.

However, Ford uses a wasted spark EDIS module that fires two coils at one time. As long as you only remove one wire at a time, there is no reason to think the EDIS module would be damaged, since the electricity will still have a path to flow to ground.

Also it sounds like your MAF sensor is indicating incorrectly. I think .94 is maxed out, indicating maximum air flow. At idle you should be seeing somewhere around .1


Lastly if you don't know when plugs and wires were last changed, they are a wear item and can cause rough running. I would suggest pulling your plugs and looking inside the nose for a cracked insulator.. if you find cracks, that's your problem cylinder.. if you don't find cracks, you could still do a spark test with an HEI ignition spark tester from any parts store.