I have a 1999 Mazda B3000 (3 litre of course).
I came across this video where the guy unplugs his MAF sensor (not a Ford/Mazda) and the engine immediately dies.
He said that if it keeps running then the MAF sensor is it fault.
How accurate is this test, after my engine is fully warmed up, I'm going to try this and I'll report back ?

I'm still chasing my slight hesitation problem; does it worse when it's cold. It's not really detectable when it's warm.
I'll drive away carefully and as I carefully depress the gas peddle as not to kick up any gravel, it hesitates slightly and then it takes off.

As far as I can tell there are no vacuum leaks, engine idle drops when I unplug the IAC valve, but not below 500 rpm.
I have to unbolt the IAC valve and block off the ports on the throttle body to test for a vacuum leak.
The IAC valves on these trucks never close all the way when they are unplugged, so the idle never drops below 500 rpm when it's unplugged.

 

That has to do with software in the computer as to whether or not it will run on after sensor is disconnected while running.
If engine dies then leave sensor unplugged and turn key off and then back on, reboot computer, and see if it will start without that sensor

No it wouldn't indicate a MAF sensor issue, because it died, if it runs a bit BETTER with MAF sensor unplugged then I would suspect failing MAF sensor.

All engine computer are "programmed" for the engine size they are to operate, so they already know how much air will come in at any given RPM and throttle position, really is just simple math, which is all these computers can do, simple math but very very fast, for air/fuel mixes on the fly.
MAF sensors main function is "air WEIGHT"

Air/fuel ratio of 14.7:1 is a WEIGHT ratio, so the WEIGHT of the gasoline and of the air is very important.
Gasoline's weight doesn't change all that much when cold or warm
Air weight, on the other hand, changes ALOT when it's cold air or warm air or at sea level or 3,000ft elevation

Thats what the MAF sensor is for, but it only samples about 10% of the air coming in, so if its off by 1% that's a 10% error in the computers calculation, so smaller errors get big fast.
Cleaning the MAF sensor gets it back to "normal" so its reading correct air WEIGHT again

IAC valve may not close all the way, but as long as RPMs drop below say 600 then thats OK, not likely to be a vacuum leak.
The throttle plate has a small hole in it, so even if IAC valve closes all the way engine may still idle, stumble at low RPMs but still run.
Most warm engines idle at 625(manual trans), to 750-800(automatic), as long as RPMs drop below that, 600, then IAC Valve can be used by the computer to set warm idle.
And any 'vacuum" leak at 600rpm idle would be negligible to engine operation

PCV valve hose can be plugged, it is a "known" vacuum leak all the time on most engine, never closes all the way

 

Unplugging the MAF sensor resulted in a CEL and very little change in the way it ran.
It did hesitate at the stop sign when pulling away (it almost stalled), but it didn't seem to make any difference when driving down the road, either on acceleration or just cruising, eg. holding the accelerator peddle steady.

I did check the TPS sensor with an analog ohm meter too, and the needle rose and fell steadily when tested.
I also set up it's voltage via the adjustment screw on the throttle body plate.
I think the range is supposed to be at .7 and 1 volt _ I set it at .77 volts.

Even if the TPS sensor tests OK can it still be at fault, causing the hesitation (stalling) at the stop sign when the MAF sensor was unplugged ?

If this was a carbureted truck with points, I would be checking the ignition system, specifically the plugs and wires. (or a condenser going bad in the distributor)
When piddling down the road it acts like the insulation is worn out on the wires and the spark is jumping to ground somewhere, but that is not the case.

And about the PCV valve, I've disconnected it until I repair the crank shaft oil seal, I'm also pretty sure there is a vacuum leak around the oil dip stick.
I've blocked off the PCV valve vacuum port as well as the port on the air tube where the oil breather is on the oil filler cap.
The crank case gasses are just venting into the atmosphere for now.
The truck runs better and has a more stable idle with theses systems disconnected, although that still wonders a round a bit, not really enough to notice by ear, but only from live data.
When it's cold it's very noticeable.

 

MAF sensor data is used for air WEIGHT so unless you are at higher elevation or very cold or very hot outside temps, unplugging it shouldn't make much of a difference.
Sea level to 2,000ft and 40degF to 80degF should be what computer can handle without MAF data
And computer may have a separate Air Temp sensor(IAT) so even air temp lower or higher may be OK

Computer will run engine slightly Rich without MAF sensor data, just to be on the safe side

When you have tested everything else then you are left with...........the computer itself, have a look at the connector, signs of water intrusion, its past any warranty so you can open it up and look for any signs of problems on the circuit board.
Sensor voltages are all under 5volts so even a little corrosion is an issue

 

I did check the ECU and cleaned off all the factory white grease on the connections. Things on the circuit board looked good too, no swollen caps.
What prompted me to do this was there was white grease on the wiring harness connection back to the fuel tank, lights etc.
This caused the gas gauge to stop working and produce a CEL in the EVAP system.

After I cleaned the ECU's main plug I did squeeze the female connectors in slightly so they would bite on the male ends.
I used a clear silicon grease, but maybe that's causing a bad connection even if the contacts were squeezed shut a bit.

I'll have to go through and methodically check all that again.

My gut tells me it may not be the ECU, but who knows, it's always had this annoying symptom.
It's so slight that I don't think most people would even be bothered by it.
I can tune my carbed vehicles to run like a swiss watch, but not this thing.

 

Putting a scanner on it while driving and capturing live data, trims, etc., may help find/eliminate potential issues...

 

I just have one of those cheap 10 dollar scanners that hook up to a computer.
It does some live data, but I'm not sure if it would give enough info, it's pretty basic, plus I would have to teach myself how to read it all and know what it all meant. I have no experience in that since this is my first EFI vehicle.
I know it doesn't stay in open loop very long from first start up, even when it's freezing out, so as far as that goes, I believe that's working.

When it's in open loop it shows the idle going up and down, it tries to go to a normal idle and the PCM realizes that it's still too cold and then it goes back to a high idle again.
It cycles through this until it's warm enough to run, this takes about a minute or 2 depending on how cold it is.
My neighbors Toyota truck doesn't do this, hers starts off at a high idle and gradually decreases to normal until it reaches normal operating temperature.

I don't know what's normal for my truck, but it has always done this in the morning and had this slight jerk to it going at slow speeds, eg around 40 or 50 km per hour. (almost like a very slight miss in the ignition)
I've discovered many things that have helped, but it has never gone away.

 

Snap throttle should give you a near 4.0-4.5v on the signal wire of the map. I check this with my picoscope. Some cars have a square wave setup that changes frequency instead of just an analog voltage feeding the pcm. I'm not sure which you have but it would be easy to check.

 

So I went through all the connections back from each major engine sensor with the exception of the oxygen sensors to the circuit board on the ECU and there was pretty much nil resistance going through the plugs.
The internal resistance of my ohm meter is .7 ohms and all connections were at least that, some even at .5, so at this point I don't think it a problem with engines wiring harness.

I plan on taking to a repair shop, the guy who owns the place really knows his stuff, even has the gear to reprogram the ECU if it comes to that, assuming the old one is the problem.
I'll post a video on what it does on first start up (open loop).
I believe that the wondering at first start up is also affecting the closed loop circuits, but just to a lesser to degree causing the slight miss/jerk while driving slowly down the road.

 

While good, the ohm test is not the best way to test the integrity of an electrical circuit. At best it will tell you if there is a complete break or open in the circuit while the engine is not running. IMO this could still be a wiring issue, but more likely on the fuel pump/injector or spark side.

 

Spark maybe, but the plugs don't show a weak spark, but then again, the problem is so small it may not show up in that fashion.
As far as fuel is concerned, it drives fine and doesn't hesitate, if it was a fuel problem, one would think that in WOT it would not respond or there would be a misfire, or lean code.

I did clean the injectors out and put pressure to them with lacquer thinner and their spray pattern is normal.
There was a fine screen in each of them partially choked with rust particles after sitting for 8 years with old fuel in the tank, I had to clean the fuel rail too, and while this helped it did not cure the problem.

 

Long shot - When the car is cold the engine is intentionally set to be rich. If the egr is sticking slightly open then it's like an extra rich condition. So.... maybe the egr is sticking? A live data scanner may be able see the command valve position vs the actual...

 

I did try a while ago and disconnect the vacuum line from the EGR valve, but I may have to blank the port right off to get a better test. (I did plug the vacuum line)
I have replaced the DPFE sensor as it was the old metal type (it was very corroded inside, yet through up no codes).
The EGR vacuum motor was also replaced with a Ford one.
The EGR valve is still the original, it always tested fine with a vacuum pump.
I also did a smoke test and the valve itself passed, at least on the outside.

The weather is supposed to get really cold and shitty here, so I may not have a chance to work on it, we'll see.
There is a car in my garage right now that really can't be moved, or at least it would be a real PITA to move it.

 

10-4 on PITA :)