New coil, plug wires, cap and rotor. I pulled the number 2 plug and it looked good, and gap was dead on.
also replaced the fuel pump, filter, and fuel pressure regulator.
At speed it bucks really nasty, can see the tach bounce when it happens. Checked the voltage and continuity and such to the TFI. All was what it should be but testing in a driveway is different from when its moving.
It is really horrible to drive but used to drive pretty well. had a skip and a buck every now and then but mostly ok.
I have all manner of test equipment as far as things go, a plethora of Meters.
where do I start?

 

I would test TPS(throttle sensor) first
Center wire Key ON
Leave it plugged in
Use sewing pin to pierce wire
Should see .69 to .99 volts, under 1v
Slowly open throttle manually and watch voltage rise
Keep opening and watch for any jumps or drops in voltage, dead spots
Should be 4.5v to 4.8v at wide open
Repeat a few times to makes sure voltage is stable at any position
If TPS voltage drops computer thinks you took your foot off the gas pedal and turns off injectors for coasting, which would cause bucking, like if you were feathering the gas pedal

MAP sensor tells computer engine load
It reads vacuum level in the intake
Its often located on the firewall behind wiring harness
It has 3 wire connector and a vacuum hose to upper intake, check that hose for cracks
Check connector for corrosion or frayed wires
Should have a brown/white and a grey/red wire, test for +/- 5v between them Key on unplugged

And last but not least, and the probable cause, the TFI module or distributor sensor
Probable because of your comment about the Tach bouncing
The TFI module and the Hall effect sensor inside the distributor times the spark and fuel injectors
This timing pulse is what you see on the Tach
If its erratic then spark and injectors are as well
An engine loses and gains RPMs, of course, but its not all that rapid, its mechanical, so happens slowly up or down
So for the Tach to change abruptly it means timing pulse changed abruptly not engine RPMs

Check distributor shaft with cap off make sure is not 'wobbly' the bushings can wear out
Not sure you can test the sensor or TFI unless they are bad, i.e. a no start
These are kind of like an ignition coil, they can test BAD but there is no test for GOOD, lol, because everything changes when they are in use at higher RPMs and higher temps

 

I was thinking of TFI as well.
I checked the TPS a few times but just got a new meter, and will check it with that. It didn't have any strangeness then but maybe some new strange can be found.
Distributor bushings are nice and tight, so no dwell is sneaking in there.
Read that PIP rarely fails.
I found a rather extended test procedure for ohming out a TFI but if there is a bad connection within it may not show on the bench, if its vibration based.
its not really heat based, it messes up right away.
also I notice its taking longer to start. Use to kick on under a half second of key turn, now takes several.

 

TFI ohmed out ok, still not counting that out, but
The new fancy meter showed some very small discrepencies with the TPS.
I took another drive and paid close attention to the patterns, and the thing runs great until you get right at where the tachometer says 3k. Then it starts its coniptions. Below it, it returns to normal.
My tach reads slightly higher rpm than it actually is, otherwise it wouldn't feel like the engine was trying to jump out of the truck at highway speeds.

If I unplug the TPS, will the computer revert to the map to figure out the throttle? Could make a good confirmation test if it does.

 

Yes, you can run without TPS hooked up, will be a bit sluggish as the computer won't have the "heads up" that driver wants to accelerate, but MAP sensor can see when vacuum drops just not as fast to react

The 3,000rpm is interesting, thats the area where an RPM limiter usually kicks in, but only when in Neutral
Automatics have a Park/Neutral switch on transmission
Manuals have it on the clutch pedal
Pin 30 on computer sees 5v when in Park/Neutral or clutch pedal down
Long shot at best

Money is still on Hall effect sensor or TFI module

 

It says 3000 but is probably closer to 2500. Using the frequency counter at idle and some math straight off the coil primary, the tach reads closer to 1200 at 1000 rpm.
This truck will rev higher than 3000 in park/neutral so I'm not thinking its that.
I will order the TPS first and the TFI next as Rockauto does not have the two items at the same "warehouse" I won't lose anything. My TPS is a little hairy looking as it is.

 

On some new TPS units you have to elongate the bolt holes to get them under 1volt at throttle closed, or to prevent them from holding open the throttle a bit, raising idle

 

I will check for that for sure

 

Well you are correct, its 1.2v closed. I'm glad I have all these drillbits lying around. I also have a new tfi and pickup but havent put them in yet. I did replace the IAC but to my dismay there is no voltage to the connector. There is 11v from BOTH wires if I ground it on the battery so either they are touching or the computer took a dump on that output.
Oh boy this truck is a riot.

 

With key on the red IAC Valve wire should be Battery Voltage, uses same Red wire as injectors, they are all spliced together, 12v comes from EEC Relay
Other wire should be a full ground, but only with Key on engine off, this is the control wire that runs back to the computer

Earlier model Rangers had an external diode between the red wire and "other" wire, at some point Ford add the diode INSIDE the IAC Valve, not sure what year that happened
If external diode failed you would get voltage on both wires with key on unplugged from IAC Valve of course

Not sure where the diode is located but its connected between those 2 wires

 

thats gonna be the first one I will test. i have a coupla diodes in my parts box, hopefully thats all it is. Im hoping the diode didnt fail open and it didnt spike the computer

 

It wouldn't hurt the computer, the IAC Valve is just a solenoid, so just a coil of wire around a metal cylinder, the 12v in runs thru that coil of wire to the computer, computer "looks" for that with key on, as part of its start up tests, if it doesn't "see" the 12v on its IAC pin then it would set a code, IAC Valve "open circuit"
Diode is there to make solenoid operation better, but if it failed IAC Valve would work

 

Alright! Itll idle now... First high and automatically down to low.
the diode is located in the wire loom wrap nearby the IAC. Or at least in my truck it was. It wasnt until I peeled back the plastic coverings I saw the duct tape. They had removed the diode and replaced it with a @/#$ plain old wire.
I have underestimated the technical skills of the tweakers that used to own this truck. It's like they knew exactly what to ruin to make it worse.

Well, with that out of the way I can move on to the ignition system.
Thank you so far for all your help!

 

Ok, truck actually drives perfectly great now. You called it early, it indeed was the TFI.

I left the old PIP in place since it I would have had to take the dizzy out and apart to change it, plus I wanted to be sure which part was exactly bad. I ended up putting the old TPS back in too, since it wasn't the cause and I could adjust the closed voltage down to 0.9V more easily.

Today was a good day.

 

Today was a good day, Good Work

Thanks for the updates and posting the fixes, it WILL help some one else down the road

 

Almost there......

Update:
It'll run perfectly for 10 minutes, then start to get worse like it was.
Since changing the TFI made a marked improvement, and it plugs right into the PIP, I believe will need to change the PIP too.

 

Got the PIP replaced. Took a 25 minute drive about town, got it nice and warm, and the truck was happy and noticably free of even the tiniest hiccup.
I guess that shows its probably best to replace both ignition items, although the PIP rarely fails, after 31 years getting one is just cheap insurance.

Now to have adventures!

 

Good work

PIP = Profile Ignition Pick-up, its a signal

The device that makes this signal is the Hall Effect sensor inside the distributor, just FYI

 

Well, I have been driving it a while. The bucking comes back seemingly at random. Sometimes right at the start of a drive and then it subsides, but usually it begins messing up once its warmer. I am beginning to suspect something else in the wiring but while studying both the truck and the diagram nothing immediately jumps out.
Check engine light isn't coming on. I admit I am a little stumped.

 

Check MAP sensors vacuum hose for cracks, also unplug its 3 wire connector and plug it back in to clean connections

If the bucking only comes with changes to gas pedal/throttle plate then TPS is still suspect, new or not
May want to test voltages with it hooked up, key on engine off

If bucking happens when cruising then "maybe" ECT or IAT sensor, these are not common failures but often replaced, lol, because they are not expensive

 

I bored out the holes and put the new TPS on and got it adjusted down to 1 volt, I have not changed the Map, ECT or the IAT.
The vac hoses have been replaced, its just the randomness of the whole thing that is throwing me here.
If the wiring harness didn't resemble a rats nest from hell I could have more confidence in it.

Dont worry, there is solder under those wire nuts....

 

weekend update
I have been noticing something, that while driving down the road and it gets running bad, if I jiggle the ignition key a bit, it improved sometimes. So I dropped the steering column and sure enough the ignition switch broke into pieces in my hand. Fret marks are clearly visible where it has been arcing for some time now. So its probably been killing the injectors and spark and fuel pump and every other thing under the sun which is why I have been struggling with a diagnosis. Hopefully I can fit it all back together and it'll be ok after that.

Result: Nope, no change. But at least the dashboard fire waiting to happen is now prevented.

Something I am noticing, while the truck will begin screwing up at 3k rpm, it will start to even out at 4k.
found some more freyed wires underneath by the tranny and repaired them.

 

I did some in-depth stuff today. I went all through the wires, one by one and check continuity from the underhood plugs to the computer. While I had the computer out I opened it up too. Found some green corrosion inside the computer on some of the ceramic modules where they are soldered into the board. Was able to get it all off with a little 91% and a toothbrush.

I couldn't find anything strange at all with the wires, they all cont'ed out fine, I have verified all of them that have to do with the ECU and the ignition. From there I found the computer grounds(pin 40 and 60) had about 100 ohms(!) of resistance to the negative battery terminal. with some new wires and some scrubbing I managed to get that down to 7 ohms.

The test drive yielded some results, when the turn signal is used, the oil pressure gauge no longer dips down in rhythm with the blinker. Its still skips a bit but seems slightly improved. I wonder if the computer is taking a dump.

I still have work to do, but I am realizing that very little about these trucks actually has to be working correctly for them to start/run.

 

My wife's van had its timing belt break, so while putting it up to be fixed I bit the bullet and bought another engine computer for it since I am forced to drive it to work. It was immediately better.
Not 100% perfect, as it every so often has a skip, maybe once every 5 minutes around the 3k rpm mark, but the result is so dramatic I can safely say that perhaps the sketchy wiring screwed up the old computer and that is what I have been fighting this whole time. I test drove it for two days in multiple conditions before I made this post as I wanted to be sure that this was no flash in the pan fix as many of the others have been.

One thing that was telling I suppose is that when even as the motor was falling on its face I never got a check engine light for a spastic misfire.

 

Interesting issues you have here. Makes me nervous. I own a 1989 4x4 5 speed with 2.9 l engine. Just spent $1200 on a rebuilt tranny, clutch, slave, and starter. Mine has 160,000 so maybe I will be okay for another 50,000 miles. Electrical, electronics. Issues make me freak out!