Help! Engine Stumble

[TheDanMan]

I cannot seem to fix this truck. I have replaced the throttle position sensor and the IAC sensor as well as all new spark plugs and wire wires and it also has a new TFI I checked for fuel pressure and I have 30 psi I also put a new oxygen sensor. I have plugged all of the vacuum leaks I can find and I also have a new MAP sensor. I cannot seem to figure out what is causing this stumbling issue. It happens a lot more when it is cold and this is the worst it has gotten in the video. Most of the time it is just a quick stumble or two when I’m accelerating. Sometimes it happens and sometimes it won’t. This video was taken about an hour and a half in to a two hour drive. So the next day I drove it on the highway a little bit and the problem disappeared, the day after that it came back. It also idles’s just fine and never has trouble starting. Please help I’ll take anything I can get. It’s an 89 Ford Ranger 2.9. Video
https://drive.google.com/file/d/1KIjgTq6_LOfHFpq-0OWU6wYAuKyQtHq-/view?usp=drivesdk

 

[Shran]

Your video does not allow anyone to view it.

We'll try to help out as best we can, there are a lot of factors involved in issues like that. Maybe a loose ground or something.

I also removed your duplicate thread in the general discussions forum, no need to have more than one.

 

[TheDanMan]

I just fixed it now. Everyone can view it. Thank you.

 

[SenorNoob]

30 pounds sounds low to me, but I'm a 4.0 guy. Those run 42 with no vacuum and as low as 35 with vacuum. Who knows how accurate your gauge is either...

That said, it kinda sounds like a miss. How long since you've had spark plugs, wires, distributor cap, and rotary button?

 

[TheDanMan]

30 pounds sounds low to me, but I'm a 4.0 guy. Those run 42 with no vacuum and as low as 35 with vacuum. Who knows how accurate your gauge is either...

That said, it kinda sounds like a miss. How long since you've had spark plugs, wires, distributor cap, and rotary button?

Spark plugs wires and TFI has been done within the past five months, but the problem was persistent before them, and I don’t know about distributor and cap and rotor, but they look new

 

[Brain75]

60 mph, July and the temp is below the N in normal... how did you get that to happen? I am between O and R all the time after the first 30 seconds. Looks like cold, "fully warmed up" and definite miss, tach drops are synched to the stumble. Not throwing any codes without a CEL?
I'd start by going for codes, and then work backwards through the last thing changed in order and when it first appeared. But like Shran started us off down the path I am thinking in the ignition system, plugs, wires, distro, cap etc - the "fire" part of Fuel+Air+Fire.

 

[rusty ol ranger]

I cant view the video but if your tach is jumping around id look at the coil plug.

Also...30 is a bit low for fuel pressure...probably not your issue but might contribute

 

[TheDanMan]

60 mph, July and the temp is below the N in normal... how did you get that to happen? I am between O and R all the time after the first 30 seconds. Looks like cold, "fully warmed up" and definite miss, tach drops are synched to the stumble. Not throwing any codes without a CEL?
I'd start by going for codes, and then work backwards through the last thing changed in order and when it first appeared. But like Shran started us off down the path I am thinking in the ignition system, plugs, wires, distro, cap etc - the "fire" part of Fuel+Air+Fire.

I did a coolant system flush, that helps

 

[TheDanMan]

I will also add the check engine light codes came back as 16, 31 and 84

 

[Brain75]

from the guide here on TRS:
16

1.9L & 2.5L – Throttle stop set too high – IDLE or Idle Set Procedures

2.3L – RPM’s too low – IDLE

(O) Electronic ignition – IDM circuit fault – Ignition Systems

​

31

EVP – (O, R, M) EVP signal is/was out of range – EVP

EVR – (O, R, M) EVP signal is/was low – EVR

PFE – (O, R, M) PFE signal is/was low – PFE

84

EGR Vacuum Regulator – Solenoids

EGR cutoff solenoid – Solenoids

EGR Vent solenoid – Solenoids

-----------------
I would check:
1) spout circuit, is it grounded out? (supposedly the spout grounded out causes a 16)

2) fuel pump dying, short in the wiring to the pump - 31 is either bad pressure sensor or bad pressure... since you are low ish (30 psi) and we see what could be fuel delivery issues it's worth considering - partially clogged line, intermittent shorted wiring (especially since it goes away one day and comes back)

3) EGR plugged, solenoid shot, wiring shot (84).

I'd also clear the codes, idle it for 5 minutes and see if only 1 comes back. especially if only 1 comes back and the stumble happens. I don't really like hunting down the 16 code cause that gets all over the place and is mainly an idle issue not a 2k rpm going 60mph down the road.

Edit: ignore my stupidity on the 31 code, listen to chatgpt below.

 

[Brain75]

finely tuned chatgpt prompt in this case seems more helpful than even me - it argues against the 31 and 84 (they aren't ignition at all - just emissions) and points us at PIP or TFI.
Chat GPT's comments:
The tachometer dip is the clue that changes the diagnostic path.

If the tach is dipping at exactly the same instant as the stumble, that suggests the engine is actually losing ignition events, not simply running lean. The factory tach on a 1989 Ranger is driven from the ignition system (negative side of the coil/TFI), so every time the ignition quits producing pulses, the tach follows.

The OBD-I codes are interesting too:

Code 16 – IDM (Ignition Diagnostic Monitor) circuit failure / SPOUT circuit issue depending on calibration. This is an ignition-related code.
Code 31 – EVP/EGR sensor below minimum voltage (or EGR not opening when expected). Common on these trucks and often unrelated to driveability at steady cruise.
Code 84 – EGR Vacuum Regulator (EVR) circuit failure.

Notice that two of the three codes are EGR-related, but only one points toward ignition.

My probability ranking
1. TFI module (very common) ★★★★★

The 2.9 Cologne is notorious for TFI failures.

Typical symptoms:

intermittent stumble
tach drops
worse after warming up
may recover instantly
often no complete stall

The module can momentarily stop triggering the coil, then immediately resume.

Classic failure.

2. Distributor Hall-effect Pickup (PIP sensor) ★★★★★

Almost tied with the TFI.

The PIP tells the TFI where the distributor is.

If the PIP momentarily loses signal:

ignition stops
injector timing stops
tach drops
engine stumbles
instantly comes back

The PIP on these trucks often becomes heat-sensitive.

One clue:

if the tach instantly falls to zero while the engine is still spinning, suspect PIP.

Unfortunately replacing it requires pulling the distributor apart. Many people simply install a reman distributor because the gear, shaft bushings, PIP, and seals all get renewed.

3. Ignition Coil ★★★★

A failing coil can intermittently stop producing spark.

Usually:

misfire under load
heat sensitive
tach sometimes flickers

Less common than TFI/PIP.

4. SPOUT connector or wiring ★★★★

These trucks are now almost 40 years old.

Corrosion around:

coil connector
TFI connector
distributor connector
engine harness near distributor

can produce exactly this.

Especially if vibration at highway speed causes it.

5. Ignition switch electrical section ★★★

Ford ignition switches of that era are infamous.

Usually causes:

radio flickers
gauges drop
complete engine cutout

Since only the tach appears affected, I'd rank it lower.

6. EGR sticking open ★★

Normally this would produce:

stumble
surging
lean cruise

But it should not make the tach drop.

That's why I think Codes 31 and 84 may be a separate issue.

7. Fuel pump / pressure ★★

Fuel pressure loss causes:

stumble
bucking

but the tach generally remains steady.

The tach drop argues against fuel.

Why the tach matters

Imagine constant throttle at 2000 RPM.

If fuel pressure drops:

RPM stays about 2000
tach steady
engine bucks

If spark disappears for 100 ms:

engine instantly slows
tach dips
combustion resumes
tach recovers

That matches your observation much better.

About those EGR codes

A failed EVR solenoid (Code 84) could also produce Code 31 because the EGR valve never receives vacuum.

That combination is common.

Even if the EVR is dead:

cruise emissions suffer
NOx rises
maybe a slight surge

but it shouldn't produce ignition pulse loss.


The highway cruise condition is another clue

You said:

constant throttle
60 mph
~2000 RPM

That's almost the perfect operating point for:

highest distributor temperature
moderate cylinder pressure
maximum EGR activity
minimal engine vibration

A failing TFI module often starts acting up under exactly these steady-state conditions because it has been heat-soaked for 20–30 minutes.