Engine misfires only slightly?

So there are a few things that can cause this issue.

1. Leaking or dirty injectors
2. Fuel pressure regulator
3. Engine coolant temperature sensor
4. O2 sensor lazy or biased rich
5. Exhaust leak ahead of O2
6. Compression issue on one cylinder
7. Base engine timing / distributor issue

Here is a guide to help fix the problem

https://www.2carpros.com/articles/why-an-automotive-engine-will-run-rich

Have you run the codes? This guide will show you how

https://www.2carpros.com/articles/ford-lincoln-mercury-obd1-1995-and-earlier-diagnostic-trouble-code-definition-and-retrieval

Also, please go over this video to help check the compression. If we have a problem here nothing above will fix it.

https://www.youtube.com/watch?v=KOxIm1F8AW0

Please go over these guides and get back to us.

I get decent looking fire pluses from coil when cranking it into a spark plug,( no engine firing). When running the fire pulses are wildly erratic, vary by 10 to 1 voltage. And they are the same on all plugs.

Did you run the codes to see if any are stored? Also I would check for vacuum leaks and I would replace the MAP sensor. What are the compression readings? Here is a video to help with the vacuum leak test.

https://www.youtube.com/watch?v=LRKWV4Are5c

Please go over this guide and get back to us.

I ran codes until I could do it properly and ended up Clear. I ran vacuum leaks by carb cleaner spray and by water flooding outside of engine, no change. Tested MAP with vacuum pump and oscilloscope, works to spec. Just finished compression: 1@ 150 2 @ 170 rest 160's. I assume I would get a code if temp sensors were bad? (I actually now remember one set a code cause I ran the scan with the engine cold) I'm going back now to do more checks on spark jitter and fuel injector pulses. Will reply after that

I have seen the EGR leak internally which can cause a symptom like an intake leak, if you can remove it here is a video on how to test it.

https://www.youtube.com/watch?v=DqUCbbKdWf4

Please let me know what you find.

At this compression, would a leak down test be really necessary? (I don't have the proper tester but could throw one together if needed

...before I put the spark plugs back in.

No, you don't need a leak down test, did you check the EGR valve for internal leaks?

I ran the engine with the EGR port on the intake plugged and the symptom didn't change. vacuum tested and found no leakdown. felt for exhaust flow with EGR off manifold and felt none with engine running. EGR not removed from exhaust tube.

I have the intake tubes and air cleaner off of the intake. The 2 big tubes are really clean, but the IAC tube is dusty inside; no sign of leaky connection. even the stub the feeds the IAC tube is clean inside. Any clue to this?

I'm planning on taking out the ECM to check pins , caps , etc.

Okay, I changed my mind on the leak down test, I think we should do one, if you have a leaky intake valve it will cause a slight misfire. Hold the throttle bore wide open when testing so you can hear if the intake valve is the cause.

1. IAC tube/hose question above.

2. Pulled the computer. Looks EXACTLY like the pictures online of blown caps. Right up to the blue corrosion on the adjacent resistor. So I fired up my iron and pulled all three caps. Then I went to test them.......and found they are not all the same value. So now is where we see if you're worth your salt. I didn't note which place the 10uF goes. Plus do you think anyone out there in auto repair land knows if the values for these caps sensitive to changes?
Thanks a Bunch

BTW, one of the caps is actually dead.

Good find, for a mid-90s Ford EEC-IV PCM, the common capacitor set is usually:

Two 47µF capacitors
One 10µF capacitor

On these Ford EEC-IV computers, the three electrolytic capacitors are commonly two 47µF capacitors and one 10µF capacitor, so they do not all go in the same location.

Look for the board markings near each capacitor location or compare your exact PCM catch code/board number to a matching repair photo.

Got the capacitors replaced. No help. I am getting so frustrated!

Can you please upload a short video from under the hood of the engine running? Also did we ever do that leak down test? I have also seen excessive gunk build up on the back side of the intake valves, that will need some disassembly though to look at.

maybe I need "another" bottle of 'smog pass' ?

I cant tell anything from the video. So lets do the leak down test to see if the intake valve are leaking. Also, I have seen a bad fuel injector cause this as well. Lets remove the parks plug and see what they look like. If one is darker than the others the injector can be bad.

when I had the plugs out for compression, they were all clean, but they only have a couple hundredmiles on them.
I have earlier pulled one bank of injectors up and applied voltage to them and they all sprayed. No sign of blowback. I have Removed the other 4 injectors, 1 was partly clogged( 1 of 4 holes) I cleaned and tested all of them with air for leakage and flow and put a little lube in/through as well. After reinstall, no change to symptoms.
I scoped again today left bank spark and all fire lines are consistent width. I scoped the injector fire pulse and found it to be about 5 with a variation of about .2, at idle
I'm recalling also seeing spark fires where the pulse went up to the normal max of about 10X but stayed there for half the whole period. I will redo both measurements and send photos.

I was hoping the fuel injectors would fix it, I would make sure you have OEM plug and coil wires, I am worried about the back side of the intake valve being gunked up blocking the intake port. You are sure the EGR is not leaking correct and you don't have a vacuum leak? Have you checked the PCV valve feed line? I have seen those collapse and cause a hard to find leak.

Did leakdown on cold engine. All ended up very good @6bar (my air runs down to 90 before motor kicks on). But on #7 initial pressure of 5-6 bar showed large leakage....started to listen around at exhaust-none...back to under hood and presto / no leakage. I recall a little leakage, then seal-up on a couple of the other cylinders also.
Scoping spark results are that all are firing the same and waveform looks to be to spec.

To confirm this engine has 73k on it correct? I would service the throttle body and IAC motor, you might want to redo the smoke test for the vacuum leak as well. The video of the engine idling is not very good and I cannot see how the engine is running. I am not sure how the video got messed up but can you please reload a new video?

Vehicle details: Standard Transmission, 4WD / AWD

continuing previous question but can't append to it
rough idle, CO meas 2-5 need

Yep, the CO will be high if the fuel injectors are leaking, have you tested them?

Please go over this guide

https://2carpros.com/articles/how-to-test-a-fuel-injector/

Let me know what happens.

I pulled my fuel rail and injectors out of the intake manifold, still have gas feed. I pulled all the feed wires from the injectors. I turn on the fuel pump -> 35psi. None of the injectors leak any fuel. I have tested them with air and a injector pulser and they seem good. Yet when the injectors were installed but without their wires, and I crank the engine, I get sputtering intermittent but ongoing firing. How can fuel be getting into the engine?

if I disable the fuel pump and relieve the pressure, the sputter/fire doesn't happen.

Yes, I think we found it, I would remove the vacuum line from the fuel pressure regulator to see if there is fuel inside of it and if so the regulator is bad.

unplug fuel pres. reg. and plug end . symptom remains. still 2 bumps on the O2. I swapped a couple injectors and the bumps don't change. There is definitely a missfire visible on my scope.I unplugged one of the likely injectors and one bump gets narrower and a bit ragged but definitely doesn't go away.

I would remove all off the spark plugs in order to inspect, see if one or more are black or very dark, this can tell us if an injector is bad or not. Please post the image of the spark plugs so I can see.

I went through the spark profiles again today and captured them on my oscilloscope. Some curious images, but I can't get them to attach to this reply

Here is a test image it seem to work okay, can you please test the system with an image?

I went through the spark profiles again today and captured them on my oscilloscope. Coil waveform is consistent across all 8.
Do you have any idea how I can get a double fire on only one plug out of a distributor?? Going out to do more checking on the double one.
Thanks

Now the double fire pulse is not present.
I still have more checks to do.......

Thanks for the graphs, yes this can be caused by a shorted ignition coil, rotor, cap or coil, spark plug wire, but you said you replaced all of those correct?

New plugs, wires, coil. Cap is recent, rotor looks OK.
I verified the right side: injector power, Injector switching to Gnd, sequentially pulled injector wires 1-4 and the rough idle only got worse.
Attached image for your opinion. Am I chasing a O2 condition (regular pulsing)that is not really a problem?
I measured the temp on by Cat: input 484F, outlet 418F after 20 min idle and then running ~2000RPM to test. Research says this is bad.

Yep, that does look like the cat is bad. I would try driving it though and test it again, idle might not give you an accurate test.

I will redo cat test.

the O2 waveform is the (only) upstream sensor.
What would cause the syncronis-to-spark pulses / are they enough to throw off the emissions?? (I haven't seen them on any waveforms online for reference)

An exhaust leak will cause this, can you hear any when the engine is running?

The pulses are only on 2 cylinders.
I was planning an Exhaust leak test anyway.

Yep, that will cause the issue we are having.

The pulses on the O2 are from ground bounce caused by fuel injectors firing 4 at a time.

Today, drove around town, +rural, + couple miles freeway. At home connected smog test, reading about 0.5 CO. (Almost jumped back in to drive to the Smog station) Left idling, after a couple minutes CO starts drifting 3, 1, 2...between 1 and 4 for several min. Tried running RPMs up to 1500, did a couple gooses : CO near 1-2 @1500RPM , then stayed there at idle, then back to wandering.
Back to wondering about fuel injector balance?? or what do you think?

summary:
1995 F250 4X4 460cid 72K mi
factory Federal smog Now in California

** No pass smog CO = 2 HC=54 @idle
** engine misfires
** Unknown coolant loss:
Has pressure after overnight.
No external leaks found even with air pressure applied for hours.
Combustion in coolant test not done (don’t own tool)
New Plugs New Wires New Coil
cap & rotor recently new
PIP scopes good
Spark line scopes good
KOEO pass
KOER pass
Compression 1@150 2@170 rest@165
Leak-down all about 5% {only rings heard hissing)
exhaust = 300deg CAT= 400deg (Flir)
Timing confirmed
Fuel injectors;
Found and fixed leakage when off.
Cleaned and pressure tests and pulsed but have not been tested for Volume / Balance
Tested res on both temp sensors, good @ cold pass OBD
Cold startup exhaust manifold temperature balance good (Flir)
Fuel pressure 30 / @35 w/o vacuum no gas in vacuum port
Exhaust leakage tested w/ blower and soapy water
Vacuum = 20 @ idle Tested OK by water and carb spray

test for misfire no help:
SPOUT
ICM
Unplug 1 injector
Idle Air valve
EGR {plugged}
MAP (unplugged)
PCV / Purge
ECM Caps renewed
wires @ ECM wiggled
air intake+filter pulled

HEGO scopes out normal sits at lean during idle
Thermactor air to ex manifold bypass cycles ‘normally’