Engine stutters at about 3K RPM

[KenHall1702]

I have been doing work on my son-in-law's

1988 Ford Bronco II
VIN 1FMCU12T6JUC37354
Year 1988
Style/Body SPORT UTILITY 2-DR
Engine 2.9L V6 OHV 12V

It needed a new clutch and we got that done (clutch, pressure plate, fly wheel, pilot bearing, slave cylinder) and that all seems to work fine.
Now that it can actually try to accelerate, we have a new issue.
At about 3000 rpm, the engine will stutter momentarily.
We then replaced the TFI module (thinking heat related issue) and also fuel filter, high pressure fuel pump.
Could not replace the low pressure pump in the tank due to mechanical issues from a slight accident.

No change in behavior.

I examined distributor cap and rotor (cleaned off very minor scoring on the end of the rotor), timing was set correctly and advance was working.
No change in behavior.

Today we replaced the plugs and wires.
No change in behavior.

The only thing I can think of now would be the pickup coil inside the distributor. I am a little hesitant to start that task as it looks like taking off the intake plenum and a ton of connectors.
Is there anything else that it could be and how would I diagnose them?

Oh yes, also measured coil resistance primary and secondary and they are fine.

This idles perfectly, it just starts to hiccup between 2000 and 3000 rpm. This can be while it is sitting in the driveway (no load) or pulling up a hill (under load).

Any thoughts or suggestions are welcome and appreciated.

 

[Jazzer]

The distributor can be removed itself by removing the holdown bolt and plate conveniently buried deep next to where the distributor shaft seats, if it’s anything like my ‘87 2.9. Get a light and inspection mirror on it and you’ll see.
Hard to get to, creativity required to get a wrench on it.
Others with more experience will be along with suggestions, no doubt.

Welcome to TRS!

-Jazzer

 

[RonD]

Welcome to TRS

Does it only have a vibration when driving or can you feel it stopped and just REVing the engine?

Check the fan clutch, near end of life it will come out of balance
Just move the fan blade in and out and see if there is play

On a COLD engine you can remove fan belt(s) from crank
Start the engine, Battery Light will stay on
REV engine and see if vibration is gone
You can only run the engine for 90seconds without the water pump, which is actually quite a long time to check things out
When done put fan bet(s) back on and start the engine again, let it run for a few minutes to circulate coolant
You can repeat this any time engine is COLD again

Test is also a VERY GOOD WAY to find engine noises, engine bay is amazingly quiet with no fan or accessories spinning

 

[KenHall1702]

Update
So the stuttering is misfires and they get bad between 2k and 3k rpm.
Today I replaced the distributor (thinking it may be the pickup coil) there was no change in behavior.
Distributor also included new TFI module, new rotor, new cap.
Othering things I have done...
Replaced Fuel Filter
Replaced High Pressure Fuel Pump
Verified fuel pressure at rail is steady 40psi
Verified fuel pressure regulator is functioning (disconnected vacuum and noted change in psi)
Changed all plugs
Changed all wires

Latest suggestion was possibly fuel injectors.
I am not sure how to diagnose those or what remediations are available (gas additives, direct cleaners introduced into fuel rail, removing and cleaning injectors, replacing injectors)

20220419_153624.mp4

drive.google.com

 

[KenHall1702]

Update, changed all 6 fuel injectors with no change in behavior. Still runs a little rough and then between 2.5 and 3K it sputters terribly.

 

[gaz]

Check your coolant level, if it's low determine whether it is losing coolant or just improperly serviced.

 

[Uncle Gump]

What brand plugs?

If they aren't Motorcraft or Autolite... I would try that.

Have you tried tweaking the timing a couple degrees?

You say it roughs a bit rough and really bad at midrange. How does it run at WOT?

 

[RC2]

Mine was/is doing that too same thing. Have not had it running yet since valve covers and new injectors. have leaky PS lines that need replaced before i run it. I noticed my throttle body the butterfly is very dirty probably backside too so I will clean that too. Linkage to TB looks like no one has ever cleaned and lubed it. Also wondering how long this sat around before I bought it could have bad/old fuel.

 

[RC2]

Update, changed all 6 fuel injectors with no change in behavior. Still runs a little rough and then between 2.5 and 3K it sputters terribly.

Did you get the six pack of injectors for 60 bucks on Amazon? I could not get myself to buy them I had same issue and got Standard Motor Products injectors. They are several different specs too, my old injectors had a code stamped on them I had to match up with the ones on Rock Auto mine was an E67E-B1B. 42 bucks a piece. But my miss and shake are gone. I was out on the road this evening runs like new.

 

[KenHall1702]

Check your coolant level, if it's low determine whether it is losing coolant or just improperly serviced.

Coolant level is steady and full.

 

[KenHall1702]

What brand plugs?

If they aren't Motorcraft or Autolite... I would try that.

Have you tried tweaking the timing a couple degrees?

You say it roughs a bit rough and really bad at midrange. How does it run at WOT?

What brand plugs---- this was key. The brand was Autolite and is now Ac Delco, but the Autolite set that my assistant picked up was not the right set for the B2. The threading only went half way so the spark was not even in the combustion chamber. Now that I have the right plugs in play, I can get on with setting proper timing.

 

[Uncle Gump]

Well... that will do it.

I always mention the plugs... I've had bad experiences with other brands.

 

[ecgreen]

Mine did the same thing, same RPMs. Changed cheapplugs with no effect. Bought expensive high dollar plugs. Worked great.

 

[KenHall1702]

I did a temperature check this morning and found that cylinder #3 is cooler than the rest by at least 80 degrees and #5 is running hotter than the rest.
I was thinking maybe an issue with injector wiring so since #3 and #6 are physically close and on the same bank, I swapped the connectors. The problem stayed with the cylinder, not the connector.

 

[RC2]

I did a temperature check this morning and found that cylinder #3 is cooler than the rest by at least 80 degrees and #5 is running hotter than the rest.
I was thinking maybe an issue with injector wiring so since #3 and #6 are physically close and on the same bank, I swapped the connectors. The problem stayed with the cylinder, not the connector.

here is a thread someone is talking same thing. Carburetor though and different car https://www.yenko.net/forum/showthread.php?t=105659

Probably other threads out there too