Engine stutters at about 3K RPM

[Cees Klumper]

Sounds like there are issues with cylinder 4 - did you check compression yet (sorry if you reported but I missed it)? If there's an issue with the rings or a valve it could be burning oil, not enough compression etc. Secondly, perhaps the injector is leaking. You could try swapping injectors with a known good cylinder and see if the problem moves to that cylinder. If you have a node tester (correct term?) you can check whether the injector is getting a consistent pulse also. Same for injector 3 - if that cylinder does not appear to be working, again I would check spark, compression and functioning of the injector. Keep at it!

 

[KenHall1702]

latest update - still need help.
Found that I had a disconnected vacuum line that went from the air filter housing down to the vacuum motor that controls the CWM (puts warm air into the air intake to speed the warm-up process) don't think that is really needed when the temp is 95+ degrees, but mystery solved about the vacuum line.

I found that the "new" injectors we bought were not very good. #3 was not firing (injector wasn't opening) and #4 was leaking (fouled the plug in 10 minutes).
BTW, changed plugs to Motorcraft version that this beast rolled off the line with and gapped them myself and then took them to OReily for a sanity check on gapping.

Decided to rebuild my old injectors so changed the filters, O rings and spacers, and pintel caps on all 6.
Now #3 is firing - haven't run it long enough to know about #4 leaking, but I think that is fixed also.
It idles like a dream now. The engine doesn't; die when the ac compressor kicks in. It has excellent power now up until the time that it hiccups.

BTW since I was taking the injectors out, I took the opportunity to clean out the intake plenum (Oven Cleaner did a fantastic job). It looks brand new inside and out.

Cleaned the throttle body (did not hit it with oven cleaner) since I had it removed also so now it can breathe much easier.

It seems to me to be something electrical given how quickly it cuts and returns.
Given the luck I had with "new injectors" I am wondering now about the "new distributor" which came with a TFI module.
What have been the symptoms of a bad TFI? Mine is the Grey one but to be honest, I don't recall what color my original one was. I didn't learn until later that black vs grey was significant. If I have the wrong one, what would be the impact?

Any other electrical issues that would cause hiccuping?

 

[KenHall1702]

If your engine is equipped with EGR, try unplugging the vacuum line and driving it. Sometimes the spring in the valve gets weak with age and allows the valve to open too far. Excessive exhaust back pressure can do the same thing.

No EGR on this model....

 

[KenHall1702]

After refurbishing the old injectors and pulling out the trashy "new" injectors, it idles wonderfully now.
I also cleaned out the intake plenum with Easy Off Oven Cleaner (after removing all sensors), it looks brand new now inside and out.

It has markedly more power now, but still glitches at about 2500 rpm. As long as I shift a little early, I was able to get it up to 65MPH today, but if I go much over 2500 rpm, it will stutter (regardless of gear).

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


3/25/2022
Clutch, Pressure Plate, Flywheel, Pilot Bearing, Rear Main Seal, Clutch Slave Cylinder, O2 sensor

4/9/2022
Fuel filter, TFI Module, high pressure fuel pump

4/17/2022
Plugs and wires - new

4/21/2022
Distributor (includes new TFI Module, Rotor, Cap, Pick up Coil)

5/17/2022
All 6 fuel injectors replaced with new units.

5/18/2022
Regapped spark plugs
Ohmed out plug wires all withing spec for carbon fiber

Verified cylinder compression - all were 150 psi
Verified fuel pressure. 40 psi when key on, 32 psi when running.

5/21/22
Replaced Idle Air Control valve
Cleaned Throttle Body
Replaced Manifold Absolute Pressure (MAP) sensor

5/30/22
Replaced plug wires. This pair has lifetime warranty.
Replaced TPS after KIER test threw 73 code twice.
Replaced in tank fuel pump with one with lifetime warranty. This got gauge working.

6/15/22
Replaced plugs with Motorcraft versions which are the type it rolled off the assembly line with.
Motorcraft Plug Number AWSF42CA Spark Plug

6/16/22
Figured out where the mystery vacuum line went to. Part of the Cold Weather Modulator system and it went to the vacuum motor bypass on the air intake which was totally obscured by the battery platform. Took it all out, replaced the collapsed breather hose from manifold to intake.

7/3/22
Replaced the negative battery cable and cleaned ground connections to frame and firewall as well as engine block.