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Engine stutters at about 3K RPM

KenHall1702

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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
Thanks for the heads up. Fortunately for him, he was not having misfires, only curious. I am having one cool cylinder and misfires. But after reading that thread, not sure that my that my "cool" pipe is outside of the realm of norm.
 


KenHall1702

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Mine did the same thing, same RPMs. Changed cheapplugs with no effect. Bought expensive high dollar plugs. Worked great.
We were running "cheap" ($2) plugs initially - actually, my buddy had picked them up and they were the wrong plug entirely. I went and got a set from O'Reilly about $8.99 each but the symptoms have not changed.
 

KenHall1702

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So update, the problem still exists, between 2500 and 3000 rpm I get misfires.
I finally broke down and created an access panel to get to the rear (in-tank) fuel pump.
I was very surprised at how clean the tank was. Zero sediment but the fuel gauge had stopped working and the pump was making no noise so change it I did.
The gauge is good to have back, and I hear the new pump working. There is no change on the pressure measured at the fuel rail, but I am not able to measure it while I am driving, only in the garage.
I also checked the fuel filter that is between the tank and high-pressure fuel pump and turns out for my 88 that it has no filter, only an empty cup so no blockage there.

So replaced items now include
Rear (low pressure) fuel pump
Front (high pressure) fuel pump
Front in-line fuel filter
all 6 fuel injectors
all 6 spark plugs triple checked gapping of .044
all plug wires
ignition coil
distributor (to include cap/rotor/pick up coil)
MAP sensor
Idle Air Control valve
Throttle Position Sensor
PCV valve (and grommets that had the consistency of gummy bears)
Inertia switch bypassed

Key On Engine Running gives 11 (System OK)
I have checked timing many times and it is set at 10 degrees TDC base timing with SPOUT removed.
I cleaned the throttle body and intake as well as I could with cans of Throttle Body Cleaner.
Checked cylinder compression and all are 150 (I don't know how to do a leak down test)
I ran a can of Seafoam through the system with 1/4 tank (maybe less) so pretty concentrated.
I opened up the computer and it is beautifully clean, no corrosion on connectors or anywhere on pcb.
Wiring harness near computer looks good, no abrasion, ground is secure.

Access panel creation:
 

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KenHall1702

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If a standard combustion test shows 150psi and 101psi is the minimum, does that mean I do not need to perform a leak down test or do they test different characteristics?
 

RonD

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If you mean compression test, then there is a big issue if 150psi is average and there are any cylinders below 135psi

You can put a teaspoon of oil in the low cylinders and see if they come up to 150psi, if not then you have a bad valve, if it comes up to 150psi then bad rings

Leak down test does similar, but you can HEAR the leaking valve at tail pipe or intake, OR oil dipstick tube in the case of bad rings, you can hear the air escaping
 

Uncle Gump

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I should just do mine while I'm working on the fuel system.
 

RC2

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So update, the problem still exists, between 2500 and 3000 rpm I get misfires.
I finally broke down and created an access panel to get to the rear (in-tank) fuel pump.
I was very surprised at how clean the tank was. Zero sediment but the fuel gauge had stopped working and the pump was making no noise so change it I did.
The gauge is good to have back, and I hear the new pump working. There is no change on the pressure measured at the fuel rail, but I am not able to measure it while I am driving, only in the garage.
I also checked the fuel filter that is between the tank and high-pressure fuel pump and turns out for my 88 that it has no filter, only an empty cup so no blockage there.

So replaced items now include
Rear (low pressure) fuel pump
Front (high pressure) fuel pump
Front in-line fuel filter
all 6 fuel injectors
all 6 spark plugs triple checked gapping of .044
all plug wires
ignition coil
distributor (to include cap/rotor/pick up coil)
MAP sensor
Idle Air Control valve
Throttle Position Sensor
PCV valve (and grommets that had the consistency of gummy bears)
Inertia switch bypassed

Key On Engine Running gives 11 (System OK)
I have checked timing many times and it is set at 10 degrees TDC base timing with SPOUT removed.
I cleaned the throttle body and intake as well as I could with cans of Throttle Body Cleaner.
Checked cylinder compression and all are 150 (I don't know how to do a leak down test)
I ran a can of Seafoam through the system with 1/4 tank (maybe less) so pretty concentrated.
I opened up the computer and it is beautifully clean, no corrosion on connectors or anywhere on pcb.
Wiring harness near computer looks good, no abrasion, ground is secure.

Access panel creation:
So update, the problem still exists, between 2500 and 3000 rpm I get misfires.
I finally broke down and created an access panel to get to the rear (in-tank) fuel pump.
I was very surprised at how clean the tank was. Zero sediment but the fuel gauge had stopped working and the pump was making no noise so change it I did.
The gauge is good to have back, and I hear the new pump working. There is no change on the pressure measured at the fuel rail, but I am not able to measure it while I am driving, only in the garage.
I also checked the fuel filter that is between the tank and high-pressure fuel pump and turns out for my 88 that it has no filter, only an empty cup so no blockage there.

So replaced items now include
Rear (low pressure) fuel pump
Front (high pressure) fuel pump
Front in-line fuel filter
all 6 fuel injectors
all 6 spark plugs triple checked gapping of .044
all plug wires
ignition coil
distributor (to include cap/rotor/pick up coil)
MAP sensor
Idle Air Control valve
Throttle Position Sensor
PCV valve (and grommets that had the consistency of gummy bears)
Inertia switch bypassed

Key On Engine Running gives 11 (System OK)
I have checked timing many times and it is set at 10 degrees TDC base timing with SPOUT removed.
I cleaned the throttle body and intake as well as I could with cans of Throttle Body Cleaner.
Checked cylinder compression and all are 150 (I don't know how to do a leak down test)
I ran a can of Seafoam through the system with 1/4 tank (maybe less) so pretty concentrated.
I opened up the computer and it is beautifully clean, no corrosion on connectors or anywhere on pcb.
Wiring harness near computer looks good, no abrasion, ground is secure.

Access panel creation:
If a standard combustion test shows 150psi and 101psi is the minimum, does that mean I do not need to perform a leak down test or do they test different characteristics?
 

RC2

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Better fix for the access panel is to go to the junkyard take your cutout with you and cut out a new piece about an inch bigger all the way around (lay yours on the other deck so lines match up) from a junker then use short sheet metal screws. and some non hardening silicone. Lot more solid if you go throwing stuff in that trunk. If you can find the same year B2. Looks like you did something for reinforcement there with the screws though
 
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Cees Klumper

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I would check all the vacuum lines on the throttle body and intake manifold - nothing loose or leaking, no uncapped vacuum ports? Routing of the lines: are the ones that are supposed to be on the throttle body actually connected there, vs the ones that are supposed to be on the intake manifold? Especially if you have an uncapped port or an unconnected vacuum line can cause bad running issues.
Also, maybe check the electrical ground to the engine: temporarily connect a starter cable from - to the block, body, to make sure everything is getting the full 12.X volts
 

RC2

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Looking around online i saw someone else said they had this problem and it ended up being the gaps on his plugs not being set right just throwing that out there for ya he had a 4.0 liter. He said it smoothed right out for him.


That is the one your engine left the factory with

I looked at the pic u have of the autolite and ac delco side by side, that ac delco looks bent over way too far unless my eyes are playing tricks .044 should be your gap
 
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Mechrick

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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.
 

KenHall1702

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So update on the shaky b2.
since the last update, there were a few suggestions.
Check coolant - yep, coolant present and accounted for.
Spark Plugs - I switched to Motorcraft, the part number that this thing rolled off the factory floor with, had gap independently audited by my friends at O'Reily as a sanity check. - it is spot on.
Noticed that with only 10 minutes of usage, the #4 cylinder plug is fouled (matt black carbon, looks like running way too rich).
Tried my version of power balance check and noticed two things.
When I removed the injector connection for #3 cylinder, there was no change in idle - so now I am thinking there is an issue with #3.... but what?
Then I removed the spark plug wire from #4 (can't get to the injector connector, buried under cables) and the idle shake disappeared.
So right now I have 3 and 4 spark plug wires removed, and it idles very smoothly.

I don't know if this is just the Bronco Gods messing with me or what. I don't initially know the relationship and why disabling 2 of 6 cylinders would improve things.



Recap on the Bronco Chronicles:
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.

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.
Not sure how to verify that the bi-metal sensor is working but everything is hooked back up at least. Verified vacuum motor bypass operates when vacuum is applied.
 

KenHall1702

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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.
0 results for EGR Valve for your 1988 Ford Truck Bronco II 2WD 2.9L MFI OHV 6cyl.
I don't think my 88 has an EGR
 

KenHall1702

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also verified no backpressure in the exhaust so cats are not constipated.
 

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