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Discussion Starter #1 (Edited)
I have looked around on this forum, other forums, & googled everything I could think of for this without finding anyone with the same symptom.

I have a 90 Bronco 5.8L auto.

The problem I'm having is under very slight throttle I get a hesitation/stumble/bucking. The best way I can describe how much throttle is the amount of throttle used to go through a drive thru. This is where I notice the problem mostly. If I keep the throttle in this position it seems to do this indefinitely. Anywhere else in the throttle range and it runs great. Also, sometimes when I go uphill really slow, if I let off the throttle quickly, it will stumble/hesitate and sometimes die.

Stuff I Have Done In The Last 1,000 Miles or Less(this problem was occurring prior to changing anything):

Spark Plugs, Cap & Rotor, Plug wires, Fuel Filter, Ground/Starter /+ Cable Upgrade to 2 gauge, O2 Sensor, Air Filter, Coolant Hoses, Change All Fluids, PCV valve, TPS, ignition coil, coolant temperature sensor, fuel pressure regulator, clean out foam filter in vacuum bleed sensor, cleaned out IAC, new throttle body gasket, cleaned throttle body and butterflies

I checked the EGR Valve with a vacuum pump, it holds vacuum & appears to operate normally, I did the same with the Fuel Pressure Regulator it appears fine. I checked vacuum hoses for leaks, & changed out most of the rubber vacuum hoses. Timing is at 10* btdc with the pill out. Checked fuel pressure during start up & idle, both are fine.

I'm thinking maybe bad BAP, or fuel pump anyone else ran into this before?

Video of problem:

 

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Discussion Starter #3
Check your TPS and fuel pressure. See this link, read to the end:

88 351 idles fine, stumbles under load
Looks like that was a fairly similar problem. I haven't looked yet, do you know if my fuel rails have a Schrader valve that's easy to access? I have a fuel pressure gauge I can hook up.

Just popped the hood, Schrader valve is right there it in the open, easy to get to. I'll probably check fuel pressure later today.
 

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I'd also do a visual inspection of the throttle cable on top of the thorttle body to see if it's binding in any way inside that spring and maybe clean and lubricate that big spring on the throttle body........easy peasy!

Good Luck ~ :thumbup
 

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Could be fuel related. Fuel pressure check and new fuel filter will eliminate that as an issue.

You almost completed the 6 litre tune up - replacing the coil would be the only item I see that was left of the tune up to do list. Consider investing another $30 to complete this.

You might also consider removing the MAS/MAF throttle body. Mine was yellowed and grimey. Some Seafoam cleaned it up. Might be sticking and causing the stumble.

You are really close to having a well running Bronco don't get discouraged.
 

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Discussion Starter #7
I'd also do a visual inspection of the throttle cable on top of the thorttle body to see if it's binding in any way inside that spring and maybe clean and lubricate that big spring on the throttle body........easy peasy!

Good Luck ~ :thumbup
Will do, throttle cable seems pretty smooth, but you never know.

Could be fuel related. Fuel pressure check and new fuel filter will eliminate that as an issue.

You almost completed the 6 litre tune up - replacing the coil would be the only item I see that was left of the tune up to do list. Consider investing another $30 to complete this.

You might also consider removing the MAS/MAF throttle body. Mine was yellowed and grimey. Some Seafoam cleaned it up. Might be sticking and causing the stumble.

You are really close to having a well running Bronco don't get discouraged.
Today 11:28 AM

I was thinking about the coil too, it's relatively inexpensive & I think they even offer lifetime warranty.

When I looked inside the throttle body during my K&N FIPK install it looked pretty darn clean. I'll check it again anyway. I changed the fuel filter less than 1k miles ago, so unless the tank was really dirty it should be ok. Plus the problem didn't change at all when I changed the fuel filter
 

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Dude, you haven't finished the initial recommendations. Check the Fuel Pressure to see if it holds pressure or falls during acceleration. You are looking at the fuel pump, not the new filter. Also test the TPS as they have been known to develop dead spots causing hesitation. Rule these basic items out first and then look at other areas.

See this link: Throttle Position Sensor Testing, Replacement and Adjustment

If all is within parameters, then test your coil, spark plug wire condition, spark plug wire routing, timing advance, spark plug gap and ICM. All should be per manufacture's recommendations with the 5.8L.

BTW, what part of Sacto are you from? I grew up in Wilton with a few years in Greenhaven.
 

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Discussion Starter #9
Dude, you haven't finished the initial recommendations. Check the Fuel Pressure to see if it holds pressure or falls during acceleration. You are looking at the fuel pump, not the new filter. Also test the TPS as they have been known to develop dead spots causing hesitation. Rule these basic items out first and then look at other areas.

See this link: Throttle Position Sensor Testing, Replacement and Adjustment

If all is within parameters, then test your coil, spark plug wire condition, spark plug wire routing, timing advance, spark plug gap and ICM. All should be per manufacture's recommendations with the 5.8L.

BTW, what part of Sacto are you from? I grew up in Wilton with a few years in Greenhaven.
I plan on doing everything I can for free first, then replace things as necessary. Right now I just need some free time, been working a lot lately, so free time hasn't been there. I'm off this Sunday & may take some time out of my day for this stuff. Most of it can be done in an hour or so. Although replacing things that may not seem bad doesn't bother me. The O2 sensor & ground cables seemed perfectly fine, but after I changed them my Bronco runs so much better, I still can't believe it. Feels like an easy 30hp was gained.

Timing is good to go. Checked and adjusted that when I first got this bronco. The distributor was loose when I got it so timing would randomly move a little. I set it for whatever the manual said 8 or 10 degrees btdc if I remember correctly.
 

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Discussion Starter #10
Finally got around to changing my TPS. It runs a lot better, but didn't solve this problem.
 

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Discussion Starter #14
Just changed the ignition coil today and it made part of the problem(seen in the video) almost non existent. It still does what's seen in the video, it's just not as dramatic. I also cleaned out the foam filter in the sensor to the left of the ignition coil.
 

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Discussion Starter #17
Fuel pressure I can only test at idle, and prior to start up, both were fine. Haven't checked while under load since I dont have a way to check that currently my tester doesnt have a long enough hose unfortunately.
 

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Discussion Starter #19
Try a different distributor.
If it's the same as 5.0 distributors from that era I have my old one from my 5.0 Mustang that I could try. Autozone now has new ones for like $70, not a bad idea to replace it anyway.
 
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