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EFI Bucking and hesitation problem

77broncodriver

Full Member
Joined
Nov 21, 2009
Messages
344
Hey guys I am trying to get my bronco running right after swapping to mustang EFI a few years ago. It has always had this issue but after a while I just lived with it and it was parked for about a year. Now I done a lot of other work to it and have been driving it a lot and now am noticing the problems of the EFI again.

Main issue:

It starts up perfect overtime no matter how long it sits. Driving around town, it has no issues but once I give it some gas it will hesitate when under load. Its not like it is bogging down but more that the spark cuts off for a split second. I really notice it when Im on the highway, it will constantly buck and hesitate cruising when I have the throttle more than 20% down.

This makes me believe that this is a spark problem and not a fuel problem

Here is the parts list I have for the EFI:

- 5.0 crate engine from ford (306 with aluminum heads and cam ~20k miles)
- explorer intake
- 30lb injectors with matching pro m maf sensor (previously had stock injectors and stock maf with same problem)
- RJM harness
- Replaced all other sensors including o2 sensors
- New MSD distributor
- New coil
- In tank fuel pump

Issues Ive had and fixes Ive tried

- First I thought the injectors so I got the 30lb injectors (what ford recommends for this engine) with matching maf
- Then I got a new distributor, spark plugs, wires, and coil
- Then I thought I needed a tune so I had it dyno tuned which was a waste of money. It ran really strong but still hesitated, turns out my harmonic balancer spun on crankshaft so after putting a new balancer on the tune wasn't right anymore so put it back to stock tune
- Converted my inline fuel set up to intank set up
- Checked timing
- Checked fuel pressure

There are probably another couple things I have tried but I've been tracing this down for several years so I probably forgot a few things.

I am thinking this has to be an electrical issue but don't know where to start looking. If anyone has experienced this and has any ideas let me know!

I really want to get this thing running right, Im really temped to sell all the EFI parts and buy a holley EFI unit or one of the other new TBI packages.

- Evan
 

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ransil

Bronco Guru
Joined
Sep 6, 2003
Messages
8,122
Dyno tune should help in pointing to what might be wrong if the guy is good.

to rule out fuel monitor fuel pressure while driving when the condition occurs anything below 30 psi I would look into it.

may need a wide band O2 installed and log test runs, something that can be done at the dyno.

aftermarket trinkets could be an issue.

30 lbs look big for a 306, I would think 19 would handle maybe 24 if something more is needed.
 
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77broncodriver

77broncodriver

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Nov 21, 2009
Messages
344
Any suggestions on how to check fuel pressure while driving?

I’ve gone back to stock injectors and maf and it is the same, I put the 30’s back one just because they are brand new/maf is brand new. For the dyno tune, it would cost another $600 for the tune plus another $100-200 for gas to get to the tuner. Might as well just upgrade to one of the new efi units for that price.


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77broncodriver

77broncodriver

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Nov 21, 2009
Messages
344
It does seem to run really rich, I don't have an AFR gauge but just from the smell and what the exhaust pipe looks like. I am going to order a fuel pressure gauge and will check to see what it is reading, if so ill have to order an adjustable fuel pressure regulator.
 

Viperwolf1

Contributor
electron whisperer
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Messages
24,316
Make sure the O2 sensor wires are on the correct sides.
 

73azbronco

Contributor
Bronco Guru
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Nov 11, 2007
Messages
7,796
First steps as always in diagnosing EFI: fuel pressure, and then fuel flow. Next and simplest issue, you said it sat for a year, you are sure the fuel is good?

What computer you using? Have you tried another one?
Have you tried moving wires around under the hood while the motor is on? Be careful:) It sounds like you might have an intermittent issue with wiring/computer.

I'd think ignition would be the next most likely area if it's hesitating on launch, again, ignition switch and wiring all good? You using an MSD AL6? Whats controlling spark, the computer? Once it's running I'd run a small wire from batt hot to ign hot, so you know you have power and go for a short drive. That would bypass the ignition switch wiring.

I agree that 30lbs sounds larger than needed but if it idles and runs right mostly, I'd not worry about that right now.
 

gavindg

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Feb 6, 2017
Messages
283
Loc.
Westlake Village
Easily overlooked but have you changed the fuel filter?
Mine had the same issue when it was plugged.
O2 sensors would cause popping noise if they were bad.
Was the TFI replace with the distributor?
 
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77broncodriver

77broncodriver

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Nov 21, 2009
Messages
344
First steps as always in diagnosing EFI: fuel pressure, and then fuel flow. Next and simplest issue, you said it sat for a year, you are sure the fuel is good?

What computer you using? Have you tried another one?
Have you tried moving wires around under the hood while the motor is on? Be careful:) It sounds like you might have an intermittent issue with wiring/computer.

I'd think ignition would be the next most likely area if it's hesitating on launch, again, ignition switch and wiring all good? You using an MSD AL6? Whats controlling spark, the computer? Once it's running I'd run a small wire from batt hot to ign hot, so you know you have power and go for a short drive. That would bypass the ignition switch wiring.

I agree that 30lbs sounds larger than needed but if it idles and runs right mostly, I'd not worry about that right now.


I think the computer is the a9p? its the manual transmission one. I have tried another computer before and it was the same.

I have a feeling this is because of wiring, maybe grounding to the coil?

I am using msd distributor that is ran off of the computer.

I only did the 30lbs over the 24lbs because thats what fords manual says, I thought it seemed a bit much for a stockish motor too.
 
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77broncodriver

77broncodriver

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Nov 21, 2009
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Easily overlooked but have you changed the fuel filter?
Mine had the same issue when it was plugged.
O2 sensors would cause popping noise if they were bad.
Was the TFI replace with the distributor?

I will have to look at what filter is on there, I have changed the fuel system so many times in the past 5 years I dont remember what is on there. If its from a while ago it could be plugged.

o2 sensors are new and the tfi is new as well. Went through about 5 of the oem ones then switched to an msd unit and it has held up since.
 

gavindg

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I will have to look at what filter is on there, I have changed the fuel system so many times in the past 5 years I dont remember what is on there. If its from a while ago it could be plugged.

o2 sensors are new and the tfi is new as well. Went through about 5 of the oem ones then switched to an msd unit and it has held up since.

Definitely replace the fuel filter then and keep us posted
 
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77broncodriver

77broncodriver

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I just looked up when I did the fuel filter. I put it on when I converted to an in tank fuel pump last year. I haven’t driven it too much since then so that should be fine and it was doing this before the fuel pump swap.






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gavindg

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Have you looked into vapor lock?
Where are the Fuel lines running?
How much fuel in the tank?
Have you been driving on hot days?
 
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77broncodriver

77broncodriver

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It happens even with a full tank, and the lines don’t go by the exhaust or anything. And doesn’t matter the temp outside. It happens when it’s 105 out or if it’s cold. The trip that reminded me of how bad it runs was driving early in the morning a couple days ago. Probably 50 out.

Wouldn’t quick cutting out make you think a spark problem rather than fuel?


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73azbronco

Contributor
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Messages
7,796
It happens even with a full tank, and the lines don’t go by the exhaust or anything. And doesn’t matter the temp outside. It happens when it’s 105 out or if it’s cold. The trip that reminded me of how bad it runs was driving early in the morning a couple days ago. Probably 50 out.

Wouldn’t quick cutting out make you think a spark problem rather than fuel?


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So would faulty connection to injector, and bad gas.

You keep saying it sat for a year and that the fuel is OK. Fuel can go bad in less than a year these days with ethanol.

Pull the fuel filter and cut it open and look.
 

EFI Guy

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Apr 14, 2013
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Assuming you're not feeling a misfire, it could definitely be tune related, or rather failing to properly tune for the cam. I'd like to know what cam that engine has. When you start getting carried away with the camshaft it can cause bucking, especially with a M/T.

Bucking is one of the hardest things to try to tune out, but it can be done. Unfortunately, all too often the typical dyno tuner focuses on those peak HP numbers and not making it a pleasure to drive. I've met quite a few that could squeeze every last drop of power out, but don't know how to properly tune idle on a Ford. I'm not trying to bash your guy, or anyone in particular, but it's something that I see a lot, especially if it's the kind of shop that tunes everything rather than specializing in Fords.

[Shameless plug] I do quite a bit of remote tuning where I ride shotgun with you via laptop. My cost is better than what you threw out there, and if your current chip is a brand that I can write to it can save you even more. PM me if you want to know more. [/shameless plug]
 
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