Hesitation under load

[Cleckner]

I’ve used this forum countless times and finally joined to try and get some specific help. I hope I can return the favor someday.

I’ve been searching for info on this topic and think I have tried everything in other posts.

ISSUE: Hesitation under load. This includes acceleration and steady speed (both high and low). However, the issue is not noticeable, regardless of RPM, when in neutral (no load).

ORIGIN: Bronco has been driving great! I’m fact, I’ve been using it as a daily driver lately. However, during a drive, I noticed some sluggishness start out of nowhere. I can hold the accelerator still and the engine will struggle, then jump up to speed, then struggle.

WHAT I’VE DONE SO FAR:

Fuel filter: this was my first guess as it was behaving exactly like it had a clogged filter. Cleaned it out, then replaced the on carb filter after snapping off the threads. There was no in-line filter so it’s effectively been getting unfiltered gas (except for the small screen in the on carb filter). In line filter installed.

Vacuum Plug: Next I thought there may be a vacuum issue. I found a leaking/cracked plug on the carb so I replaced it. All others a nearly brand new.

Coil: Seeing as issue was only under load, I thought it might be spark and the coil looked original so I replaced it.

Spark plugs: Following on from above, they were disgusting (and AC Delco - can’t have that in a bronco) so I replaced them all with nice NGK plugs.

Accelerator pump: I’ve taken the top off my motocraft 2100 and can see the two strong streams of fuel from moving the throttle/pump. It is adjusted so that upon any throttle movement, they squirt.

Oil change: it was close to due anyway and it was a little over a quart low. Was concerned a valve stuck?

Alternator: I’ve checked the alternator to ensure it’s putting out power.

Fuel pump: I’ve measured the pressure on my mechanical pump and I’m getting 4.75 lbs (at 7,500 ft elevation).

Adjust carb mixture: I’m at two full turns out based off of a vacuum gauge at the carb.

Vacuum check: Getting 20 in of pressure (corrected for altitude) at intake manifold tree. Needle is jumping 1 in or so quickly - it’s not much and could just be from vibration.

At this point, I’m thinking of rebuilding carb (was just rebuilt 1 year ago professionally) because it may have crud from no real filter. Also thinking of replacing ignition control module.

Also, starting to wonder if it’s something with drive train bogging the engine down - but the hesitation seems like an engine issue.

Except for finding a vacuum leaked that I haven’t found yet and my next steps above, can anyone think of something else to check? Remember, it just started mid-drive.

Edited to add: It happens without air filter attached (so it’s not starving for air) and I’ve re-calibrated the choke and it’s happening at full-open choke. And I’ve also checked the vacuum advance in the distributor.

 

[Dylan_Shaffer]

I once had a hesitation/misfire only under load, replaced everything I could think of that could cause it and it didn’t fix it. Even swapped from efi back to carb with no luck. Ended up being my grounds. Regrounded everything and it went away. Fire it up in the dark and see if you can see arcing across your spark plug wires (very faint but visible). If it’s the same thing I had you can actually see the electricity jumping from the spark plug wires and arcing onto anything except for the spark plugs. They were grounding out all over the block, from one wire to another, and from plug wires to my Fitech unit. Would hesitate and fall on its face under load but could go through the whole rpm range at neutral.

 

[Cleckner]

I once had a hesitation/misfire only under load, replaced everything I could think of that could cause it and it didn’t fix it. Even swapped from efi back to carb with no luck. Ended up being my grounds. Regrounded everything and it went away. Fire it up in the dark and see if you can see arcing across your spark plug wires (very faint but visible). If it’s the same thing I had you can actually see the electricity jumping from the spark plug wires and arcing onto anything except for the spark plugs. They were grounding out all over the block, from one wire to another, and from plug wires to my Fitech unit. Would hesitate and fall on its face under load but could go through the whole rpm range at neutral.

This sounds promising - thank you!

I wonder if there’s a way to check for this with a multi-meter?

 

[Dylan_Shaffer]

This sounds promising - thank you!

I wonder if there’s a way to check for this with a multi-meter?

You can check for continuity, which you likely have since it runs, and measure the ohms. Though most multimeters don’t have the resolution needed to check for a the quality of the ground as you’d want it to be on the order of milliohms.

 

[Cleckner]

It sounds like I should just go through in confirm grounds wherever I can find them. Even if it's not the culprit, it seems like something good to do anyway.

 

[Cleckner]

Dylan, to where do I send the beer?

I think you helped me find it! I went out to the Bronco with my multimeter set to measure resistance and just started working with it.

Started with negative battery terminal to other end of cable bolted to block… 0.1 ohms. Then, worked my way around the engine.

Realizing I could measure the positive wires too, I ended up finding out that there was an issue on the positive side of the coil.

Worked my way back and found the culprit (I hope). A few inches down the “I” wire from the starter solenoid was a complete separation. It was hidden in the wire loom and was a previously crimped connection.

The weight of the loom and the bouncing around lately pulled the wire out. I reconnected it and… fingers crossed… the problem seems fixed!?!?!

What I don’t understand is how the engine ran at all with a complete break in that wire.

 

[Cleckner]

And…. I posted too soon. Test drive showed problem is not fixed and some more research showed me that wire is only used during starting.

Back to trying to find the problem.

 

[gr8scott]

Did this behavior start recently, or has it always kind of been there?

Has your carburetor been tuned for 7500 ft? If not it's probably running rich and why your plugs looked disgusting.

Also, please put some of your Bronco's information into your signature. Year, engine size, transmission type, and ignition type for starters.

One last thing, if you become a contributor for $12 a year you can upload pictures directly into you posts.

 

[DirtDonk]

How did you check for vacuum leaks, other than visually? Do you spray WD-40, or carb cleaner, or brake cleaner, or even just water, around the engine and carburetor area?
Looking for any change in idle at all.
And check your PCV valve for function. Next time you have the engine idling, pull it out of the grommet and see if the idle changes or even dies. T basic test is to shake it to see if it rattles. But I found it once they get old enough that’s not an accurate test of whether they’re good or bad. Sometimes they just need to be replaced.
Unlike a coil that was working!

Do not throw away your original coil, as they are usually still good.
Do some tests on it with your ohm meter following the instructions in the shop manuals.
And you’re right, that break in the I wire would not cause any trouble unless it was shorting out.
With the engine off and the key on (for just a moment while testing) measure voltage at the positive side of the coil. Then do the same thing with the engine running.
Then do it again while the engine has been running for a few minutes.
Report back with the voltages at the coil.

Which model NGK spark plugs do you have?

The two filters that you described are all that there ever was from the factory.
One filter inside the fuel tank, and one screwed into the front of the carburetor.
Sounds like your old one was in there for a long time if it broke off. Glad you got it fixed without having to replace the carburetor

How did you check the distributor for timing? Do you have a timing light?
And is your vacuum advance connected to ported, or full intake vacuum?

 

[EPB72]

I’ve used this forum countless times and finally joined to try and get some specific help. I hope I can return the favor someday.

I’ve been searching for info on this topic and think I have tried everything in other posts.

ISSUE: Hesitation under load. This includes acceleration and steady speed (both high and low). However, the issue is not noticeable, regardless of RPM, when in neutral (no load).

ORIGIN: Bronco has been driving great! I’m fact, I’ve been using it as a daily driver lately. However, during a drive, I noticed some sluggishness start out of nowhere. I can hold the accelerator still and the engine will struggle, then jump up to speed, then struggle.

WHAT I’VE DONE SO FAR:

Fuel filter: this was my first guess as it was behaving exactly like it had a clogged filter. Cleaned it out, then replaced the on carb filter after snapping off the threads. There was no in-line filter so it’s effectively been getting unfiltered gas (except for the small screen in the on carb filter). In line filter installed.

Vacuum Plug: Next I thought there may be a vacuum issue. I found a leaking/cracked plug on the carb so I replaced it. All others a nearly brand new.

Coil: Seeing as issue was only under load, I thought it might be spark and the coil looked original so I replaced it.

Spark plugs: Following on from above, they were disgusting (and AC Delco - can’t have that in a bronco) so I replaced them all with nice NGK plugs.

Accelerator pump: I’ve taken the top off my motocraft 2100 and can see the two strong streams of fuel from moving the throttle/pump. It is adjusted so that upon any throttle movement, they squirt.

Oil change: it was close to due anyway and it was a little over a quart low. Was concerned a valve stuck?

Alternator: I’ve checked the alternator to ensure it’s putting out power.

Fuel pump: I’ve measured the pressure on my mechanical pump and I’m getting 4.75 lbs (at 7,500 ft elevation).

Adjust carb mixture: I’m at two full turns out based off of a vacuum gauge at the carb.

Vacuum check: Getting 20 in of pressure (corrected for altitude) at intake manifold tree. Needle is jumping 1 in or so quickly - it’s not much and could just be from vibration.
to me this is a red flag that should be inspected further ,, If you indeed have a bouncy needle that would indicate a engine mechanical issue and probable a valve train issue,.. ..try a power balance to see if you can isolate a single cylinder .does the engine Idle smooth or is there some roughness?? point, I’m thinking of rebuilding carb (was just rebuilt 1 year ago professionally) because it may have crud from no real filter. Also thinking of replacing ignition control module.

Also, starting to wonder if it’s something with drive train bogging the engine down - but the hesitation seems like an engine issue.

Except for finding a vacuum leaked that I haven’t found yet and my next steps above, can anyone think of something else to check? Remember, it just started mid-drive.

Edited to add: It happens without air filter attached (so it’s not starving for air) and I’ve re-calibrated the choke and it’s happening at full-open choke. And I’ve also checked the vacuum advance in the distributor.

 

[Cleckner]

You guys are amazing. I appreciate all of the help.

I think it’s all fixed.

Replaced the ignition control module and rebuilt the carb (it was a good experience for me to learn from).

 

[Cleckner]

Did this behavior start recently, or has it always kind of been there?

Has your carburetor been tuned for 7500 ft? If not it's probably running rich and why your plugs looked disgusting.

Also, please put some of your Bronco's information into your signature. Year, engine size, transmission type, and ignition type for starters.

One last thing, if you become a contributor for $12 a year you can upload pictures directly into you posts.

Carb had a busted gasket but no other indication of something wrong.

I think it’s all fixed now between carb and ignition module.

Thank you!

 

[Cleckner]

How did you check for vacuum leaks, other than visually? Do you spray WD-40, or carb cleaner, or brake cleaner, or even just water, around the engine and carburetor area?
Looking for any change in idle at all.
And check your PCV valve for function. Next time you have the engine idling, pull it out of the grommet and see if the idle changes or even dies. T basic test is to shake it to see if it rattles. But I found it once they get old enough that’s not an accurate test of whether they’re good or bad. Sometimes they just need to be replaced.
Unlike a coil that was working!

Do not throw away your original coil, as they are usually still good.
Do some tests on it with your ohm meter following the instructions in the shop manuals.
And you’re right, that break in the I wire would not cause any trouble unless it was shorting out.
With the engine off and the key on (for just a moment while testing) measure voltage at the positive side of the coil. Then do the same thing with the engine running.
Then do it again while the engine has been running for a few minutes.
Report back with the voltages at the coil.

Which model NGK spark plugs do you have?

The two filters that you described are all that there ever was from the factory.
One filter inside the fuel tank, and one screwed into the front of the carburetor.
Sounds like your old one was in there for a long time if it broke off. Glad you got it fixed without having to replace the carburetor

How did you check the distributor for timing? Do you have a timing light?
And is your vacuum advance connected to ported, or full intake vacuum?

Thank you for this.

The coil was getting 4.8 volts with engine on and 6.1 with engine running.

I did not check timing - have never had to do that yet.

I think it’s all solved with the ignition control module and carb rebuild.

 

[Cleckner]

An update for anyone that was wondering: a carb rebuild made her run a little smoother but replacing the ignition control module solved it.

With the rats nest of wires, I decided to just yank them all out and re-wire her. All done except for the tail lights and fuel tank sending unit.

 

[DirtDonk]

Sounds like the snowball effect in full affect!
But usually once it’s up and running with new wiring, you don’t regret all the extra work it took to get there.

 

[Wyattman2015]

You guys are amazing. I appreciate all of the help.

I think it’s all fixed.

Replaced the ignition control module and rebuilt the carb (it was a good experience for me to learn from).

Hi Cleckner.
I have a 71 Bronco and what you described is happening to me. So the ignition control module was the issue ?
Thanks
Robert

 

[DirtDonk]

Your 71 won’t have an ICM, unless it was added by a previous owner. They only first started appearing from the factory on 74, or maybe 75 on Broncos.

 

[Wyattman2015]

Thank you for that

Your 71 won’t have an ICM, unless it was added by a previous owner. They only first started appearing from the factory on 74, or maybe 75 on Broncos.

Thank you for that - I was going crazy trying to find it. As you can tell I am not well versed yet.

The issue I am having is it starts to sputter like it is running on 4 cylinders instead of 8 ( 302 ) then it kicks back for a short time to running normal. I changed the vacuum hoses but no better.

Was thinking of changing the coil, fuel pump but that is out of desperation.

Any ideas ?

 

[cldonley]

Thank you for that

Thank you for that - I was going crazy trying to find it. As you can tell I am not well versed yet.

The issue I am having is it starts to sputter like it is running on 4 cylinders instead of 8 ( 302 ) then it kicks back for a short time to running normal. I changed the vacuum hoses but no better.

Was thinking of changing the coil, fuel pump but that is out of desperation.

Any ideas ?

Hello Wyatt-

Start with a timing light. Check your iginition timing, that is often the culprit with hesitation. Second, with the truck running, spray some starter fluid or brake cleaner around all the vacuum areas and see if you get an rpm change-you should be able to hear it. If so, you have a vacuum leak, another frequent hesitation culprit.

Finallly, you will probably get more responses, perhaps from people smarter than me, if you start your own thread.

Good luck!

Chuck

 

[DirtDonk]

How about some more information too.
Or you can wait till you start your own thread and include information.
Such as, what other things have been done to the Bronco that you know of?
And is this Bronco new to you? Or have you owned it for a long time?
When was the last time it was given a full tuneup? A bad PCV valve can cause this as well, as can points that are out of adjustment or deteriorating.
Your original 71 distributor would’ve had points and condenser, however an original one could’ve been converted using one of the three or four available electronic conversion kits at the time.
These days Pertonix is the main go to, although in the old days there was Accel, Mallory, and a couple of names I am forgetting at the moment.

Absolutely check for vacuum leaks as was previously described. This is a critical first step I think.

And as usual, pictures would come in immensely handy.
To post them directly here, you have to become a contributor for 12 bucks a year. Or you can use a free hosting site elsewhere, and link the pictures to your post.
Or you can also upload them to your garage or gallery up above.