Electrical Bucking under load - no starting or idle issues - Maybe electrical?

90NATO

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Nov 20, 2018
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So my (stock engine 5.0) 90 Coupe has had a bucking issue for a long, long time now. I've tested and/or replaced many/most of the sensors, swapped EEC's after replacing caps, checked charging system and grounds, fuel filter, etc. I've been searching forums forever but this doesn't seem to match any other issues. Most are no start or cutting off type issues.

The last thing I've done is taken apart and cleaned the 10 pin connectors and separated the pins for a more snug fit. After doing this, the idle is amazing, but the car still bucks and jerks while driving. Which kinda led me down the electrical rabbit hole even further - not my strong suit.

Background:
-Car starts, runs, and idles fine (the 10-pin connector work ironed out the little bit of a surge it had), and has yet to cut-off while driving or stopped.
-Pump primes (Obviously, but seems kinda loud IMO - subjective, I know)
-Fuel pressure is 41psi at idle with vacuum off. I am waiting on a gauge extension, so I can actually check while driving and the issues occur to see if pressure is cutting out.

Issue:
The car runs strong up to about 3.5-4K @ 3/4 to WOT with no electrical load, then will buck and jerk moderately through that range, then smooth the rest of the way to redline. RPMs go spastic when it happens.
BUT - when I have an electrical load on it - headlights, fan, wipers - it is much, much, worse (severe bucking at random RPM ranges) and doesn't like me to give it much throttle at all. If I let off the gas it calms down and will stay running still.

That is what leads me to an electrical issue. Like, perhaps there is resistance somewhere in the circuit that becomes too great when accessories are on or fuel demand is greater, and the pump can't keep up? Where should I check first in this circuit? I've read that EFI primer, but it's difficult for me to figure where to start since the symptoms are strange and I'm not very experienced with electrical.

Thanks!!
 
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
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Well I would start with the charging system, make sure the voltage is steady/constant. throughout the rpm range.
then I would take a digital volt meter and check the sweep on the tps (throttle position sensor) instructions are in the surging idle checklist. I would do the complete list, you may have more than one issue.
if you go to the surging idle checklist in the technical/how to thread you'll find it at the bottom of the list. Start by reading through it to get an understanding then do it step by step, don't skip around, take notes and post any questions you have in this thread so members don't have to ask questions that were already covered.
Is it bone stock or is there something you've left out like it's a converted 4 cyl car? Anything rewired? Have you owned it long?
 
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90NATO

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Well I would start with the charging system, make sure the voltage is steady/constant. throughout the rpm range.
then I would take a digital volt meter and check the sweep on the tps (throttle position sensor) instructions are in the surging idle checklist. I would do the complete list, you may have more than one issue.
if you go to the surging idle checklist in the technical/how to thread you'll find it at the bottom of the list. Start by reading through it to get an understanding then do it step by step, don't skip around, take notes and post any questions you have in this thread so members don't have to ask questions that were already covered.
Is it bone stock or is there something you've left out like it's a converted 4 cyl car? Anything rewired? Have you owned it long?
Thanks for the response, I've owned it since 2014. Factory 5.0 with AOD. It was converted to a T5, but the issue I describe was present before that. Engine-wise it's bone stock. Aside from the tranny, only real "mods" are 2G alternator, gears, headers, catted midpipe and catback, so nothing to the engine. The rest were just normal maintenance type items. I have checked and gone through the majority of the surging idle checklist and everything is within specs according to the checklist. Nothing rewired. I did have some vacuum lines break previously (EGR and Smog stuff), but I replaced them, ran a smoke test, and everything seemed good. Only things I hadn't done from the list was the ECT/ACT, PCV (which I will try to do this weekend).

Would it be possible for ECT/ACT, PCV to create the issue I described?

Once I change the PCV, I'll run another charging system test with a DMM while driving. I didn't get any major fluctuations at idle throughout the RPM range under load when I tested initially. Alternator, battery, and grounds tested good at that point.
 
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7991LXnSHO

wanna catch the space herp
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Well I would start with the charging system, make sure the voltage is steady/constant. throughout the rpm range.
then I would take a digital volt meter and check the sweep on the tps (throttle position sensor) instructions are in the surging idle checklist. I would do the complete list, you may have more than one issue.
if you go to the surging idle checklist in the technical/how to thread you'll find it at the bottom of the list. Start by reading through it to get an understanding then do it step by step, don't skip around, take notes and post any questions you have in this thread so members don't have to ask questions that were already covered.
Is it bone stock or is there something you've left out like it's a converted 4 cyl car? Anything rewired? Have you owned it long?
If these check out, I suspect the coil or it’s connections, no matter the brand.
 
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90NATO

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UPDATE: So when I put an electrical load on the car, I can re-create the bucking or miss by hand controlling throttle. I re-checked the charging system and fuel pressure during the symptoms. It happens quickly, but it looks like the alternator output never fluctuated - still 14.3-ish. Fuel pressure with the vac line connected was about 37. It waivered a bit (by maybe 2 psi), but the gauge readings don't change as quickly and its just a blip, so I can't really tell if fuel is totally cutting during the symptom. I'll dig a little deeper on the coil.

Does anyone know of the best way to test the coil, while recreating the symptom? Seems like it happens in a flash and will be difficult to detect like the fuel pressure, but idk.

Thanks again ya'll!
 

7991LXnSHO

wanna catch the space herp
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An oscilloscope can be used to measure ignition patterns.
A multimeter can static test the coils’ resistance. That’s a standard way of bench testing.
The bucking at a certain RPM range sounds like the possibility of the TPS, but if it is worse under electrical load, but voltage is the same, I’m stumped.
 

Mustang5L5

Put lubricant all over the balls
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Right now its an accel. The first one was the original motorcraft it looks like. Both seem to work, or both are/were failing, because the symptoms remain.

Same symptoms with both?

Reason I ask is because I had similar symptoms with an accel coil. I swapped an OEM coil in to test and the problem was solved.
 
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LX Dave

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Does it do this when the engine is cold too? If it's just when it's hot, read on...

Had a car years ago (fuel injected) that had a bad bucking problem. I replaced a couple of components that I though may be bad. No change. Took it in to the Ford garage and the tech had it diagnosed within a few minutes. When the engine is cold, the fuel mixture is enriched, which makes it easier to ignite the mixture. When it's warmed up, the mixture is leaner then when cold. A lean mix is harder to ignite than a rich one. Turns out the problem was ignition wires. Running the engine in the dark, you could see a spark run from the boot to the exhaust manifold once in a while. Replaced the wires and the car never did it again. Also, you could see spark trails on a couple of the boots.

Whether this is the problem or not, it costs nothing to check out.
 
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90NATO

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Same symptoms with both?

Reason I ask is because I had similar symptoms with an accel coil. I swapped an OEM coil in to test and the problem was solved.
Yes, it was the same symptoms with both. I am starting to hone in on ignition again now and might just try with a new OEM. Perhaps the old OEM and the new accel both are bad. Not sure how common this is, but I'll try anything at this point.
 

90NATO

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Nov 20, 2018
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Augusta, GA
Does it do this when the engine is cold too? If it's just when it's hot, read on...

Had a car years ago (fuel injected) that had a bad bucking problem. I replaced a couple of components that I though may be bad. No change. Took it in to the Ford garage and the tech had it diagnosed within a few minutes. When the engine is cold, the fuel mixture is enriched, which makes it easier to ignite the mixture. When it's warmed up, the mixture is leaner then when cold. A lean mix is harder to ignite than a rich one. Turns out the problem was ignition wires. Running the engine in the dark, you could see a spark run from the boot to the exhaust manifold once in a while. Replaced the wires and the car never did it again. Also, you could see spark trails on a couple of the boots.

Whether this is the problem or not, it costs nothing to check out.
It doesn't normally do it when cold. I checked the plug cables a while back and didn't notice any sparking, but I wasn't able to check with the symptoms present in the dark. I'll try this again since I can now recreate the symptoms without driving. I did visually inspect the boots when I swapped headers, and they looked ok. I'll update with findings.
 

7991LXnSHO

wanna catch the space herp
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It doesn't normally do it when cold. I checked the plug cables a while back and didn't notice any sparking, but I wasn't able to check with the symptoms present in the dark. I'll try this again since I can now recreate the symptoms without driving. I did visually inspect the boots when I swapped headers, and they looked ok. I'll update with findings.
Grab a squirt bottle of water and mist tap water around the cables in the dim light while checking for spark leaks.
 

90sickfox

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Also, check the pickup under the rotor button inside the cap. I had a similar problem and the pickup had turned to powder. The rotor button sits on a plate. That plate has windows cut out of the side that the pick up reads to fire injectors and read timing of the distributor. If its bad it'll cause all types of weird stuff to happen. This part is what the TFI module plugs into.

Usually, they corrode and turn into powder when they are bad. The distributor would have to be removed and taken apart to replace it. There have been people here with misfires due to the windows on aftermarket distributors being incorrectly made. Caused misfires at higher rpms though.

Screenshot_20220109-232009_Chrome.jpg
 

90NATO

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Augusta, GA
Also, check the pickup under the rotor button inside the cap. I had a similar problem and the pickup had turned to powder. The rotor button sits on a plate. That plate has windows cut out of the side that the pick up reads to fire injectors and read timing of the distributor. If its bad it'll cause all types of weird stuff to happen. This part is what the TFI module plugs into.

Usually, they corrode and turn into powder when they are bad. The distributor would have to be removed and taken apart to replace it. There have been people here with misfires due to the windows on aftermarket distributors being incorrectly made. Caused misfires at higher rpms though.

Screenshot_20220109-232009_Chrome.jpg
Thanks for the suggestion. I'll take it off and look once I get back in town this weekend.