1986 F150 interesting problem
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4wd Low
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1986 F150 interesting problem
Starting a new thread on my 1986 F150 5.0 auto EFI: Been chasing the extremely rough idle.Ran codes, no luck. Decided to get back to basics.
First:
When check the timing with timing light, put clamp on no.1 wire, No light flickered. Tried on no.2, and flicker so I know timing light works. Pulled wire of no.1 plug and nothing. Checked the others and all gave a good spark except no.4. So 1 and 4 no spark. Same thing at distributor cap terminals nos. 1 and 4. What would cause only 2 out of 8 not firing. ?
Does the computer have anything to do with which plugs fire.
First:
When check the timing with timing light, put clamp on no.1 wire, No light flickered. Tried on no.2, and flicker so I know timing light works. Pulled wire of no.1 plug and nothing. Checked the others and all gave a good spark except no.4. So 1 and 4 no spark. Same thing at distributor cap terminals nos. 1 and 4. What would cause only 2 out of 8 not firing. ?
Does the computer have anything to do with which plugs fire.
Last edited by saemetric; Dec 21, 2025 at 04:52 PM.
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From: Virginia
Take the distributor cap off, grab the rotor and push it back and forth. Does it have a lot of play in it? The bushing may be worn out. There is also a wheel with teeth on it under the rotor. Make sure it has all the teeth and they have not been rubbing.
The computer does not know what cylinders are firing. This system is batch fired, it is not sequential fired like the mustangs. The computer gets a signal from the distributor, and it tells all the injectors to spray half the amount of fuel the engine needs, regardless to what cylinder is firing. So some of the fuel just sits behind a closed intake valve. The next go round it fires half the fuel again on all cylinders. So when a particular intake valve opens, the right amount of fuel enters. This is what is meant by batch firing.
The only thing that differentiates one cylinder from another as far as the sparkplugs is where the rotor is pointed at the time the coil gives out a spark.
The computer does not know what cylinders are firing. This system is batch fired, it is not sequential fired like the mustangs. The computer gets a signal from the distributor, and it tells all the injectors to spray half the amount of fuel the engine needs, regardless to what cylinder is firing. So some of the fuel just sits behind a closed intake valve. The next go round it fires half the fuel again on all cylinders. So when a particular intake valve opens, the right amount of fuel enters. This is what is meant by batch firing.
The only thing that differentiates one cylinder from another as far as the sparkplugs is where the rotor is pointed at the time the coil gives out a spark.
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From: Angier, NC
I would swap them 2 plug wires with other cycl. and see if the no spark follows the wires or stays with #1 & 4.
If it stays do as DaveF said and see of the shaft has play side to side and of the "star" is hitting the pickup coil.
Check the pickup coil over really good as I remember someone having a crack in the coil and it was causing strange things to happen.
With out looking up the firing order where does #1 & #4 sit in the order?
Dave ----
ps what is a 3.0 EFI motor
would that be a 5.0 EFI I am guessing?
If it stays do as DaveF said and see of the shaft has play side to side and of the "star" is hitting the pickup coil.
Check the pickup coil over really good as I remember someone having a crack in the coil and it was causing strange things to happen.
With out looking up the firing order where does #1 & #4 sit in the order?
Dave ----
ps what is a 3.0 EFI motor
would that be a 5.0 EFI I am guessing?
Last edited by FuzzFace2; Dec 20, 2025 at 07:53 AM.
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FTE Legend
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From: Angier, NC
So not across from each other.
Someone installed a 300 six point star wheel in place of the 5.0 / 5.8 8 point star wheel
I would still swap wires with ones that are firing and if that did not work check over the pickup coil really good.
Before MAX says it, pull the horseshoe connector off the coil and check its connection.
Man grabbing straws here
Dave ----
Someone installed a 300 six point star wheel in place of the 5.0 / 5.8 8 point star wheel
I would still swap wires with ones that are firing and if that did not work check over the pickup coil really good.
Before MAX says it, pull the horseshoe connector off the coil and check its connection.
Man grabbing straws here
Dave ----
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And you know the wires are good because...
Have you swapped wires with a known-good cylinder?
Can you please elaborate how you are testing for a spark at the distributor cap? Are you holding the wire a short distance away from the post? If so, the electrons still flow through the plug wire and respective spark plug. Have you tried holding the wire from a known-good cylinder to the suspect post on the distributor? Have you tried swapping or replacing the #1 and #4 spark plugs?
Hold old was the first suspect cap? Was it recently installed as part of a tune-up or similar work? When you "tried a new cap" was it from the same source? I have been there, done that and even got the T-shirt. On another vehicle, the engine started running poorly after a tune-up. As many parts were changed, it took some serious head-scratching to deduce the new distributor cap was bad. Went back to the same store and unknowingly got another bad cap from the same defective batch. Talk about a troubleshooting nightmare...
Have you swapped wires with a known-good cylinder?
Can you please elaborate how you are testing for a spark at the distributor cap? Are you holding the wire a short distance away from the post? If so, the electrons still flow through the plug wire and respective spark plug. Have you tried holding the wire from a known-good cylinder to the suspect post on the distributor? Have you tried swapping or replacing the #1 and #4 spark plugs?
Hold old was the first suspect cap? Was it recently installed as part of a tune-up or similar work? When you "tried a new cap" was it from the same source? I have been there, done that and even got the T-shirt. On another vehicle, the engine started running poorly after a tune-up. As many parts were changed, it took some serious head-scratching to deduce the new distributor cap was bad. Went back to the same store and unknowingly got another bad cap from the same defective batch. Talk about a troubleshooting nightmare...
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4wd Low
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I pulled the wires off the distributor cap one at a time while running badly. Each one had a spark except 1 and 4.
Today I’m not getting any spark from the coil when I crank it.
So I’ve ordered a new distributor w/cap,rotor,modules installed plus a new coil. If that doesn’t work I’ll be totally lost. Not much else to do. Truck has only 125,000 miles on it
More to come!!
Today I’m not getting any spark from the coil when I crank it.
So I’ve ordered a new distributor w/cap,rotor,modules installed plus a new coil. If that doesn’t work I’ll be totally lost. Not much else to do. Truck has only 125,000 miles on it
More to come!!
Cargo Master
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From: W (BY GOD) V
Before you load and fire the parts cannon at it ...
https://mechanictimes.com/how-to-dia...removing-them/
If a DIST SHAFT was bent or had a worn bushing, you would see damage on the rotor button and/or DIST CAP points.
It has now many miles and years on the wires? How long ago were the spark plugs replaced and were they quality or BOX STORE SPECIALS? Have you pulled the plugs on the affected cylinders?
FTE Legend
Joined: Nov 2015
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From: Angier, NC
I pulled the wires off the distributor cap one at a time while running badly. Each one had a spark except 1 and 4.
Today I’m not getting any spark from the coil when I crank it.
So I’ve ordered a new distributor w/cap,rotor,modules installed plus a new coil. If that doesn’t work I’ll be totally lost. Not much else to do. Truck has only 125,000 miles on it
More to come!!
Today I’m not getting any spark from the coil when I crank it.
So I’ve ordered a new distributor w/cap,rotor,modules installed plus a new coil. If that doesn’t work I’ll be totally lost. Not much else to do. Truck has only 125,000 miles on it
More to come!!
Pop the cap and when you crank the motor dose the rotor spin? If not the distributor gear roll pin may have broken, I take it the motor turns over the same speed it always has?
Do you have power to the + side of the coil? How bout the - side of coil and what happens on the - side when you crank the motor?
It could be the module took a dump and it dropping spark to #1 & #4 was a warning?
Dave ----
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I see you've moved on to a new problem. Don't panic, as now you have a hard fault. That's generally easier to troubleshoot, versus a partial failure only affecting two cylinders.