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Power Loss / Burning out Coil Packs...

Gene79

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1991 Ford Ranger 4.0 XLT
204,000 KMS

Starts and idles fine when cold. Once I move in to drive it will switch intermittently between working normal and huge power loss/running rough like its on 3 cylinders. Eventually it will heat up the coil pack until it blows.
I've replaced the fuel pump, filter and pressure regulator. Had a diagnostic done, the fuel system is all working fine.
Replaced all the spark plugs/wires/coil pack.
I cleaned out the MAF sensor and initially it seemed to run better so I replaced it with another one. No change though so just a fluke i guess.
 


RonD

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Welcome to TRS :)

You should run Motorcraft AGSF42FM spark plugs gapped at 0.054(they should come with that gap)

But in a pinch you can use Autolite or NGK, also gapped at 0.054, they won't last as long but not a big deal
The 4.0l OHV literally EATS other brands

The main thing is the GAP, 0.054, did I mention that, :)

You have a Waste Spark system
This means there is a spark in each cylinder at its TDC(top dead center), since this is a 4-stroke engine that means only 1 of these sparks is used when a cylinders piston is at TDC, the other spark is "wasted"
Its just a simpiler setup, actually first spark system ever used in the late 1800's

Because of this Waste spark system there are only THREE coils are in the coil pack, not 6
3 and 4 share a coil
2 and 6 share a coil
1 and 5 share a coil

That's why coil is wired that way
3 4
2 6
1 5
front

Its for the way the engine is balanced

There are 3 parts to the 1991 4.0l spark system
Crank sensor, its on the lower front of engine by the crank pulley, it reads a Tone Wheel(looks like a gear) tone wheel has teeth that rotate with the crank and that causes a signal to be generated in the Crank Sensor and TIMES the spark

EDIS-6 Module, this gets timing signal from crank sensor
It uses that timing signal to ground and unground the 3 coils in the coil pack
It also sends out the timing signal to the engine computer(PCM) so it can time fuel injectors

The coil pack, when ungrounded each coil sends out its spark, just like any ignition coil
If you are burning out coils then thats usually a spark plug or spark plug wire issue

This is an autonomous system, does not need engine computer to work
BUT.........it does get Load Advance suggestions from the PCM
So the PCM could cause mistimed spark and loss of power, and a 1991 PCM could be in need of 3 capacitors that only lasted 25 years or so, nothing is built to last anymore, lol, they are easy to replace
Pull out computer and open it up, you will see the 3 cylinder type capacitors, they leak and the pins corrode and break, wiggle with finger
But I would just replace them on speculation, cost is under $5

Other than the spark plugs and wires not sure what else to suggest?
Double check the coil pack wiring, I still get the 5 6 4 side mixed up, either at the coil end or spark plug end of the wire
 
Last edited:

Gene79

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Welcome to TRS :)

You should run Motorcraft AGSF42FM spark plugs gapped at 0.054(they should come with that gap)

But in a pinch you can use Autolite or NGK, also gapped at 0.054, they won't last as long but not a big deal
The 4.0l OHV literally EATS other brands

The main thing is the GAP, 0.054, did I mention that, :)

You have a Waste Spark system
This means there is a spark in each cylinder at its TDC(top dead center), since this is a 4-stroke engine that means only 1 of these sparks is used when a cylinders piston is at TDC, the other spark is "wasted"
Its just a simpiler setup, actually first spark system ever used in the late 1800's

Because of this Waste spark system there are only THREE coils are in the coil pack, not 6
3 and 4 share a coil
2 and 6 share a coil
1 and 5 share a coil

That's why coil is wired that way
3 4
2 6
1 5
front

Its for the way the engine is balanced

There are 3 parts to the 1991 4.0l spark system
Crank sensor, its on the lower front of engine by the crank pulley, it reads a Tone Wheel(looks like a gear) tone wheel has teeth that rotate with the crank and that causes a signal to be generated in the Crank Sensor and TIMES the spark

EDIS-6 Module, this gets timing signal from crank sensor
It uses that timing signal to ground and unground the 3 coils in the coil pack
It also sends out the timing signal to the engine computer(PCM) so it can time fuel injectors

The coil pack, when ungrounded each coil sends out its spark, just like any ignition coil
If you are burning out coils then thats usually a spark plug or spark plug wire issue

This is an autonomous system, does not need engine computer to work
BUT.........it does get Load Advance suggestions from the PCM
So the PCM could cause mistimed spark and loss of power, and a 1991 PCM could be in need of 3 capacitors that only lasted 25 years or so, nothing is built to last anymore, lol, they are easy to replace
Pull out computer and open it up, you will see the 3 cylinder type capacitors, they leak and the pins corrode and break, wiggle with finger
But I would just replace them on speculation, cost is under $5

Other than the spark plugs and wires not sure what else to suggest?
Double check the coil pack wiring, I still get the 5 6 4 side mixed up, either at the coil end or spark plug end of the wire

Thanks for the help! I replaced all the spark plugs with NGK plugs. I double checked the wiring, and it all looks good.. I made sure the order matches up. At this point I think i'll pull the PCM and check that out. I'm actually pretty handy with the soldering gun, so I'll keep my fingers crossed on this being the main problem. Thanks again!
 

Gene79

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So I pulled the PCM. It’s EEC-IV and only seems to have 2 capacitors on it. They look in real good shape. There’s a couple questionable areas on the board I noticed though. Some green death on a couple resistors and a chipset. Also looks like some issues with the connection on IC23 maybe. I’ve attached photos the best I could. I’ll go ahead and clean up what I see and try it again. Unless you suggest I replace those caps anyways. ?
CBC46D03-84E7-448D-9511-C57FF8EFE959.jpeg
BDC456F8-9039-4A4C-A566-22C12988CE85.jpeg
574D4F0E-DCC2-407F-ACCB-D5913F880F8F.jpeg
33B069FA-7C9E-4996-9AB5-119C123188B5.jpeg
56E18860-03EE-40E1-AC1E-C45F72ECEE75.jpeg
13830D08-C93A-4F5E-8CD1-F414972AFA64.jpeg
 

RonD

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I would get some isopropyl alcohol and Q-tips and clean it up

There has been some moisture in there
If you can get the 2 new caps then I would replace them just because they are 25 years old
 

Gene79

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I replaced the capacitors and cleaned up the board etc. No change. I found some exposed/ broken wires on the coil pack electrical plug. I rewired and tidied all that up as well as the ground. Still doing the same thing.
it starts right up and idles real good! Once I put it into drive it starts crapping out. Where should I look next?
 

RonD

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Do you have a vacuum gauge and a timing light

Its reading like its engine LOAD related

Expected vacuum is 17-20 inches at idle
Expected timing at warm idle 18-22deg BTDC, if you see 10-14deg then computer is not sending LOAD advance to EDIS module, or EDIS module is not using it

LOAD advance was Vacuum Advance on older distributors, with distributorless fuel injection the computer has the LOAD information so sends that to external spark module
The EDIS can only do RPM advance, so would be sluggish and lack power without LOAD advance
 

Gene79

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Ok. I don’t own either tool but I can get my hands on them and check this out soon. so if I’m seeing 10-14 deg that means I’m replacing the EDIS module or the PCM? Is there a way to test either one to be certain?
thanks for all your help!
 

RonD

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No, not really, you can look up SPOUT signals and see if computer is outing a correct SPOUT signal to EDIS module or if EDIS is ignoring the signal
 

Gene79

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I found a black wire coming off the harness for the EDIS module was destroyed. Someone had done a real real poor splice and tape job on it in the past.
once I rewired she’s working great again! Thanks for your help Ron! I wouldn’t have known where to look without you.
CC67D386-CB84-472C-ACC3-243E2BF74EE4.jpeg32ED4E0E-DB92-4DEC-AAF9-70A9C89D5A98.jpeg
 

Eddo Rogue

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Mine hated NGK's, ran like crap. I switched to the Autolites, and it has ran great since. I may try the Motorcraft next.
 

RonD

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