1991 f-150 intermittent issues
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4wd Low
Joined: Jan 2022
Posts: 11
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1991 f-150 intermittent issues
1991 f-150 5.8w automatic
intermittent stalling issues.
losing spark verified that
(whats new)
distributor
tfi
pickup coil
ignition coil
ecm
battery
starter
tfi pigtail
factory and replacement ecm are giving the same issues, so dont know wether or not its ecm related.
known good 5.0 ecm the truck runs like a top, but stalls randomly regardless of engine rpm, like i turned the key off. other "5.8" ecms do the same thing, but it runs bad when it does
intermittent stalling issues.
losing spark verified that
(whats new)
distributor
tfi
pickup coil
ignition coil
ecm
battery
starter
tfi pigtail
factory and replacement ecm are giving the same issues, so dont know wether or not its ecm related.
known good 5.0 ecm the truck runs like a top, but stalls randomly regardless of engine rpm, like i turned the key off. other "5.8" ecms do the same thing, but it runs bad when it does
Fleet Owner
Joined: Oct 2005
Posts: 20,786
Likes: 1,751
From: Kentucky
Check the ignition switch on top of the steering column. At this age they are known to split open and/or burn the internal contacts. Very common issue.
Logistics Pro
Joined: Aug 2014
Posts: 3,559
Likes: 852
First, have you pulled codes?
Second, you say it looses spark but it runs bad when it does?
Either it looses spark or it does not. How have you determined it is a spark issue, and not a fuel pressure issue?
Last, why are you using a 5.0 EEC? What automatic transmission?
Have you opened up the EEC to check for old/blown components?
Thread Starter
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4wd Low
Joined: Jan 2022
Posts: 11
Likes: 0
Quite a few issues here that you are overlooking.
First, have you pulled codes?
Second, you say it looses spark but it runs bad when it does?
Either it looses spark or it does not. How have you determined it is a spark issue, and not a fuel pressure issue?
Last, why are you using a 5.0 EEC? What automatic transmission?
Have you opened up the EEC to check for old/blown components?
First, have you pulled codes?
Second, you say it looses spark but it runs bad when it does?
Either it looses spark or it does not. How have you determined it is a spark issue, and not a fuel pressure issue?
Last, why are you using a 5.0 EEC? What automatic transmission?
Have you opened up the EEC to check for old/blown components?
"runs bad when it does it" means it runs crappy on the 5.8 ecm, but still loses spark
when it dies and wont restart it loses spark, verified no spark to plugs
determined by one pressing schrader valve and fuel spraying out, two the lack of spark
using a 5.0 ecm to see if it changed how the engine ran (it does) not trying to drive the truck with it just trying to fix engine problems
yes, factory and reman unite 5.8 look good.
Thread Starter
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4wd Low
Joined: Jan 2022
Posts: 11
Likes: 0
not super tore up about the firing order. sometimes the truck runs great with the 5.8 ecm in it sometimes it dont. it always runs great with 5.0 ecm
Mountain Pass
Joined: Mar 2024
Posts: 111
Likes: 12
From: MN
"runs bad when it does it" means it runs crappy on the 5.8 ecm, but still loses spark
when it dies and wont restart it loses spark, verified no spark to plugs
determined by one pressing schrader valve and fuel spraying out, two the lack of spark
using a 5.0 ecm to see if it changed how the engine ran (it does) not trying to drive the truck with it just trying to fix engine problems
yes, factory and reman unite 5.8 look good.
when it dies and wont restart it loses spark, verified no spark to plugs
determined by one pressing schrader valve and fuel spraying out, two the lack of spark
using a 5.0 ecm to see if it changed how the engine ran (it does) not trying to drive the truck with it just trying to fix engine problems
yes, factory and reman unite 5.8 look good.
And does it die more often under load? Does it die on idle? How does it sputter out and die? Does it shut off randomly then fire up later? Need a little more info man.
Last edited by DentsideFan; Feb 28, 2026 at 03:25 PM.
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Logistics Pro
Joined: Nov 2013
Posts: 3,928
Likes: 1,198
From: Mojave, Ca
My first thought. Check your ignition switch. Or hook the "run" wires up to an Incandescent lights bulb and see if the bulb dies when the engine died.
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