Flashing check engine light on acceleration
Thread Starter
|
5th Gear
Joined: Jan 2026
Posts: 5
Likes: 0
Flashing check engine light on acceleration
Hey guys, recently picked up a 1990 F350 with the 5.8l 351 EFI. On acceleration I get a flashing check engine light but I don't feel any stuttering or hesitation common with a misfire. Most of my Google searches are telling me I have a misfire under load with the AI telling me I'm experiencing a catastrophic failure that I apparently cant feel. I plan on trying to read stored codes tomorrow, but was curious if anyone else has had this. I plan on doing an ignition tune up and replacing wires/plugs because it has been about 5 years since they were done (based on what I can tell from scattered records in the glove box). Also curious if there are any other common culprits. Thanks!
Cargo Master
Joined: Mar 2002
Posts: 3,132
Likes: 218
From: W (BY GOD) V
Hotshot
Joined: Jun 2015
Posts: 12,836
Likes: 2,437
^^Start here ^^ That computer is not capable of detecting misfires. You are guessing at the problem if you don’t check for Key On Engine Off (KOEO) codes first.
Fleet Mechanic
Joined: Oct 2025
Posts: 1,772
Likes: 388
If he had a unicorn that had a working knock sensor, could that not detect one? I would imagine it would need to be bad enough for OP to feel though… I know they are designed to retard timing, just curious!
Checking codes is vital!
Checking codes is vital!
Hotshot
Joined: Jun 2015
Posts: 12,836
Likes: 2,437
which does not have a knock sensor and nowhere in the block to install one.
Last edited by My4Fordtrucks; Jan 30, 2026 at 09:34 AM.
Fleet Mechanic
Joined: Oct 2025
Posts: 1,772
Likes: 388
Trending Topics
Cargo Master
Joined: Mar 2002
Posts: 3,132
Likes: 218
From: W (BY GOD) V
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
10 Things Every Truck Owner NEEDS (2026 Edition)
Michael S. Palmer
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
Verdad Gallardo
Top 10 Most Expensive Ford Trucks Ever Sold on Bring a Trailer
Joe Kucinski
2027 Ford Super Duty Buyer's Guide (Every Model, Engine, & Package)
Brett Foote
Top 10 Ford Truck Tragedies
Joe Kucinski
AEV FXL Super Duty - the Super Duty Raptor Ford Doesn't Make
Brett Foote
Lobo Vs Lobo: Proof the F-150 Lobo Should Be Even Lower!
Michael S. Palmer
Ford's 2001 Explorer Sportsman Concept Looks For a New Home
Verdad Gallardo
10 Best Ford Truck Engines We Miss the Most!
Joe Kucinski
Fleet Owner
Joined: Oct 2005
Posts: 20,786
Likes: 1,751
From: Kentucky
As stated in a previous post 5.8L trucks never got one.
Irrelevant to the thread topic, but....
1996 OBD-II trucks have an external misfire detector mounted near the harmonic balancer. That balancer has a pulse ring mounted on the back of it.
Cargo Master
Joined: Mar 2010
Posts: 3,331
Likes: 292
From: Laramie, WY
A knock sensor detects pre-ignition/detonation/name it what you want, not misfires.
As stated in a previous post 5.8L trucks never got one.
Irrelevant to the thread topic, but....
1996 OBD-II trucks have an external misfire detector mounted near the harmonic balancer. That balancer has a pulse ring mounted on the back of it.
As stated in a previous post 5.8L trucks never got one.
Irrelevant to the thread topic, but....
1996 OBD-II trucks have an external misfire detector mounted near the harmonic balancer. That balancer has a pulse ring mounted on the back of it.
No knock sensor on the 1996 half tons with 5.8L.
Tuned
Joined: Dec 2005
Posts: 400
Likes: 57
From: Saskatchewan
My 5.0 does the same thing, CEL flashes when on the gas and goes out when at steady state, or coasting. Mine has been that way for many years and does not seem to make a difference in how it runs. However, the thing only runs on seven cylinders, but it has been that way for many years now, too. It may, or may not be a significant issue, but reading the codes will be the place to start.
Last edited by 94ZR580; Feb 3, 2026 at 01:39 PM.
FTE Legend
Joined: Jun 2006
Posts: 31,930
Likes: 1,499
From: Ottawa, Ontario
I'd suggest pulling it out for an inspection, failed capacitors are common but so is water damage from leaking windshield adhesive. In this truck the main wiring harness detaches from the engine bay side while the computer has to be pulled from inside the cab via the drivers kick panel. Don't forget to disconnect the battery first.
Last edited by Conanski; Jan 30, 2026 at 03:53 PM.
Lead Driver
Joined: Jun 2021
Posts: 5,221
Likes: 805
The 5.8 likely didn’t have a knock sensor because ford never intended the engine to use one. Either because they had lower compression, less ignition advance, or something else which meant it would be less likely to knock to begin with. A knock sensor on a 5.0 could have been to use more aggressive timing advance without concern for damaging the engine , or for driveability refinement
the simple reality is that a 5.0 302 knock sensor would not translate to a 351 5.8 because the block is different and the tuning and location of the knock sensor was fairly engine specific
same thing with gm at the same era , trucks got 1 sensor and some cars got 2 but they had different rotating assembly and timing curves and such
the 4.3 got two but they were maxing that thing out for power, likely done there to use all the timing advance they could stuff in it
the simple reality is that a 5.0 302 knock sensor would not translate to a 351 5.8 because the block is different and the tuning and location of the knock sensor was fairly engine specific
same thing with gm at the same era , trucks got 1 sensor and some cars got 2 but they had different rotating assembly and timing curves and such
the 4.3 got two but they were maxing that thing out for power, likely done there to use all the timing advance they could stuff in it
Lead Driver
Joined: Jun 2021
Posts: 5,221
Likes: 805
A knock sensor detects pre-ignition/detonation/name it what you want, not misfires.
As stated in a previous post 5.8L trucks never got one.
Irrelevant to the thread topic, but....
1996 OBD-II trucks have an external misfire detector mounted near the harmonic balancer. That balancer has a pulse ring mounted on the back of it.
As stated in a previous post 5.8L trucks never got one.
Irrelevant to the thread topic, but....
1996 OBD-II trucks have an external misfire detector mounted near the harmonic balancer. That balancer has a pulse ring mounted on the back of it.
also why the 1996 trucks had a misfire sensor and a knock sensor
the misfire is more important for catalytic converter life is why the dedicated sensor came to be