Engine 1994 5.0 refuses to idle

FordFanatic1894 · Jul 20, 2025

I recently purchased a 1994 Mustang GT 5.0 convertible with a 5 speed manual. It wouldn’t turn over at first so I added gas. It would then fire a couple times, misfire, backfire, and stall. I pulled plugs and all were completely black. Swapped them with motorcraft copper plugs and checked compression while I was there. Couldn’t reach cylinder 2 and 4 but all others tested good at 120, with cyl 6 testing 130. Changing plugs did not help, so I checked fuel pressure which read 32 at priming. Checked all plugs fired at the distributor with an inline plug tester. Tested at least 2 injectors were getting 12V but dropped to 9V when cranking. Also swapped ignition module which only seemed to make the backfiring worse. I am running out of ideas and expertise. Could use help with a diagnosis.

AeroCoupe · Jul 21, 2025

Have you checked for codes? The check engine light (CEL) only flashed for emissions related codes so if it’s not flashing (or the bulb has been removed or burnt out) there still could be codes.

FordFanatic1894 · Jul 21, 2025

AeroCoupe said:
Have you checked for codes? The check engine light (CEL) only flashed for emissions related codes so if it’s not flashing (or the bulb has been removed or burnt out) there still could be codes.

I went and got an OBD1 reader for Fords. The only thing that would generate was a code 12. Which the book said rpm at idle out of range/high.

gkomo · Jul 21, 2025

Code 12 isn't a code related to the airbag system?

FordFanatic1894 · Jul 21, 2025

gkomo said:
Code 12 isn't a code related to the airbag system?

This is what the book tells me. It’s my first time doing anything OBD1 so I may have been doing something wrong. Going to try and static reset the timing when I get off, in hopes of getting it at least running.

gkomo · Jul 21, 2025

Interesting you'd get a "R" code when the car doesn't run. Is there a code 12 with "O"? when you ran the codes was the key on engine off? did you hear the car doing things/making some noises on its own? From what i understand you need the KOEO codes cleared before you can even run KOER codes, and then you need the car to at least be able to idle to run the KOER codes.

And you're probably right, i just googled "1994 mustang GT code 12" and a bunch of threads on the airbag system popped up, like low voltage to the airbag system or something.

Can you verify fuel pressure when cranking, not just priming? or would they be one in the same? I keep reading FP should read at near 39. Sorry, i'm also sort of a noob at this stuff, just thinking of things i've read on this forum over time.

FordFanatic1894 · Jul 21, 2025

gkomo said:
Interesting you'd get a "R" code when the car doesn't run. Is there a code 12 with "O"? when you ran the codes was the key on engine off? did you hear the car doing things/making some noises on its own? From what i understand you need the KOEO codes cleared before you can even run KOER codes, and then you need the car to at least be able to idle to run the KOER codes.

And you're probably right, i just googled "1994 mustang GT code 12" and a bunch of threads on the airbag system popped up, like low voltage to the airbag system or something.

Can you verify fuel pressure when cranking, not just priming? or would they be one in the same? I keep reading FP should read at near 39. Sorry, i'm also sort of a noob at this stuff, just thinking of things i've read on this forum over time.

The car “can run” with my foot on the gas. It misses and backfires while throttle input is severely delayed. It immediately dies when I stop revving or the clutch is released. I couldn’t get any codes with KOEO. I pulled that code 12 after it died when starting. Will re verify fuel pressure. The previous owner seemed to think it was the fuel pump. He bought one to replace, but said he never got around to it.

gkomo · Jul 21, 2025

When you say you couldn't get any codes with KOEO, something doesn't sound right. You should get code 11 at least. Thats the computer telling you everything is good.

From my understanding you want to do KOEO first, fix/clear any codes so you get Code 11, then move on to KOER and fix/clear any codes there to get code 11 as well.

General karthief · Jul 21, 2025

Our tech threads have stuff for this but I would check the sweep on the TPS, idle reading on DV meter should start below 1 volt and smoothly sweep up, https://www.stangnet.com/mustang-forums/threads/825424
If you bought the car not running or running poorly you need to run a system checklist, I would start by verifying timing, plugs, wires, fuel pressure, fuel pressure regulator, cap/wires and charging system.
The PO may have changed things or fiddled with stuff in an attempt to fix or mask a condition.

FordFanatic1894 · Jul 22, 2025

gkomo said:
When you say you couldn't get any codes with KOEO, something doesn't sound right. You should get code 11 at least. Thats the computer telling you everything is good.

From my understanding you want to do KOEO first, fix/clear any codes so you get Code 11, then move on to KOER and fix/clear any codes there to get code 11 as well.

Went home and pulled codes again last night. This reader drives me nuts. With KOEO and clutch pushed in, the reader was telling me I have a 4 cylinder. I read that this is usually nothing and to ignore it. It would then generate code O 12. Which after some research it lead me to the IAC. I also noticed the purge valve vac line had dry rotted off of its connector so I re made that with a new hose and connector. Swapped the firing order around and it seems to no longer miss or backfire under throttle, but it still dies while going back to idle.

FordFanatic1894 · Jul 22, 2025

General karthief said:
Our tech threads have stuff for this but I would check the sweep on the TPS, idle reading on DV meter should start below 1 volt and smoothly sweep up, https://www.stangnet.com/mustang-forums/threads/825424
If you bought the car not running or running poorly you need to run a system checklist, I would start by verifying timing, plugs, wires, fuel pressure, fuel pressure regulator, cap/wires and charging system.
The PO may have changed things or fiddled with stuff in an attempt to fix or mask a condition.

The link led me to a thread that says the cut off is for 93 mustangs. Would this still work for me since it’s basically the same platform? Or are the fox body 5.0s that much different from the sn 5.0s?

gkomo · Jul 22, 2025

FordFanatic1894 said:
Went home and pulled codes again last night. This reader drives me nuts. With KOEO and clutch pushed in, the reader was telling me I have a 4 cylinder. I read that this is usually nothing and to ignore it. It would then generate code O 12. Which after some research it lead me to the IAC. I also noticed the purge valve vac line had dry rotted off of its connector so I re made that with a new hose and connector. Swapped the firing order around and it seems to no longer miss or backfire under throttle, but it still dies while going back to idle.

What did you do to address the code O 12 related to the IAC? Why did you swap the firing order? Like the plug wires weren’t going to the right cylinders before?

Sounds like you’re on the right track and nearly there.

FordFanatic1894 · Jul 22, 2025

gkomo said:
What did you do to address the code O 12 related to the IAC? Why did you swap the firing order? Like the plug wires weren’t going to the right cylinders before?

Sounds like you’re on the right track and nearly there.

I haven’t done anything about O 12 yet, I will be cleaning out the IAC today after work.

I swapped the plug wires, because they looked out of order from a YouTube video I was watching on timing.

It does feel close, idling is the only hiccup.

AeroCoupe · Jul 22, 2025

IAC will help with the idle. Clean it up and do a base idle reset. IAC and idle reset are all in here:

Thread 'Help me create the "Surging Idle Checklist"'

Apr 10, 2005

Updated 05 July 2022 to add rough idle when cold and smooth idle when up to normal operating temperature. See the EGR tech information, section #15 for more help.

Many of you are familiar with the "Cranks OK, but No Start Checklist for Fuel Injected Mustangs" checklist and the No Crank Checklist. They are very effective in finding and eliminating the problems by use of a systematic process that checks and eliminates possible causes. This idle problem checklist will use the same troubleshooting strategy and methodology.

By the term surging idle, I mean an idle that...

General karthief · Jul 22, 2025

FordFanatic1894 said:
The link led me to a thread that says the cut off is for 93 mustangs. Would this still work for me since it’s basically the same platform? Or are the fox body 5.0s that much different from the sn 5.0s?

Yes I believe it is the same with the 94-5 5.0 SBF, the TPS adjustment to .99 whatever the latest fad is is a old wife's tail that should go away.
As I often state in 'no idle' issues there are several yards of vacuum lines and are suspect in vehicles of this age. Check all the vac lines.

Engine 1994 5.0 refuses to idle

FordFanatic1894

New Member

AeroCoupe

lube between the nut and the face. I know my lubes

FordFanatic1894

New Member

gkomo

now i can hopefully expect to receive the shaft

FordFanatic1894

New Member

gkomo

now i can hopefully expect to receive the shaft

FordFanatic1894

New Member

gkomo

now i can hopefully expect to receive the shaft

General karthief

wonder how much it would cost to ship you a pair

FordFanatic1894

New Member

FordFanatic1894

New Member

gkomo

now i can hopefully expect to receive the shaft

FordFanatic1894

New Member

AeroCoupe

lube between the nut and the face. I know my lubes

Thread 'Help me create the "Surging Idle Checklist"'

General karthief

wonder how much it would cost to ship you a pair

Similar threads

Engine 1994 5.0 refuses to idle

New Member

lube between the nut and the face. I know my lubes

New Member

now i can hopefully expect to receive the shaft

New Member

now i can hopefully expect to receive the shaft

New Member

now i can hopefully expect to receive the shaft

wonder how much it would cost to ship you a pair

New Member

New Member

now i can hopefully expect to receive the shaft

New Member

lube between the nut and the face. I know my lubes

wonder how much it would cost to ship you a pair

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