Engine Surging idle on start-up

keel · Aug 11, 2025

I started a similar thread about a year ago on this topic but I'm still having idle surge issues and I have some more information so I would like to start with a clean slate.

Here is what I know:
- Car has a terrible surging idle upon startup that will go away after feathering the throttle for a few seconds (bad enough that the car wants to die)
- Car starts up with zero surging when the MAF is unplugged
- No codes other than smog stuff because I deleted all the emissions equipment
- Already tried replacing the sensor and the connector, did not change the problem
- Car runs fine once the idle settles down

Another weird symptom that has come up is that when driving at part throttle the car will have a sudden surge of power at around 1700-2000 RPM. It doesn't feel like it is lacking on power or misfiring or anything at low RPM, it just feels like I'm pressing the pedal a little bit more even when I am holding the throttle at the same position. Unsure if this is related to the idle surge but it could be.

What could all of this mean?

AeroCoupe · Aug 12, 2025

So I am going to assume that you have checked for vacuum leaks, have the throttle stop correctly adjusted, and have done a base idle reset. Obviously you have checked for KOEO and KOER codes. I know all of this was in the other thread but I think will help here.
Year of car
Auto/Manual
Engine and mods (be as specific)
Have you fixed anything else on the car while trouble shooting this issue?

I know this is frustrating but I am going to say it anyway. Starting a new thread always seems like a good idea as it allows you to clear your head and start the process over. What is does for everyone else is cause chaos as we don't have all the previous information. My suggestion here would be to go back to the other thread and do a summary post of all the things that have been done or looked at and then restate what the car is incorrectly doing. You can even change the title of the thread to better reflect the current issue(s).

I had to go back and read both pages again to remember what all had been done. What bothers me the most is that the base idle reset did nothing which I find very odd. I do not believe you have sent the ECU off for any kind of repairs and I would strongly encourage this. I am not saying this is the fix but they are 30+ years old and there are multiple parts in them that have basically timed out and need to be replaced. I want to say that one member sent theirs off just as a maintenance things, The ECU Exchange replaced several parts, and after he got it back it fixed several really small things that were not keeping the car from running but were not "right".

Its up to you but having mulitple threads on the same issue is kind of confusing to those of us that are trying to help so we try and discourage it.

gkomo · Aug 12, 2025

AeroCoupe said:
I want to say that one member sent theirs off just as a maintenance things, The ECU Exchange replaced several parts, and after he got it back it fixed several really small things that were not keeping the car from running but were not "right".

Will second this. AeroCoupe may not be referring to me, but i sent my ECU out to ECUexchange just for maintenance since i didn't know the condition of the ECU. Car ran fine, no 'reason' to send it out for repair. They found multiple issues that they fixed. The big one which i'm surprised didn't affect drivability was some power supply on the board wasn't outputting the incorrect voltage (too low of voltage if i recall). Don't know which one or what it did, but they replaced it, fixed physical board damaged, traces, capacitors, etc. Now, at least i know my computer is running as best as it can for hopefully a long time to come.

General karthief · Aug 12, 2025

AeroCoupe said:
So I am going to assume that you have checked for vacuum leaks, have the throttle stop correctly adjusted, and have done a base idle reset. Obviously you have checked for KOEO and KOER codes. I know all of this was in the other thread but I think will help here.
Year of car
Auto/Manual
Engine and mods (be as specific)
Have you fixed anything else on the car while trouble shooting this issue?

I know this is frustrating but I am going to say it anyway. Starting a new thread always seems like a good idea as it allows you to clear your head and start the process over. What is does for everyone else is cause chaos as we don't have all the previous information. My suggestion here would be to go back to the other thread and do a summary post of all the things that have been done or looked at and then restate what the car is incorrectly doing. You can even change the title of the thread to better reflect the current issue(s).

I had to go back and read both pages again to remember what all had been done. What bothers me the most is that the base idle reset did nothing which I find very odd. I do not believe you have sent the ECU off for any kind of repairs and I would strongly encourage this. I am not saying this is the fix but they are 30+ years old and there are multiple parts in them that have basically timed out and need to be replaced. I want to say that one member sent theirs off just as a maintenance things, The ECU Exchange replaced several parts, and after he got it back it fixed several really small things that were not keeping the car from running but were not "right".

Its up to you but having mulitple threads on the same issue is kind of confusing to those of us that are trying to help so we try and discourage it.

You're likely referring to me, this is done do to the age of these computers, First thing I do is send them to get checked out, while I'm not that tech savvy but I understand things, like the compter 'sees' things from the senors and reacts per installed parameters, when we start up cold the computer does things that are different when hot so if a capacitor is malfunctioning in cold mode then the same behavior will not be present in 'hot mode.
Best I came explain

Mcmahst · Aug 12, 2025

I also sent mine to ECUExchange, even though I could drive my car, but it also had weird surges and not a great idle. They replaced the 4 capacitors and the sensor voltage regulator. Car now runs great. Good luck.

keel · Aug 12, 2025

First I'll just say that in the future I'll keep it to one thread, sorry about that.

I can't say I am surprised to hear that it might be the ECU, I have been dealing with a lot of weird quirks with the car for years that seem to come and go and have no real diagnosis. Never anything catastrophic that caused that car to break down but definitely annoying. This confirms my suspicions that the ECU might be malfunctioning.

I do know for a fact that the parts car I pulled the computer from had sat for at least 10 years without being driven, possibly a lot more. There was damage from moisture all over everything on the interior. Hopefully the inside of the computer isn't corroded.

I'll send it out and report back with results eventually.

General karthief · Aug 13, 2025

keel said:
I do know for a fact that the parts car I pulled the computer from had sat for at least 10 years without being driven, possibly a lot more. There was damage from moisture all over everything on the interior.

Noobz347 · Aug 13, 2025

keel said:
I do know for a fact that the parts car I pulled the computer from had sat for at least 10 years without being driven, possibly a lot more. There was damage from moisture all over everything on the interior. Hopefully the inside of the computer isn't corroded.

keel · Aug 13, 2025

Not a flood car thankfully... The previous owner had rear ended somebody pretty hard at some point and parked the car. I was told it was stored inside while it sat but it was probably outside for a while at some point.

Even if the computer is bad I still got a ton of goodies from it

keel · Oct 29, 2025

Alright finally got the car back up and running. ECU Exchange informed me that they did repair some parts in the computer (some board damage and power supply) but the repairs did not seem to have a major effect on the issues I'm having.

However, it did trip the CEL for a few minutes when I was driving and the ECU stored code 41, 91, 95, and 96.

Any idea where to go from here with diagnostics?

gkomo · Oct 29, 2025

https://lmr.com/products/fox-body-mustang-ob1-codes

EEC Codes - Figure Out What Your Fox Body Codes Are

The fox Mustang's EEC IV computer has two connectors for self-testing and located on the driver-side firewall. The large connector contains the self test output (STO) and ground (SIG RTN)....

www.foxstang.com

AeroCoupe · Oct 30, 2025

So is this still the 93 4 cylinder car converted to a 5.0 using the motor and trans from an 89 car? I am going to assume that since you got it running you used a 92.5-93 ECU harness out of a 5.0 car.

Code 41 - passenger O2 reading lean
Code 91 - driver O2 reading lean

Post in thread 'Codes 41 & 91 With New 02 Sensors'

Apr 3, 2014

Code 41 or 91. Or 43 Three digit code 172 or 176 - O2 sensor indicates system lean. Look for a vacuum leak or failing O2 sensor.

Revised 20-Jan-2014 to add code 43 to test collection

Code 41 is a RH side sensor, as viewed from the driver's seat.
Code 91 is the LH side sensor, as viewed from the driver's seat.

Code 172 is the RH side sensor, as viewed from the driver's seat.
Code 176 is the LH side sensor, as viewed from the driver's seat.

Code 43 is not side specific according to the Probst Ford Fuel injection book.

The computer sees a lean mixture signal coming from...

95 - Fuel Pump Circuit Open-PCM To Motor Ground (CM,O)
96 - Fuel Pump Circuit Open-Battery To PCM (CM,O)

95 KOEO FPM signal indicates circuit failure FPM senses fuel pump voltage is out of range.
96 KOEO Fuel pump secondary circuit failure FP secondary circuit is bad or fuel pump relay is bad.

Either a wiring issue or a bad fuel pump relay.

keel · Oct 30, 2025

Yeah I used a harness from a '93. Do you think the codes are related to the way the car is behaving? I checked fuel pressure and it seemed fine, reading 40 PSI at idle with the vacuum hose disconnected from the fuel pressure regulator and it dropped to about 35 PSI with vacuum.

AeroCoupe · Oct 31, 2025

It’s running lean and having a fuel pump problem. Pressure at idle is good but what is it under load? Should not change so if it’s dipping then it’s going to go lean as indicated by the codes.

Have you changed the fuel pump at any point? Is the fuel pump relay original or has it been changed?

Mcmahst · Oct 31, 2025

I wonder if it could as simple as a clogged fuel filter? Could also be the pump, as @AeroCoupe says.

AeroCoupe · Oct 31, 2025

It could be the rubber hose between the pump and the hanger but you won't know until you can see the pressure under load or just pull the tank and take a look but that is some work. If the pump has never been replaced then maybe its time.

keel · Oct 31, 2025

I changed the fuel pump out a couple years ago to a 155LPH pump when I did HCI so my pump should be fine. I think it's been about 5 years since I did a fuel filter so maybe that's due. And I've never changed the fuel pump relay since I got the harness so who knows how old that is. So that will probably be the first thing I address (and it's cheap).

Engine Surging idle on start-up

keel

Active Member

AeroCoupe

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

gkomo

now i can hopefully expect to receive the shaft

General karthief

wonder how much it would cost to ship you a pair

Mcmahst

keel

Active Member

General karthief

wonder how much it would cost to ship you a pair

Noobz347

Stangnet Facilities Maint Tech... Er... Janitor

keel

Active Member

keel

Active Member

gkomo

now i can hopefully expect to receive the shaft

EEC Codes - Figure Out What Your Fox Body Codes Are

AeroCoupe

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

Post in thread 'Codes 41 & 91 With New 02 Sensors'

keel

Active Member

AeroCoupe

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

Mcmahst

AeroCoupe

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

keel

Active Member

Similar threads

Engine Surging idle on start-up

Active Member

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

now i can hopefully expect to receive the shaft

wonder how much it would cost to ship you a pair

Active Member

wonder how much it would cost to ship you a pair

Stangnet Facilities Maint Tech... Er... Janitor

Active Member

Active Member

now i can hopefully expect to receive the shaft

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

Active Member

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

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

Active Member

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