Misfiring and running rich

[JoshM579]

I recently bought a 2001 Gt 5 speed with a known running issue. It is stock other the exhaust, long tube headers and side exits with the roush body kit. It will idle rough at startup and runs slightly better after about a minute but still surging a little. While driving it misses if I give it throttle too quickly and when I let off the throttle. It seems to run best around 4k rpm while driving. It is running rich for sure, you can smell it and sometimes see it out of the exhaust. I am getting a handheld tomorrow but I am not sure exactly how to interpret the live data. I have experience with mercruiser CAN BUS and their language for what the data is listed as, but I know ford has their own. My first thoughts are vacuum leak in the intake or exhaust, but I wanted to put out a post because I know there are people that are much more experienced than me are on here. I will update with any codes I see on the handheld tomorrow. If anyone could let me know how to read the 02 readings, if they use delta pi or stoich. And how the fuel trim reads on the handheld I would be extremely grateful. Thanks

edit: I pulled codes P1401 diff pres feedback EGR circ and P1409 EGR vacuum regulator. This instantly makes me think the vacuum leak is the problem, however the prev owner knew enough to replace the bottom end of another mustang he had, so i would assume he followed those codes. Regardless im going to check out the vacuum lines and see what i can find.

https://drive.google.com/file/d/1Fo5kNrWAfkeHzmEncBW4mBkb1hatuqdq/view?usp=drive_link Here is a video of me going through live data. I am a bit confused on the labeling, SHRTFT1 is at -20.3 and LONGFT1 is at -24.2. I assume this means that it is trying to trim fuel down to make it lean because it is seeing rich. SHRTFTb2s1 is at -20 and SHRTFB2S2 is at 99.2. This seems like an exhaust leak on the second bank at the second o2 sensor but i'm not sure if i am right in assuming the negatives are lean and positive is rich.

The tuner says that it is running rich and trying to pull fuel but still running rich. He mentioned he worked on the car when the previous owner had it and pulling back fuel on the main table didnt change the rich condition.

 

[ttocs]

o2 sensors still in there? No check engine light?

 

[JoshM579]

There is a check engine light but it the headers may be causing it because the o2 are in a different spot. Im getting the handheld from a tuner tomorrow with a tune to make it not throw the code for o2 response time

 

[JoshM579]

o2 sensors still in there? No check engine light?

I replied to the post instead of to you

 

[ttocs]

o2s are what the motor uses to know what to do. If they are not in the right place and working correctly it can cause the motor to do funny stuff.

 

[96blak54]

Fuel rail pressure sensor causes all kinds of dumb issues on 99 up cars. Id suggest swapping in a new one regardless of condition since they were factory inconsistent parts. And dont think youll get a new one and it not be a bad part and thats aftermarket parts store types. Hope you figure out your issue

 

[JoshM579]

Is there any way to test the sensor? I did notice the pressure drop between injectors got up to 60 at 3kish rpms. But seemed consistent otherwise

 

[ttocs]

what err code did you get for the o2 sensors?

 

[Alexi]

Since you're dealing with EGR codes (P1401, P1409) and extreme negative fuel trims (-20%+), the ECU is pulling fuel hard because it thinks the engine is running lean but your nose and O2 readings say otherwise. First, verify the EGR valve isn't stuck open (causing unmetered air) and check for vacuum leaks, especially around the intake. That wild +99.2 on Bank 2’s rear O2 suggests an exhaust leak *before* the sensor or a failing sensor itself inspect headers for cracks/loose bolts. If the tuner already tried pulling fuel with no change, this isn’t a tune issue. Rule out:
1. Faulty EGR valve/vacuum solenoid (unplug it temporarily to test).
2. Exhaust leaks (listen for hissing, spray soapy water at joints).
3. Dead O2 sensors (swap sides to see if the +99.2 follows the sensor).

Keep it simple chase the EGR/vacuum first, then exhaust leaks.

 

[JoshM579]

Since you're dealing with EGR codes (P1401, P1409) and extreme negative fuel trims (-20%+), the ECU is pulling fuel hard because it thinks the engine is running lean but your nose and O2 readings say otherwise. First, verify the EGR valve isn't stuck open (causing unmetered air) and check for vacuum leaks, especially around the intake. That wild +99.2 on Bank 2’s rear O2 suggests an exhaust leak *before* the sensor or a failing sensor itself inspect headers for cracks/loose bolts. If the tuner already tried pulling fuel with no change, this isn’t a tune issue. Rule out:
1. Faulty EGR valve/vacuum solenoid (unplug it temporarily to test).
2. Exhaust leaks (listen for hissing, spray soapy water at joints).
3. Dead O2 sensors (swap sides to see if the +99.2 follows the sensor).

Keep it simple chase the EGR/vacuum first, then exhaust leaks.

Thanks for the reply and sorry for the lack of response work has been busy enough that i havent had much time to work on it. First of all i realized that the egr is deleted so thats the source of those codes. Smoke tested the intake and found there was a leak on top of the IAC coming from a black cap that was over what seems like a vacuum port. The pcv was also leaking at the valve cover. I replaced the coming out of the pcv and put a properly sealing cap on the IAC. It now runs much better however it still has a rough idle and runs rich. As i was taking the smoke tester out of the intake i noticed there is a rough drilled hole in the throttle plate. In the little research i did it seems people would do that to increase idle rpm for various reasons. I feel as i should just replace the blade and see how it runs from there.

 

[ttocs]

you could probably put a piece of aluminum/duct tape over the hole for a temp fix to see if it helps. I certainly would not take it out on the road for long to test it but I think for a short test it would not hurt.

 

[95opal]

Thanks for the reply and sorry for the lack of response work has been busy enough that i havent had much time to work on it. First of all i realized that the egr is deleted so thats the source of those codes. Smoke tested the intake and found there was a leak on top of the IAC coming from a black cap that was over what seems like a vacuum port. The pcv was also leaking at the valve cover. I replaced the coming out of the pcv and put a properly sealing cap on the IAC. It now runs much better however it still has a rough idle and runs rich. As i was taking the smoke tester out of the intake i noticed there is a rough drilled hole in the throttle plate. In the little research i did it seems people would do that to increase idle rpm for various reasons. I feel as i should just replace the blade and see how it runs from there.

If it's running rich and the pcm is trying to dial back fuel I doubt the vacuum leaks are the problem as the pcm would be trying to add fuel not pull it. I would look at plugs, wires, coil packs, fuel pressure sensor.

 

[ttocs]

The plugs will tell the story you/we need to hear.

 

[JoshM579]

plugs were black as can be, so it is running rich. the prev owner offered me a spare blade he has so im going to swap that and see if it makes a difference.

 

[95opal]

plugs were black as can be, so it is running rich. the prev owner offered me a spare blade he has so im going to swap that and see if it makes a difference.

Put it this way, even with the vacuum leaks the ECM would only be able to fatten it up to a limit before it throws a code for out of adaptive limit. That's not enough to turn your plugs black. Your issue is elsewhere