Erratic Idle after header replaced - Mustang Evolution

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Old 10-02-2019, 05:47 PM   #1
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Erratic Idle after header replaced

Hi guys, I'm having an issue that I cant seem to remedy after replacing my drivers side header. The story started last week when my check engine light came on and I went to get the codes scanned, I cant remember the code but it had to do with my driver side bank running too rich, funny enough once I got home my engine sounded like a lawn mower and I immediately suspected an exhaust leak. After checking I had found that the collar of the header that attaches to the mid pipe had cracked clean off, at this point I'm assuming a hit a bad speed bump bending the exhaust and slowly causing it to crack.


So I order a new header pull out the old one and replace it with the new one, here is where I screw up royally and I'm still kicking myself, when I put the header in I removed the spark plug cables and when I went to put them back I screwed up the order like an idiot and scratched my head for all of 5 min when I was getting nasty backfires, to make matters worse I noticed my error and rewired the spark plugs only the screw up a second time (I know right how the hell does someone screw up placing 3 wires into their proper holes twice).


Finally I get the order right and the backfires are gone, however at this point my cat is glowing red from all the fuel being dumped from the firing order being off, so I give it a few hours to all cool down before I come back. So now I'm faced with idle being completely erratic, while in park it is only slightly rough but rough enough to notice, when the car is put into drive the rpms immediately plummet to a near stall but recovers, this plummet occurs anytime the break is pressed when the car is in gear. If the car is in park and I turn on the AC the rpms will begin to dance up and down like a yo-yo repeatedly, never stabilizing.


I'm thinking that there is a problem either with air or fuel ratio at this point, Ive pulled off the throttle body, idle air control valve, and mass airflow sensor cleaning all parts scrupulously, Ive done several checks to make sure no hoses are lose or wires unplugged. At this point I'm stumped on what to do next, I suppose maybe the IAC valve or MAF sensor could have gone bad, but I'm sorta lenient on playing the game of buying new parts until the issue gets found. The thing that really puzzles me though is that this issue occurred right after replacing the header, unless of course my screw-ups with the firing order and backfires managed to damage something else.


My mustang is the 04 V6 Base Model.


Note:
The code that it was spitting out prior to me discovering the cracked manifold was P0174 indicating bank 2 being too lean which was most likely due to the exhaust being open to atmosphere. There no longer is any check engine light present once the spark plugs were properly wired.
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Old 10-02-2019, 09:39 PM   #2
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I suspect that the code is related to the O2 sensor, but you need to retrieve whatever codes that are displayed and write them down. Without knowing the Diagnostic Trouble Codes, any diagnosis is largely just a guess.
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Old 10-02-2019, 09:51 PM   #3
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I suspect that the code is related to the O2 sensor, but you need to retrieve whatever codes that are displayed and write them down. Without knowing the Diagnostic Trouble Codes, any diagnosis is largely just a guess.

Trouble codes are gone, after correctly wiring the spark plugs the check engine light remains off. I unplugged the battery and held the brake for a few seconds as well as letting it sit for a a few hours after to try and rid any residual energy. Then plugged it back in to see if a relearn would get it working, nope.


The oxygen sensors were replaced last year, both up and down stream, so there are essentially newish. Is it possible that the misfiring caused enough heat to damage the sensors and cause the incredibly rough idle? Rough is somewhat of an understatement, as ive said, simply holding the break in gear will bring the car to the point of near stalling with incredible shaking.


The code that it was spitting out prior to me discovering the cracked manifold was P0174 indicating bank 2 being too lean which was most likely due to the exhaust being open to atmosphere.
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Old 10-02-2019, 10:03 PM   #4
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You say that the catalytic converter got red hot?
I suppose that it is possible that it might have been damaged, and it is disrupting the exhaust flow.
Also, it would be a good idea to go back and double, or triple check all of the work that was done to the car before this problem occurred. When a problem like this occurs after working on the car, it is likely that something was done incorrectly, something didn't get plugged back in, damaged, etc.
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Old 10-02-2019, 10:14 PM   #5
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You say that the catalytic converter got red hot?
I suppose that it is possible that it might have been damaged, and it is disrupting the exhaust flow.
Also, it would be a good idea to go back and double, or triple check all of the work that was done to the car before this problem occurred. When a problem like this occurs after working on the car, it is likely that something was done incorrectly, something didn't get plugged back in, damaged, etc.

Yup the cat was glowing cherry red and the floor inside the car became very hot to the touch. Ive given it several look overs and cant seem to find anything damaged or unplugged, only 8 nuts in total were removed and replaced.


I do get where your coming from though, since the car was having no issues prior to the exhaust cracking, I would assume that somehow that during the process of repairing it something was altered or damaged to cause the issue.


On a whim I've read that with all the electronics off the car should be able to maintain a base idle with the IAC valve unplugged. When I unplug my idle air control valve the engine immediately dies and is unable to start again unless its plugged in or I apply some pressure to the gas. Is this something I should investigate further with the set screw, Ive found a few articles on reestablishing the base idle.
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Old 10-02-2019, 10:27 PM   #6
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If the cat was cherry red I would say that the cat has plugged up and needs replacing. When the back firing happened it was most likely hot enough to melt the honeycomb material that is inside the cat. You could also remove the cat and with a big screwdriver or small pry bar and a hammer try to break all of the insides up and shake them out of the cat. I did this once and the car still passed CT. emissions. As long as the cat was visible the inspector didn't know that it was hollowed out. If the cat is glowing it is plugged and causing a restriction.
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Old 10-02-2019, 10:32 PM   #7
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DO NOT ADJUST THE SET SCREW ON THE THROTTLE BODY!!!

That's not what it is for.
The IAC valve controls the idle speed.

The catalytic converter has a ceramic block inside of it that can become damaged from excessive heat. If it melted, it can partially clog the exhaust and cause problems such as difficulty idling, loss of power, etc. It won't be detectable by looking at the outside of it.You can purchase a test pipe, and replace the cat with it to test it. The "test pipe" is also referred to as a "off-road pipe", or exhaust.

And yes, the engine should be able to run in "open-loop" without receiving information from the 02 sensor, or the MAF sensor.
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Old 10-02-2019, 10:34 PM   #8
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If the cat was cherry red I would say that the cat has plugged up and needs replacing. When the back firing happened it was most likely hot enough to melt the honeycomb material that is inside the cat. You could also remove the cat and with a big screwdriver or small pry bar and a hammer try to break all of the insides up and shake them out of the cat. I did this once and the car still passed CT. emissions. As long as the cat was visible the inspector didn't know that it was hollowed out. If the cat is glowing it is plugged and causing a restriction.

Ok this makes sense, the cat actually got hot enough that the inside floor of the car became incredibly hot to the touch as well as there being a very unpleasant odor.

Ive sorta been looking at replacing the entire exhaust, a new pair of high flow cats would be a nice start.
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Old 10-03-2019, 08:24 AM   #9
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So this morning I had the bright idea to remove the cap that came with the replacement header to let exhaust gas flow into the engine bay and I started it up. Idle problem gone completely, so I believe at this point I can confirm there is indeed a blockage.
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