Engine 1990 fox car runs great unti it goes into closed loop runs vary rough popping backfire no power please help

Vaughnmx

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Apr 23, 2019
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Ok guys new member here I bought a 90 coupe last summer for $500 did a few mods the car ran great before the mods drove the car for a few months before winter right before I put it away it started acting up it runs good before going into closed loop then it back fires badly and popping and missing also vary badly to where it’s not even drivable I pulled the codes and came up empty handed with only smog code but all has been deleted long before the issues arose ..but I had a pile of parts waiting for the car so I put them on anyway maybe a bad decision but the isssue is still present
Mod list (issue started before these mods)
System max intake
Gt40 heads valves springs 1.7 rockers
75mm tb
24lb injectors with maf (had 19s with stock maf same issue)
B21 cam (push rod length is correct) also checked lifter preload
Adjustable fpr (40 psi with vacuum off)
Headers h pipe cat back
5 speed swap
A9p computer
Cai
I had a lot of good parts laying around fresh off my other car I’ve swapped distributors,ect,act,tps,map/bap,3 tfi modules,2 mafs new coil tried others new plugs new plug wires,fresh gas timing is set to stock ive set it twice to rule it out also have checked firing order and plug wire orientation but the car ran great up until the day it did not only thing I haven’t touched is the o2s but I’m getting no codes for them ive also checked the salt/pepper shakers,maf plug,coil plug,and tfi plug for corrosion all is good I’ve also upgraded the alternator but the problem was present with old one as well
 
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Mustang5L5

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Usually when it runs poorly in closed loop, it points to a sensor issue as the car runs off various tables during open loop.

Did the car run well before the t-5 swap, and poorly after? Or did it just randomly start one day?
 
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Vaughnmx

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It ran fine both before and after the swap ..during the swap it was bone stock and then one day it decided to act up before mods
 
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Vaughnmx

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Apr 23, 2019
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Update..unhooked 02 harness and it runs good but obviously I want to hook it back up I do not want a constant lean mix so what should I do here replace o2s or strip harness back and inspect?
 

jrichker

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You should have gotten the engine not running memory codes and the engine running codes.
41/91 are for the O2 sensors; dump the codes again and see what you get.
 
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Vaughnmx

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I got the not running codes I will check the running codes ...last night I unhooked the battery and plugged the o2 harness back in and all is fine which is vary odd
 

JimZim

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That is really great! You actually opened the case of the ECU and found the problem? WOWIE ZOWIE!!! I had an old ASUS motherboard that needed the electrolytics replaced. A new installation of Windows would last a month or so until it was so corrupted by bad writes to the hard drive it would stop functioning.
 
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Vaughnmx

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That is really great! You actually opened the case of the ECU and found the problem? WOWIE ZOWIE!!! I had an old ASUS motherboard that needed the electrolytics replaced. A new installation of Windows would last a month or so until it was so corrupted by bad writes to the hard drive it would stop functioning.
Yes sir I’ve never been so happy to discover a issue lol
 
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Vaughnmx

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update changed caps out fixed the surging idle but not the popping and and miss fire in closed loop ...moving on to fuel pump planned on changing it anyway for the nitrous ‍♂..any input is appreciated
 

Vaughnmx

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Apr 23, 2019
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I do have a a9l also but the whole o2 harness bs scares me I don’t want to fry anything I have a 93 5 speed o2 harness as well but I don’t know if the harness will interchange with the 90
 
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