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Discussion Starter #1 (Edited)
I have a 2006.5 Optima w/ the 2.7L engine. I've been working on it and replacing parts for two weeks but cannot find the source of my misfires.

It all started in traffic. I pressed on the accelerator and the car started shuddering and was very slow to accelerate. I plug in the fault code scanner:

Fault code: Misfire on Cylinder 1, bad O2 sensor right bank, downstream. I buy a replacement O2 sensor. I pull my intake manifold and inspect the #1 spark plug. It was a little darker than the other 5 plugs but didn't look 'bad'. After gapping the spark plugs to .044, installing the O2 sensor. Put it all back together again but...the misfire is still there!

2nd reading: Misfire on cylinder 1. I suspect a bad coil pack and switch the location of the bad coil pack to an easier location just to see if the problem goes away on cylinder 1 and shows up on the other location. I change the coil pack with the #2 cylinder (on the front of the engine). The reader said my engine is now misfiring on #1 and #2. The problem moved to #2 cylinder. Replaced w, new coil pack but it still misfires. Any suggestions? I'm out of clever ideas. All vacuum hoses appear to be intact.
 

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Discussion Starter #2
Anyone? I'm still desperately searching for solutions. Pulled, inspected and cleaned the CPV valve and replaced the rubber tube that connects it with the intake manifold (it had a split). All other rubber tubes around the intake look solid and intact.

The last reading I had showed fault codes for all 3 of the original O2 sensors. That was certainly new and unwelcome info. It is difficult to believe the other sensors would all go bad at the same time.
 

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Discussion Starter #4
Thanks, @SpongeBob,
The codes are:
P030 - Random Multiple Misfire
P0131 - Circuit Low Voltage
P0151 - Circuit Low Voltage
P0157 - Circuit Low Voltage

I changed the Bank 1, downstream O2 sensor a few weeks ago when it was the only one that registered a fault code. Changing another 3 @ over $150 each is a LARGE financial risk if that is not the problem.

I tried the old trick of spraying starting fluid over and around the intake, throttle body and air lines but nothing happened. I appreciate any thoughts as to the problem!
 

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Discussion Starter #6
Have tried the ignition wires?
I may not understand the wires you are asking about. With the direct ignition, I don't really see any wires to change. I just ordered an entire set of O2 sensors made by NTK. Later this week, I will install them and see if this is the missing link in the misfire mystery.
 

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U need to find out the "firing order". Check the spark plug wires and make sure they are in order in relation to the ignition coils.Also, anytime sensors are replaced or hooked/unhooked etc ., Disconnect (-)terminal for 15 min this will reset the ECU and if CEL (check engine light ) comes back it will be more accurate as to what the issue is.
 
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