Its a 2013 GS automatic. My buddy has scanner the does live reading.

 

The easiest first step is usually to change the coil over plug coils for 1 and 3 to different cylinders and see if the misfire moves to the new cylinders.

 

Yes, weak or broken valve springs can cause misfires at higher RPM's but these misfires didn't happen till the O2 sensor was changed....why was the O2 sensor changed initially and were all 4 changed ??...for the P0420 ??...good your friend has a scan tool that can read live data...can he graph with it and what does Mode 2 "freeze frame" data saying for those 2 DTC's ??...what is bank 1 upstream O2 sensor doing from idle to 5000 RPM and then above 5000 RPM ??...is it switching between 200-800 mv's ??...what do the fuel trims look like at those RPM's ??...if the O2 sensor is bad and just because it's NEW doesn't mean it's GOOD !!...I went through 3 new O2 sensors last year before I got a Denso that finally worked...if it is "stuck" over 800 mv it will be "pulling" fuel and that bank will be lean which will give you misfires...the upstream O2's should go rich at WOT...also as far as the P0420 is the downstream O2 sensor fairly steady around 700mv's or is it oscillating like the upstream when the cats are hot...if bank 2 fuel trims are positive and the bank1 is negative under load your cat may be starting to plug up...if I can get some numbers from you during a pull I can check the VE of the engine to make sure it is pumping air correctly...if the VE is good it's not an "air" side problem ...I've been using this cool scan tool recently that will let me know by just going around the block if I have an air or fuel issue with no guessing...the chart on the left is the calculated vs. actual VE...if both are the same the air side is not your problem.
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4 days after I bought it a valve spring broke on #7 intake. It has green valve springs in it. I would barely run, I had no idea what it was I had my local diagnostic folk try to figure out what it was. After scanning it and trying to figure out what it was the car had to be running and it was dumping alot of fuel trying to compinsate. Which gave the P0420 bank 1. It had also melted the intake manifold becouse tne vavle was stuck open. I replaced both springs on #7 as well as the intake manifold, but the code came back on. I bought all 4 02 GM sensors and was advised to replace the ones on the manifold first. Thats when all the missfires started at 4800+ rpms, I put the old one from the right side back in the ln tje left side but have yet to test it.

 

OK, if it was dumping fuel your O2 may be “stuck” below 200mv and that can “add” fuel and then destroy your cat…there is your P0420…fix the fueling issue and then concentrate on the cat…when my O2 went bad it stuck lean at 100mv and the exhaust stuck and was missing on all 4 cylinders…luckily I was in the garage so I quickly shut it down and diagnosed it to a bad O2…if you can graph your O2’s and fuel trims at idle and under load we’ll try to assist if needed…so is it still misfiring at those higher RPM’s ??…trying to figure out when you have right now.

 

I haven't tried it yet, Ive been thinking all along that it messed up the cat. Im going to order a replacement and replace it irregardless. I appreciate the help and will take it out this weekend. In the meantime im enjoying driving my 90 ZR1.

 

That rich mixture most likely killed the cat so that is the effect and not the cause…fix the fueling issue first…I believe Arkansas is a non emissions state but you still want to buy a cat which is as close to the OE spec as far as “cells per square inch” go which is around 400 I think.

 

I talked to my friend he said the fuel curve was good, it only dumped all that fuel when the valve spring was broken. He said the the #7 cylinder didn't show any missfires. #3 had the most then #1 and a few in #5. #7 i replaced both valve springs.

 

Ok, so fuel trims look good but I don’t know if they have been checked under load…and 1,3,7 still have misfires at high RPMS on bank 1 still ??…with either an injector, a mechanical or ignition misfire your fuel trims won’t be good…you still have a P0420 I’m assuming…like I mentioned you have to look at both O2 sensors on that bank and make sure the downstream is not oscillating or switching like the upstream…also the fuel trims on bank 1 would be negative if that cat was restricted and you would have a loss of power…can’t offer you any more help than this.

 

We seafomed the top end, and put the original top O2 sensor no change. Swapped #1 and #2 coil and wire, no change.

 

I see you’re in open loop so I’m guessing the engine was misfiring at that time…only a single cylinder misfire ??…were the upstream O2 switching between 200-800 mv’s ??…in open loop the O2’s aren’t used in the fuel calculation…I’d swap injectors if it’s a single cylinder misfire and see what happens…I see you have the MX808 and I don’t know if it has injector balance testing under special functions but if it does and you have a fuel pressure gauge you can do that test instead of swapping injectors…you can do a quick test for mechanical integrity of the engine with a “cranking cadence” test…gas pedal held to the floor while you start the car…it will only crank but listen if the engine has a steady cranking sound.

 

#1 and #3 missfiring pretty bad i think #3 missfired 300+ #1 200+ and #5 just under 100. Bank 2 is as near perfect as it can get. Now the missfiring is coming in at 4500 RPM's.

 

Can you graph the O2’s being your in OL and fuel trims don’t give us any guidance ??…looking at just a number doesn’t tell us anything…I would still do a VE test…you can record with that scan tool and you would normally use the highest MAF reading and RPM in the calculation at WOT…do you show an fuel injector pulse width data PID on that scan tool for bank 1 and 2 ??…that is a good one to look at.

 

I may have figured out why its missing. I know it need a cat but thats not the culprit for the missfires. The only cylinder its not missfiring on is #7, the one I put the vavle springs on. It was at tdc when I torqued it down, I had several people tell me i didnt have to put the other ones at tdc just torque them down. After watching several videos they all say to rotate each clyinders to tdc and torque. Iwill check this out this weekend. What are your thoughts. Tia

 

Each cylinder has to be at TDC !!…that is correct.