Random misfire after a few hard pulls
Thread Starter
Staging Lane
Joined: Apr 2019
Posts: 80
Likes: 6
Random misfire after a few hard pulls To preface I should mention that I just installed 1 7/8 long tubes and true duals about a week ago, I changed all spark plugs, wires, and O2 sensors and it's been running fine until now, no tune yet. Yesterday I was racing my buddy's car and did a few hard pulls from 2nd to the top of 4th, ran fine during those pulls but was cruising at about 70 mph and noticed on any throttle my car would shake pretty badly. Parked and scanned it and its getting a random misfire code (along with all the other codes associated with deleting cats, EGR, and AIR). I know typically this is a spark plug/wire problem but I checked them and they all seem fine. What is most likely to go bad and cause this after straining the engine like that? Starts and idles fine still, only noticeable under hard acceleration.
Thread Starter
Staging Lane
Joined: Apr 2019
Posts: 80
Likes: 6
Ngk tr55 and the wires are MSD. I'm not very good at interpreting DTCs so correct me if I'm wrong but wouldn't a random misfire code make it unlikely that it's the plugs since it's being detected on multiple cylinders? Unless something has simultaneously fouled multiple plugs
Random misfires just means more than one hole is not happy. Example, you burn up #4 plug you get #4 misfire. You burn up #4 and 2, you get random misfire.
That said, may be as simple as fuel filter plugged.
That said, may be as simple as fuel filter plugged.
Thread Starter
Staging Lane
Joined: Apr 2019
Posts: 80
Likes: 6
It just seems unlikely to me that I wouldn't notice a misfire before and then suddenly two or more plugs cause misfires. But I did just double check all plugs and wires including gap, no problems there. I'm going to replace the fuel filter and see if that helps, hasnt been replaced since I got the car 10k miles ago and I have no idea if the previous owner ever replaced it.
If you have access to a tuning suite you can disable the cylinders 1 by 1 and listen for the change in sound. If you have a cylinder intermittently firing this will help narrow it down. Otherwise can unplug 1 injector at a time.
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Staging Lane
Joined: Feb 2020
Posts: 80
Likes: 19
I had a similar issue on my LT1 car back about 7 years ago. I know it's not a LS, but it started and ran fine when cold and in open loop. Once it got into closed loop it started misfiring like crazy. After going through the plugs, opti, fuel filter, injectors etc, it wound up being something wonky in my PCM. We re-flashed my tune and the car never misfired again. I would say that it's possible that your car just didn't like having all those codes popped. The PCM is already trying to make fairly large adjustments for the new exhaust on the stock tune, and then you max performed it.
Thread Starter
Staging Lane
Joined: Apr 2019
Posts: 80
Likes: 6
I had a similar issue on my LT1 car back about 7 years ago. I know it's not a LS, but it started and ran fine when cold and in open loop. Once it got into closed loop it started misfiring like crazy. After going through the plugs, opti, fuel filter, injectors etc, it wound up being something wonky in my PCM. We re-flashed my tune and the car never misfired again. I would say that it's possible that your car just didn't like having all those codes popped. The PCM is already trying to make fairly large adjustments for the new exhaust on the stock tune, and then you max performed it.
Joined: Nov 2003
Posts: 6,216
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From: Wichita KS / Rancho San Diego
But if the misfire is randomly skipping all over the place from cylinder to cylinder, then you may have a PCM, electrical, or grounding problem.
Originally Posted by greaseman69
Well the misfire is back already. You think because resetting the PCM fixed it theres something wrong with the PCM?
Thread Starter
Staging Lane
Joined: Apr 2019
Posts: 80
Likes: 6
Alright I'm gonna get a quick mail order tune with new plugs and see how it does. Was gonna get one once I got my intake but it couldn't hurt to do it now. The plugs are a little black around the rim which I thought was normal but after only a couple hundred miles maybe it shouldn't look like that. Thanks for the advice
