01 SSEi. I posted on this not long ago and thought the problem was a bad Injector as we had the P0204 Open Injector Circuit Code appearing and the car was shaking when idling and accelerating. First started when I entered the freeway and accelerated hard to avoid some idiots and immediately after letting off the pedal and coming to a consistent speed I noticed a bad misfire/shake. Ordered some injectors from Harofreak (thanks again for the awesome price) and we replaced the injectors for Cyl 2 & 4. No change in the shake at all. Had a mechanic put his code reader on it and the P0204 Injector in Cyl 4 issue was no longer present but there was a constant misfire in Cyl 2 now. Pulled the plugs, all looked good, 2 was a little black and wet around the threads at one point, cleaned it up, checked for spark and spark was good in all the plugs we pulled, wires he said were fine and the wiring for all 3 rear injectors tested at the proper voltage, he said it's most likely internal mechanical damage such as a blown valve tip or the like. Although I didn't want to hear that, it seemed logical to assume internal damage, however today, it started without a shake, drove almost as if the problem never existed and the SES light was on but no longer blinking. It did seem to idle slightly high for a few seconds when started on another occasion today and my wife said it surged slightly as if trying to go faster with no input from her on her way to work but just slightly and intermittently. I started it again at a different time and the shake was back a bit on start-up and when driving and the SES light began to blink again but soon after in the same drive the light stopped blinking and just remained on steady and the car accelerated like a champ again, very smooth. the shake seems hit and miss now for some reason. An improvement from before but still there. I am guessing this improvement is either great news for me or VERY BAD? Below is a little input from a buddy of mine that has worked on the car before. Aside from the obvious things to do like changing out the wires and all the plugs which should be done regardless in the very near future, does anyone have any ideas on starting points for diagnoses or care to venture a guess as to what might be happening? Does anyone know the firing order for this car? Sorry for the long post, just trying to provide detailed info on what is happening and what has been tried or suggested already to save us all time. Thanks in advance!!! I greatly appreciate any input as always

Friend's comment: I really don't know what to tell you, other than if it is intermittent it can't be a valve. Sounds like fuel (Injector, regulator, filter, wiring issue) or spark (Plugs, wires, coils, Computer, wiring issue). Other than that it could only be air, but that wouldn't cause a problem in only one cylinder unless it's due to a closed valve. Your plugs looked fine so I am leaning towards fuel. If it was just spark, the plug in the dead cylinder should have come out wet from unburnt fuel. If the intake valve doesn't open, that would explain the dry plug, if the fuel isn't squirting in through the injector, that would explain the dry plugs. What's causing the lack of fuel to the cylinder is what we need to diagnose. I have no way of checking the fuel pressure, but I've been told if the pressure is below normal, the injector to fire last in the firing order is the one that starves for fuel. I thought the firing order for a 6 cylinder was 165432.

Sounds like the injectors need a little cleaning.
You describe a problem that I had before I threw in a half can of Seafoam in the gas tank. I'd try that first.

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Mark
The Familyman's Trans-Am
Intense 3.4 MPS, Intense FWI, Intense PCM, copper NGKs TR55's, MSD wires, MSD coils, BWD ICM, MagnaFlow High Flow Cat, drilled 180 'stat, solid front mount, Monroe Max Air rear shocks, Moog rear mounts, Moog suspension bushings, cleared front corners, 190,000 miles

Start with your plan of plugs and wires. If the problem is still there, then swap out the number 2 injector with another cylinder. I'd choose something other than 4, where you already had problems. Check the fuel pressure by either renting or buying a pressure tester. Do a cylinder compression or leak-down test to check for mechanical damage. Have the intake gaskets ever been changed?

Edit - The firing order is correct.

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*Gone, but not forgotten* - Black 2000 SSEi, HIR Headlights, Angel Eyes fogs, 3rd brake light overlay, hi-flo cat, 180 degree thermostat, HS 1.9 rockers, LSx yellow springs, Intense FWI, PCM, shift kit, push rods, and 3.4 Pulley. ZZP Power Log.

I disagree. I've had a bad misfire, but still had dry plugs, as long as the misfire is somewhat intermittant. I've had a misfire even after doing plugs and wires before. Someteims those wires are hard to seat properly. Also, when you do the plugs/wires, make sure you smear some dielectric grease inside each plug and coil boot on the wires.

You're misfire could be due to something else. I know it happens, but I've never had a misfire due to a bad or dirty injector, or by plugged fuel filters.

We replaced the intake manifold gasket about 2 years ago and I think the valve cover gaskets need replacing now. Should have done them at the same time I guess, possibly extra pressure created from the new intake gasket seal caused them to let go earlier but there are small puddles of oil in the drive (has been for about 2 years now), started happening soon after we did the IMG, but not large amounts, pretty small. I have extra injectors as we bought 6 used injectors, should I try one of those or just swap with another already installed? Thanks for the input.

Instead of throwing another unknown into the mix, I'd suggest swapping one that is on the car from a cylinder that is not, and has not, given you a problem.

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*Gone, but not forgotten* - Black 2000 SSEi, HIR Headlights, Angel Eyes fogs, 3rd brake light overlay, hi-flo cat, 180 degree thermostat, HS 1.9 rockers, LSx yellow springs, Intense FWI, PCM, shift kit, push rods, and 3.4 Pulley. ZZP Power Log.