So awhile back I bought an 89' GTA that had been sitting about 6 years. Hopefully I'm not forgetting anything but it's a 350 TPI reman engine with 30k miles (supposedly .40 over but cant verify that), has a comp xr269hr cam, accel 21 lb injectors (all ohm'd good), bbk adjustable fuel pressure regulator, edelbrock high flo base, accel runners, flowtech headers, and hypertech underdrive pulleys. The issue I'm having is a misfire that's most noticeable at idle, when driving you dont really notice it. The header tubes on cylinder 1 and 7 are way colder than the rest, like 300 degrees or so colder. Replaced both those injectors with no results. It had misfired like this since I bought it, didnt think anything of it since it had sat 6 years. So I changed the plugs (the ones I pulled were brown as they should be), new Taylor 8mm wires, pertronix flame thrower coil, new ICM, new pickup coil, new distributor cap, and new rotor. None of the above fixed it obviously. Compression check showed 165 psi and it also passed a coolant pressure check. It doesnt run terrible but I wouldn't say its dialed in perfect. It doesnt have a catalytic converter but it does smell rather rich and I wouldn't say the gas mileage is all that great. Once you get past 4000 rpm it starts to vibrate, pulling timing possibly? Starting to wonder if its ECM related but thought I would post here and see if anyone has any input before I keep throwing parts at it

 

If the misfire is only noticeable at idle, I'd run a leak down test. the compression test may not be perceptive enough.

I battled an idle misfire for a while and it too passed the compression test. it wasn't until I ran a leak down test where I determined it was my piston rings (the air hissing sound was coming from the crank case as opposed to out the intake or exhaust which would have indicated the cylinder heads).

at low engine speed there's sufficient time to lose compression, whereas at higher rpms there's less time, hence why it is only noticeable at idle.

I also went after the injectors and ignition system to no avail.

ended up running some CD2 additive in the oil which evidently freed up a stuck piston ring after about 100 miles of driving, as the problem cleared up and never returned. this was back in like 2008 or so...

btw, the short block is a ZZ4 and it only had probably 25k miles on it, so it's not beyond the realm of possibility that you may have a similar situation.

 

Wow that's a scary similar situation! Mine is definitely most noticeable at idle, seems like when its warmed up it slightly improves but kinda varies. Past 4000 rpm I can start to feel a vibration. In your case did yours smoke or have any blow by? I have neither but a leak down test may not be a bad idea. I haven't replaced my electronic spark control module, but before I buy one I'd rather test it if possible. Unplugging the EST when timing it didnt made it idle any smoother

 

Went and bought a fuel pressure tester yesterday so I could check it. I'm running a bbk adjustable regulator and Accel 21lb injectors. I'm almost positive with the accel injectors it should be set around 48psi. Turned the key on and it got to 20 psi then stopped. I let it sit for a couple mins and did it again and it got to around 36 psi. With it running it didnt get much past 36. I gave it a couple revs past 3,000 rpm and didnt see it go over 40 psi one time. So I'll check back after I play with the regulator

 

Ordered a new adjustable regulator with the diaphragm from South bay and finally got it installed tonight. Got it set to 48 psi and it runs much smoother. Past 4000 rpm it doesnt vibrate anymore. Not sure what was going on with the old regulator. They had the bolt backed almost all the way out, weird.