Been scouring this forum for days now (I'm a long time occasional lurker), as I've been trying to track down the route of this problem. And I'm struggling, so I thought I'd just sign in and ask for your advice.
The Symptoms
Over the past week the car has started misfiring / shuddering. The problem comes and goes at the drop of a hat. And it seems to becoming more regular.
At idle, the car may be running fine. Smooth as ever. Then suddenly it'll drop maybe 100 revs, down to 600rpm rather than 700rpm. The rev counter will sit perfectly still at those revs, but there'll be a pronounced shudder in the car and burbling through the exhaust like it's not firing on all cylinders. It feels very much like sitting in an old diesel van.
When you pull away, it becomes even more pronounced and sounds like what I've read on here, an old WRX. I couldn't put my finger on that though as I've never been in a WRX. I could even go one step further and say it sounds like an old american V8 muscle car! Except without the power
Then instantly, it's gone, power shoots up like the turbo's just kicked in (not the case, I can hear it spooling when it's running rough) and the engine's totally smooth again.
This problem comes and goes purely at random. No better or no worse at any rev range (though it's being driven easy a. because I don't want to break it, b. because the weather doesn't allow otherwise here at the moment).
ECU
Checked for fault codes on the ECU. Two slow flashes, followed by one quick flash. Which I'm led to believe is the ignition signal.
The Car
Pretty much totally standard RB25DET series 1 Stagea, with the exception of Yellow Jacket Coil Packs, NGK Iridium Plugs (both less than a year old), K&N panel filter and 3" exhaust.
So Far..
From what I've been reading, the symptoms pointed to the AFM. I've had that out today and re-soldered the pins. This made no difference at all. I'm yet to try contact cleaner on it. I haven't done anything to the car recently. Later checked the error codes and got 21, ignition signal (I believe? Though 12 is the AFM..! Am I reading these right?).
This is my daily driver and need it to get to work, so would appreciate any help.
Thanks