The engine has always run a little rich but it's never affected the performance of the car. I'd have the occasional single afterfire, but only if i took my foot *completely* off the throttle when shifting to accelerate. The car has also had an inconsistent idle, but never stalls or dies, it just surges about 500 rpms, settles back down, repeat repeat repeat.
Within the last week, the car has become completely undriveable. It began with a series of sputters if I got on the throttle too hard while accelerating. The idle became rough and somewhat "lopey" if that's a common term. You can probably imagine what I'm talking about- Brum-brum-brum-brum-brum. Still doesn't stall out or die at idle though (unless the RPMS fall to fast, say, if I get surprised by a red light and have to hit the clutch and the brake at the same time and just let the throttle drop). Over a matter of days, these symptoms increased to the point where the car will only accelerate if you do it gently. If you try and get on the throttle too hard it'll surge and seize like when someone isn't giving it enough gas. every surge is accompanied with black smoke. oddly, this only seems to happen consistently under heavy acceleration in first, second, or third gear. If I'm on a highway, and I can get up to cruising speed, the engine behaves fine. In fourth or fifth gear, I can go to WOT and the turbo will spool up, the car will accelerate smoothly.
I've also noticed a *severe* drop in MPG. I typically get between 10-12 mpg in the city and up to 16 highway. Now I'm down to 6-8 on average. The exhaust smells heavily of unburnt gasoline, even when it's puffing black smoke. Consistent with the sudden rich fuel conditions, i've opened up the engine and cleaned everything I can get to. The spark plugs were covered in black carbon (i think) so I cleaned them gently with a wire brush.I think that does it for the symptoms, but it's likely I'm forgetting a detail.
What I've done so far:
- Cleaned the MAF with MAF cleaner spray. It's an ISR and i've heard they tend to crap out spectacularly and suddenly. I don't have access to a known good Z32 MAF to swap them out and check. The MAF is about five years old
- removed and cleaned the AICV.
- removed and cleaned the 02 sensor. I actually have another 02 sensor around here somewhere, but damned if I can find it!
- removed, checked, and then for good measure insulated the Coilpacks. As noted, spark plugs were cleaned of deposits with a brush.
- Ran a treatment into a vacuum hose to clean engine deposits utilizing the "sip sip sip sip choke sit drive until there's no white smoke for ten minutes" method. This did mitigate the frequency of the symptoms, and slightly improve performance, but city driving is still next to impossible.
- For shits and giggles, I ran a compression test. All cylinders were between 150-135 except for cylinder three, which was about 125.
- searched for vacuum leaks using the soap and water method. I didn't notice any bubbles or changes in engine performance.