Help please, I'm stumped! Car was running fine, then suddenly, immediately after a moderate run-up to ~7000 rpm in 2nd gear, the engine absolutely will not idle. As soon as rpms drop below 1200 rpm, it immediately dies (no fire in the hole). It also dies under any minimal load conditions (maintenance throttle cruise). But give it more throttle, and it accelerates just fine. But the problem is bad enough that the car is undriveable.
I scanned the OBD-II port, and this is what I see: the MAF, TPS, rpm, front and rear O2 sensors all appear to be working fine. When the engine wants to stall, the O2 sensors go very lean. Hard to tell if its a cause or an affect, but it does cause the short term fuel trim to go full positive (+17%). No ETC (electronic throttle control) codes, no codes period.
Here are the things I've considered:
Intake air leak: I've looked (and looked, and looked), but I can't see anything amiss. This seems most likely to me, but where could it be hiding? Would have to be a pretty significant leak, not just a bit of seepage around a snug hose clamp. Also, wouldn't the engine tend to race at idle, due to the extra air, and the ECU dumping extra fuel to make up for the lean condition? Instead the rpm's will hang briefly at about 1200, then quickly drop and die.
Exhaust leak: an exhaust leak before (or at) the front O2 sensor would cause an incorrect lean reading at idle, which would make the ECU dump more fuel. I've removed and reattached the muffler, making sure the cat-to-muffler joint is tight, and it seems fine. I was esp. suspicious of this because I had recently changed the exhaust (stock cat pipe, 2bular 8x22 muffler). The front O2 sensor is also tight. And there is no indication at the tail pipe of an overly rich condition (due to the ECU dumping extra fuel), no gassy smell, no drip.
Water in the gas: I suppose it's possible, but the condition came on suddenly at about 2/3 full tank (after previous fill-up at my usual station). Also there is no visible white smoke from the tail pipe. I'd expect the engine to stumble under load too, wouldn't it?
Exhaust constriction: I would expect the O2 signal to read rich, rather than lean. Also the engine would resist revving, but it seems to rev fine.
Intake constriction: I wouldn't expect the O2 sensors to read lean, but maybe I'm just seeing lean when the fire goes out? Hard to tell, it happens fast, and the scan rate on the OBD-II scanner isn't all that quick.
Some variable cam problem: OK, I have no idea here. Say it "got stuck" on the second cams, how would the engine behave at lower rpm's? What can I check?
Fuel pump/fuel pressure/fuel injector problem: I doubt it's a clogged fuel injector, because all 4 cylinders seem to die at once. If it was low fuel pressure, I would think I'd see the opposite of what I see (idle OK, die under load), no? What sort of fuel pressure regulator does this engine have?
Is there something else I'm missing? Where else should I look? Any and all suggestions welcome. My last resort will be to flat-bed the car up to the Lotus dealer (about 40 miles), not too bad, but I'd really prefer to fix it myself.
Thanks in advance,
