P0300 CEL, rough firing, nearly stalls

[97GreenSpyder]

I've looked through all the references regarding the p0300 Random Misfire CEL, but I'd appreciate if someone would look at these symptoms en toto.

Several days ago, my '97 GST (auto) was very hard to start in the morning (high humidity). Seemed to be running on half the cylinders when I started it, but after jabbing the accelerator a couple of times, it settled-down and ran fine. My ScanGuage indicated a p0300 CEL, which I cleared and it did not recur. No problems the rest of the day starting/running. This morning it was rough again while starting, but not quite as bad. Again, a one-time p0300 CEL. But driving home, I started losing power- it was bogging-down and would barely accelerate. Every time I’d clear the p0300, it would come back. At a stop, the needle would drop to the bottom of the tach and the engine was on the verge of stalling, so I would shift into neutral and use the pedal to bring the revs up, but the car was vibrating along with the loss of power (not all plugs firing?). Got on the expressway to limp home (barely could accelerate), and it would hold 55-60mph, but the tach was only reading 1200-1300 rpm at that speed (1/2 revs?). Don’t know if it is important, but instead of a normal instantaneous MPG on the ScanGauge (usually jumps around from 3-40mpg), the gauge seemed to almost continuously show 99999, which is what you see at decel/fuel cutoff. And the engine was running in Open Loop mode. Also, the inside of my stainless exhaust tips are now solid sooty-black (previously had very little carbon deposits inside them). The car’s ignition system is stock, having changed the plugs and wires about two months ago (NGK BPR6EKN & std OEM NGK wires). Cat and front Denso O2 sensor are about 6 months old, and the car has about 84,000 miles. Car just passed emissions last month.

Something that MAY be important- about a week ago the battery drained overnight and I had to jump-start the car from another that evening. Got some smoky-smells, my jumper cables were heating up, so I stopped and bought new. The car then started right up and I put it on a trickle charger for the night. But in the process of jumping the car, it apparently fried the Jensen power amp. I’m thinking I might have damaged something else. But the car seemed to run fine for almost a week other than no audio due to the dead amp.

Little help here, guys- what’s the most likely cause, the ignition module, the coil packs, or the ECU itself?

 

[inthemidst]

make sure your ignition module is plugged all the way in .my connector looked like it was pluuged in and did the same to me.later i pushed it in harder and it ran like a champ.

 

[97GreenSpyder]

Fixed the problem. Two weeks ago I installed a narrowband A/F gauge according to the write-up on DSMTuners. Was getting a p1791 CEL (water temp???? That made no sense!!!), so I disconnected the gauge end from the wiring harness I had spliced into the ECU plugs, leaving the harness connected, but not powering anything. A couple of days after doing that is when the p0300 started.

The hot wire to power up the A/F gage should be teed onto the ECU power supply wire at terminal number 25.
The ground wire for the A/F gage should be teed onto the ECU ground wire at terminal number 92. The air-fuel signal wire for the A/F gage should be teed onto the front O2 sensor return signal wire at terminal number 76.

Near as I can figure, the extra harness was making the O2 sensor send a lean condition signal to the ECU, so the ECU was trying to compensate by dumping in more fuel, hence the sooty exhaust tips. Pulled my plugs and the electrodes were as black as the exhaust tips' interiors.

But as soon as I disconnected the harness from the ECU (three 36" wires had been connected to.... nothing!) and tried the ignition, it fired right up as if nothing had been wrong.

Unexplained is 1) WHY I was getting a p1791 with the A/F sensor connected to #76, which was supposedly the O2 wire, white just like the wire next to it (#75, the rear sensor), and 2) WHY three wires (insulated end terminals, so they were disconnected from anything at the gauge end) would trick the ECU into running WAY rich. Does an A/T ECU use a different plug wire than #76 for the front O2 sensor? Didn't see any other paired white wires on any of the 4 harness plugs. The '97 Factory Electrical Manual says it's #76, too.