OK so here's the deal with watching the STFT on both banks as well as the 02 sensors.

P0300 set; Closed loop.

Under idle:
STFT1 is -30%
STFT2 is -8%

O2:

B1S1 .060 - .800
B2S1 .060 - .800


2500 RPM in Park

STFT1 is -7% - +7%
STFT2 is -7% - +7%

Can you do the fuel trim test with the MAF unplugged. At idle, 1500 rpm and then 2500 rpm?

Just to make sure I am reading that right.

Bank 1 is -30% fuel trim reading not +30%

If your getting -30% then the possibilities are leaking injectors, O2 sensor problem bank 1, exhaust leak before 02 that side, low compression or a FPR that is leaking to that side.

What was the LTFT bank 1?

I was thinking MAF but that would affect both banks.

I will do this this afternoon and report back. I did swap a brand new MAF Yesterday, but no change. I need to go fire a new cat in the 95 I Just fixed with this same issue. Cap/rotor/plugs/wires and spider assembly. Sold.

Yeah you are reading that right it's Negative 25-30. I suspect an O2 as well, as it seemed to be moving slowly. If I am not mistaken, bank 1 is the side where cylinder number 1 is, right?

Okay, I did the test with the maf unplugged as you asked;

Results:

@IDLE:

STFT1: -30.5%
LTFT1: -10.9%

STFT2: -25%
LTFT2: -10.9%


@1500 RPM

STFT1: -9.4 to -18.0%
LTFT1: -14.8%

STFT2: -1.6 to -10.2%
LTFT2: -1.6%


@2500 RPM

STFT1: -6.3 to -21.9%
LTFT1: -7.8%

STFT2: +0.8 to -5.5%
LTFT2: 0.0%


I did notice that maf plugged in or unplugged the O2B1S1 V moves really slow, while O2B2S1 moves very fast with the voltage variances. O2B1S2 (after Cat) moves slow, within limits.

I think the slow moving B1S1 is bad. But would this cause my P0300 code? I don't know for sure. I could say (from my memory) that a slow upstream O2 being sluggish perhaps tells the PCM that that bank is running lean, so the PCM adds more fuel? THis would make sense of my screwed up Negative STFT values on B1, right?

Yeah replace the 02 sensor. They are supposed to cycle fast .1 to .8 constantly at idle. Heat is your best tool for removing the sensor.

The PCM is thinking its running rich, thus taking away fuel, making bank 1 (drivers side, cyl #1 side) run lean. Negative fuel trim. A positive fuel trim means the PCM is adding fuel.
Its easier to see what the 02 sensor is doing if you graph it.

Do you think that O2 has anything to do with the P0300?

you could have a bad cam sensor. just had to do mine. your better off replacing the whole dizzy instead of just the sensor the bushing can go bad inside the dizzy throwing off the cam sensor

I meant to update this post. Thanks to Burned for helping me via PM.

Hooked up a live data scanner, saw my STFT1 was -25%! Watched the 02's and sure enough the upstream on bank 1 was moving very slow and pinned out around .800mv. I changed that 02, and now while its moving faster, it's not moving as fast as 02B2S1.

Upon further investigation, I did a spark check all around. All was well.

I knew my problem has to be on the Bank 1 side, since STFT1 was so damn high. IT only went positive when I induced a large vacuum leak at the PCV port. "She's not breathing right" I thought.

Hooked up a vacuum gauge and it was reading between 5 and 15 with erratic jumping at idle. Snap throttle to 2500 RPM and rapid decel shows it goes to 0 at snap, on decel goes to like 30, then back to 5-15 with the rapid erratic jumping.

Did a compression test and;

Cylinder 5: 150 PSI
Cylinder 3: 0 PSI
Cylinder 1: 150 PSI

Cylinder 2: 150 PSI
Cylinder 4: 152 PSI
Cylinder 6: 155 PSI


On cylinder 3 I pumped some motor oil in there to do a wet compression test. It didn't rise at all, still 0 PSI. I suspect valve train issues. I am going to pull the valve cover tomorrow, then probably the head. A brand new Surefire head is only $200 my cost through commercial at auto zone. I'm thinking the new head is the route to go, so long as the cylinder on #3 isn't also damaged.

So I have taken off the Bank 1 head. That wasn't enjoyable!

Isolated the no compression on #3;

https://i.imgur.com/36FiA4N.jpg



https://i.imgur.com/lBx5p1Q.jpg