What's up, fella's?

Big fan. Long time reader; first time poster. I'm a bit of a mechanical novice, but learning a lot with this project:

1991 Firebird 3.1 v6

Symptoms:
Hard start and rough idle. Sputters and dies occasionally at stops.

Repaired and replaced:
Battery
Plugs and wires
Cap and Rotor
Air filter
Fuel filter
Catalytic converter
IAC
MAP
Purge solenoid
Brake booster
Cleaned intake and butterfly
Checked for vacuum leaks
Swap tested fuel pump relay

Initially the car threw a Code 34, and once a Code 42, but now she runs rough without any SES light. What's funny is whenever I disconnect the negative battery terminal to replace a part or something, when I start it back up the car runs fantastic under default settings until the computer catches up and starts making adjustments. Then it bogs down and seems to run rich and hiccup. So, after each attempted fix I would go to test it out and would think "YES! That was it!" only to be let down 3 minutes later when the symptoms returned.

Tested fuel pressure. Zero psi with key on engine off; and can't hear any sound from fuel pump. After a couple cranks she starts and holds fuel pressure steady at ~40 psi.

What to do next?
Fuel pump
Fuel pressure regulator
ECM
ICM
TPS
O2
Knock
Timing
Timing chain slack

I also noticed a module with a plug-in on top of the charcoal canister that appears removeable but apparently not replaceable. After driving around for a bit I noticed this component was extremely hot; too hot to the touch and almost on the verge of melting the plastic casing. Looking at replacement canisters it looks like the module has been deleted and hoses run straight out of the canister lid. I tried unplugging the module and even bypassing the canister altogether with no improvement to the idle/stall issue. Anyone know what this piece does?

I've found so many fixes for similar symptoms that my brain is about to explode. And I'd really hate for it to be something stupid like "get a new gas cap" or something. Any advice on what to to next?

 

Unplug the O2 sensor and go drive it.

GD

 

Thx GD. Was this a fix or a troubleshooting test? I disconnected and drove a bit. Started out fantastic, but warmed up and bogged a bit. No more sputtering (YAY), but still running rich.

Soooo... replace O2 sensor? And regarding the fuel pressure test, not priming, pump or regulator? or something else?

Thx again for any help.

 

Fuel pressure with engine off doesn't matter.

Fuel pressure running at idle should be around 35 psi. 40 is too high. At WOT the fuel pressure should rise to 43.5

What is your indication that it's running rich?

Unplugging the O2 just prevents closed loop. Keeps the ECU from messing with the fuel trims. Might mean it's bad but need more data.

GD

 

probably not a good idea to run it unplugged all the time, huh? Would it even pass emissions like this?

Just the gut feeling of myself and a couple of "brochanics" that I have on retainer.
After reset or during cold start, she starts up and I only have to feather the accelerator and she goes. But after a few minutes warm up she feels bogged. Have to smash the pedal just to get to highway speed. And the fumes! Feels like she's consuming like 3 times more fuel than necessary.

 

Any thoughts on this O2 sensor defouler mod?

 

Definitely NOT. That's to fool computers and you only do that on rear O2's for 96 and newer models.

GD

 

Check the resistance of the coolant temp sensor. Could be the computer thinks the engine is cold all the time which would be either a bad sensor or resistance in the circuit. You should probably check all the ECU grounding points as well - on the V8 models they are located on the back of the cylinder heads. I don't know if the V6 is the same but I would imagine it's similar.

GD

 

Update:

Replaced coolant temp sensor.

Had a "Bro"chanic look at the timing. He tuned it "by ear" and managed to smooth out the idle so it doesn't sputter and die anymore, but still running sluggish and gas mileage is atrocious. My friend said to mention that when the MAP sensor is unplugged while idling there is very little to no change in performance. He says unplugging the MAP should make the engine run rough, choke out, or die. MAP sensor is new; changed out early on.

Invested in a timing strobe to verify timing and could not find timing mark on harmonic balancer. Thought I had clipped onto the wrong wire so I switched it and BAM, there was the timing mark right where it should be. But when I double checked the wire I was clipped onto the #3 wire. So is it like 120 degrees out of phase? How do I correct this?

Thanks for any help. Getting so close now...