I could use some ideas for this rig.

2001 Pontiac Montana 3.4L van needs diagnostic

Running very badly, any rpm, misfire, weak, maybe worse when warm.
Acts exactly the same when in open loop. (disconnect O2)
P0300 Random Misfire (blinking)
Fuel trim heavily negative
Motor running warm due to lean pcm

Was not burning oil and cooling system is clean and solid.

I have replaced front O2 - no result.
I have recently replaced fuel pres reg as it had failed.
I checked IAC for the heck of it.
I think all injectors are operating correctly and fuel delivery seems to be correct but I have not pressure tested fuel line.

Just feels like there is no ignition for half of the motor, so the pcm sees rich.
Could it be ignition module and/or coil pack?
Vehicle is from North East, so corrosion could be an issue.

whens the last time you did plugs and wires? how many miles? check the spark at all cyl?

Spark plugs and wires are the most likely culprits, particularly if they have not been replaced for some time. After that, it could be coils or ICM if it is spark related, or low fuel pressure from a plugged fuel filter or a failing fuel pump. Certainly not a comprehensive list of problems that can cause a misfire, but these are the first things to check.

• change plugs and wires if they are not relatively new
• change fuel filter
• check ignition coils (https://www.gmforum.com/trouble-shoo...-wires-278818/)
• check fuel pressure (https://www.gmforum.com/trouble-shoo...e-test-284304/)
• clean the electrical connector on the ignition control module (https://www.gmforum.com/trouble-shoo...g-test-284383/)
• check the ignition control module (https://www.gmforum.com/trouble-shoo...m-test-299761/)

Chances are the plugs and wires will clear the misfire, but let us know what happens.

Here is my update pulled from the document I am preparing when I give up and send it to a garage.

PROBLEM:
P0300 Random Misfire (blinking)
Running very badly, any rpm, misfire, weak
Seems worse when closed loop and worse yet when warm.
Fuel trim heavily negative, less so now with new O2 installed.
Motor running warm due to lean pcm

Was not burning oil and cooling system is clean and solid.

WHAT I HAVE DONE:
I have replaced front O2 - no result.
I have recently replaced fuel pres reg as it had failed.
I checked and cleaned IAC for the heck of it.
I checked fuel pressure at rail. ~45psi idle with regulator vacuum and ~50 without regulator. Holds through all rpm.
I think all injectors are operating correctly and fuel delivery seems to be correct.
Spark looks good with basic light tester.
Has new plugs, gapped by me. Wires are original but they look good and I dont suspect.
I did pressure test cylinders a few months back and did not detect any bad valves. But that could have changed.
Lower intake gasket was done by me last year.

Just feels like there is poor/no ignition for half of the motor, and the pcm is seeing rich and trimming back hurting the issue further.

THOUGHTS:
-Have not checked for collapsed cat. There is an exhaust leak on cyl2 that closes when warm. Maybe it runs better cold because it can exhaust better.
-Could still be ignition module and/or coil pack.
-Vehicle is from North East, so corrosion could be an issue.
-Need to check bottom two crankshaft sensors, could be corroded or oil fouled, but no errors reported.
-Could be valves

Quick History:
Last year on roadtrip to NM, I dumped a can of CRC “Guaranteed to pass” in fuel tank to burn through on the highway to attempt to maintenance clean the injectors. Immediately the vehicle began to run crappy like it is now. I made my destination with very poor fuel economy and a friend there had a Snapon code reader tool. He ran the vehicle through some tests and ended with a “purge”? test which resolved the issue for unknown reasons. Thoughts were that O2 was blocked and test cleaned it out, but now I don’t think that was it. I believe that the computer “learned” some behavior with the testing tool and counteracted an existing issue whatever that was. Been running fine since then until I recently disconnected the battery and the pcm forgot the learning. Problem could actually be the PCM itself, but whatever it is, the default programming does not seem to get past or recognize the issue.

About the only thing he could have done was probably a fuel trim reset or relearn. But even then a relearn wouldn't cause such a drastic change in drive ability.

You say wires are original, do you mean factory original? As in 12 year old wires? Change them out. Old wires is usually the leading cause of misfires.

And you said you dumped a can of mystery fix it stuff, then all these symptoms happened afterward?

Remember, a 45-50psi reading only shows that your pump is working to spec. Don't let it fool you, 45-50 psi does NOT mean you are getting the required amount of VOLUME of fuel. Lack of fuel can cause a P0300, and a lean condition.

Something somewhere between the tank, and the injectors is causing a flow issue. And I'm wondering if that mystery cleaner loosened up some gunk somewhere and either clogged the filter or the injectors?

Based on the additional information you provided my response is the same. When is the last time you changed plug wires and the fuel filter?

"Well there'* your problem!" It never ceases to amaze me how much of an idiot I can feel like. It took me a lot of effort to find this. Those of you who have ever worked on a transverse motor in a mini van will understand why finding this wasn't as easy as it should seem. I made sure it wouldn't happen again. Good luck to whomever has to change spark plugs in the future... 250,000 miles and still going.

Are we looking at broken wires? And replacing them fixed the problem?

I see spark plug hoods still attached. They often get touch to remove, resulting in the wire off but the hood still on the plug. I use dielectric grease to prevent that when replacing plug wires.

Yes, the wires against the firewall were apparently laying on the exhaust header/shield. It melted/chewed through the plastic wire loom and apparently into two wires. They were still connected with a thread but were obviously shorting out to ground intermittently and also based on temperature and rpm. An interesting side effect is that since the coils are shared, one shorted wire can easily foul up a second plug. So in essence I suspect I was running a 2 cylinder at times and that FINALLY makes sense based on what I was seeing. Two missing from six isnt what I was seeing, it was more like 4 missing from 6, and randomly weak or missing power/ignition.

Anyone familiar with working on a transverse motor in a mini van will understand why it is difficult to catch something like this. You think "duh" until you too have to get in there and see what it'* all about against the firewall.

I think the only wasted funds was the new O2 sensor. It didnt need it. It was not the first thing I tried and replacing an O2 sensor has never corrected an issue in my experience, but I thought I would try it this time as it seemed odd how much the PCM was trimming down the mixture. Well now, duh... it all makes sense. 4 cylinders of unburnt fuel and the only two firing cylinders get leaned down to where they too go unburnt mostly...

I am moving "check wires" to the top of my next list. It'* free and doable even if it may not be easy in some circumstances.

I have made sure that the new wires cannot repeat this fate.

I wonder what all of that hot unburnt fuel does to a CAT?

Thanks for the "check wires" motivation. I am a bit embarrassed that it was actually that. The front ones looked in great shape! <grin>