92 Acclaim, bucking/stumbles

...not the Maserati version, the REAL PLYMOUTH ACCLAIM/3.0L(ref. a scannerdanner video). My 90 year old ma's car with a little over 60,000 miles has developed a very bad bucking/stumble condition. Not sure what all is original on this vehicle, but before going to the usual parts cannon(plugs, wires, cap rotor, etc.)not to mention rigging up a fuel pressure gauge(no Schrader)...decided to make my first attempt into the scope world and tested injectors using the Hantek 1008c(yeah, I'm new). The following capture represents #4 injector on the left, while 1,2,3,5,6 pretty much being the same waveform on the right.

Hi Arby,

Have you noticed any change in rpm once you unplug #4 injector?

Is that change in rpm different from the other injectors?

Thanks for the reply. Unfortunately the injectors(particularly the back bank) are placed pretty deep under the upper intake, with #4 on the back bank even more shrouded...but I'll give it a try.

Edit: Correction 1,3,5 back and 2,4,6 front. I'll go ahead and at least test the front bank.

OK(at least testing the front bank)unplugging injectors #2 and #6, no (noticeable) difference/change in RPM, actually no change at all(roughness no better/worse). Unplugging # 4 however...engine dies. Tested twice, same result. Timing issue?

You can also try with plug wires (if they are more accesible).

At this stage, you'd want to confirm what you have noticed in scope and match it with symptom (i mean, is in fact #4 cylinder and no other else)

It's just my method. I try to confirm the suspicious part is the issue and also the suspicious part is not the complete issue.

Cheers

Do you mean with all injectors plugged and only #4 unppluged, the engine dies?

That is correct.

Spark/wire test... I used a test light at the distributor and pulling plug wires at "towers" #4 and #5(individually)caused engine to die before I could even see/hear arc. The others I could see/hear arc with "maybe" some rpm drop??..hard to tell with engine running so roughly...but the engine kept running checking those.

.

Bear in mind that the ones which don't change anything, once you unplug them, are the ones with problems.

After a long hiatus on this project, I've decided to jump back into it. Some updates since my original post/replies. I had the vehicle running fairly well(though I haven't actually taken it out on the road), turns out a couple spark plug wires got crossed, and I put back some OEM parts that I had replaced with after-market stuff, including the idle air controller. One main addendum to my original post is the waveform posted is from "#3" injector. I have since put in a new fuel tank/pump, fuel lines, brakes(front and back), brake lines, timing belt, water pump, other drive belts. It idles fairly well, but with occasional misfires(?), more pronounce when warm.

However, the posted waveform(for #3 injector) is still valid... I've swapped out that injector 3 times, but the pattern remains the same on injector #3. I ohm tested all injectors, all similar, I ohm tested the control wires from all fuel injectors(connector)to the PCM (connector), all similar. Connector/pins(pcm harness/injector harness) appear OK.

A new issue I recently noticed after changing out the old battery. I am reading 15.2(+/-)at the battery from the alternator. From what I understand, the PCM(guess it's technically called a SBECII for this model/year?)has a alternator field control driver(voltage regulator?)in the pcm, also the injector control driver + ignition coil control. So my question is, is it more likely that a bad pcm driver/etc. is causing this weird injector waveform...or is it still possible it's something else?....same perhaps with the alternator over-voltage? At this point I am considering trying a(compatible)salvage yard pcm(no core/keep old) to see if it corrects these issues(while hoping it doesn't add others)...but I'm open to any other suggestions. Thanks.