Misfire mis-diagnosis
Hello all! First off, I wanted to express how I am impressed I am with the level of knowledge and professionalism that exist in this network. I haven't posted anything yet, but I read the posts in here everyday, and am constantly impressed with the content.
Anyways, I posted about this Pontiac on FB in August. Thought I had it all figured out, but cars always know how to humble you. This particular vehicle knows how to kick my ass up and down the street... Originally brought to me as multiple cylinder misfire under load. I found a secondary ignition issue from worn out plugs/wires that was corrected with new parts. This solved 1 miss, but another remained. Cylinder 1 had a misfire when under a load and over roughly 4000 RPM. Under that RPM, it has no issues even under a strong load. The misfire counter for cyl 1 counts rapidly once the miss occurs, MIL flashes, and a lack of power is felt (no bucking or jerking though). After scoping injector amperage and spark I found it was not loosing injector control but secondary ignition changed dramatically during the miss. Suspecting a compression issue, I check cranking and running compression (idle), no problems found. I then hooked up a vacuum gauge, routed it through the window and re-created the miss. The gauge confirmed my suspicion of a mechanical issue. You could feel a major pulsation when the fault occurred. The gauge vibrates so much you can hear it. 2 times while test driving, the misfire continued after the engine returned to idle for a short period, during this short period the gauge was bouncing rapidly. Suspecting a sticking valve in a guide, I hooked up the first look to the intake manifold. By the pressure fluctuations during the fault I felt it was cylinder 1 intake valve sticking open. I recommended disassembly and inspection. He decided to drive it and keep it under 4k.
Fast forward a couple months, he brought it back for repairs. I removed the heads and valves. The valves and guides looked pretty worn out, not surprising at 200k, but nothing concrete that stood out as THE issue. I inspected the cam lobe and measured lift at the rocker, lifters, rockers (bearings are free), Cylinder walls, and push-rods. Nothing that seemed out of the ordinary. So we decided on re-manufactured heads and a set of lifters. I felt confident the problem was in the valve-train, and the customer did not want to reassemble with old parts. And as you can guess, nothing changed. Same miss, Same cylinder.... So now at this point I'm second guessing all my tests and wondering where I went wrong. Frustrated at the end of the day, I swap injectors, plugs, coil, perform a crank relearn. Nothing changes. I teed in my vacuum gauge and received the same pulsation during the fault. I decided to call it a day and re think this at home after clearing my head... So here I am! I think I need to attack this problem from the beginning, with no assumptions. Cover my basics and re-do all my tests. My feeling is that I am missing something obvious. I'm definitely willing start start in any directions or accept that my test interpretations were invalid. Any Ideas from the great minds in here would be greatly appreciated. Thank you!
Apologies The last 2 pics are not annotated,
Pic 4 A- Crank B-Crank C- Cylinder 1 secondary ign D- Cylinder 4 secondary ign You can see about half way through the screen where the problem occurs and the firing line for 1 drops off.
Pic 3 A- first look sensor in the intake B- Cylinder 1 secondary ign.
I can send anyone the Pico files if you'd like.