I've been chasing a sneaky misfire issue and wanted to get some opinions before I start tearing deeper into the engine.
~150k miles
Rough idle
P0300
Cylinder 3 misfire on scan data at idle, sometimes clears up around 1600 RPM

The current parts cannon list:
New fuel filter
MAF cleaned
New Ac Delco plugs and wires
New Delphi fuel pump
New Delphi distributor (retard at 2)
New Standard MFI spider
Cleaned throttle body
New EVAP components

Compression tested relevant cylinders before injectors Cylinder 3: 160 psi
Cylinder 1: 155 psi
Cylinder 4: 150 psi
Compression appears healthy on the missing cylinders

Still sets P0300
Misfire data is almost exclusively on cylinder 3 at idle
Around 1600 RPM the misfire almost completely disappears
Above ~2000 RPM the misfire returns
During a road test the engine began slightly missing on more than just cylinder 3, but i couldn't pin it as I was driving. I can hear a hissing sound on the passenger side of the engine bay but have not located it yet, sprayed some brake clean around the vacuum ports and intank plenum but nothing is increasing idle.
While driving, the brake pedal becomes super stiff as, if vacuum assist was lost.Once parked, the brake pedal feels normal again
STFT for both banks are around 0-3, LTFT are around -9 which throws me away from vacuum issues or single cylinder problems.
O2 sensors are sweeping in the graph, exhaust leak post cat due to terrible install, 02 spacer on that sensor

Any ideas are appreciated and im not afraid of diagnosis but this one has me stumped. Maybe crank sensor? It is covered in oil but giving me no reasons to replace. I'm trying to avoid throwing more parts at it and would like to get pointed toward the next logical test.

 

1. Do you feel the misfire?
2. Try to get cam retard closer to zero?
3. what is your ten min fuel pressure leak down?
4. did you replace your crank sensor without a relearn?
5. mist your engine with water in the dark at idle while it’s misfiring and look for arcing?
6. The O2 sensor that was jacked with is post cat?
7. Did you replace all 6 plugs?

George

 

George posted while I was looking the two attachments up. Attached is factory info on the P0300. Stuff to check that you may not think about. George will get you taken care of! Consider this additional info.

Good luck and best wishes for a successful repair.

 

1. Can hear it and feel it but I can't nail down a cylinder physically, not even with a stethoscope, both manifolds sound the same.
2. I don't know how to change it with the scanner I was using but the shop has a pair and I wanna try the other one (both snapon)
3. Haven't done any leak down testing since I replaced the original poppet spider with a new Standard MFI conversion
4. I have not replaced the crank sensor, but I don't know what the previous owner was up to.
5. I'll have to try the misting and see what happens, when I was toying around the distributor it definitely stung me though.
6. Yeah, the post cat sensor has a spacer on it, I suspect it's a bandaid fix for a bad cat but I haven't cut it out to check yet
7. All 6 plugs, wires, and the distributor. along with the fuel pump, filter, and injectors.

I want to try the other scan tool tomorrow and see if live data says anything different, just seems odd to me it would throw a p0300 instead of a 303 while only counting cylinder 3. I'm gonna check grounds soon and I'll record the live data at idle, 1500, and 2500.

Some other things worth noting
The truck sat in a field for 6 years, when I got it there were some odd quirks, but no apparent rodent damage.
Swapping plugs doesn't move the miss, nor does moving wires
when I pulled the plugs pre-distributor replacement, cylinder 3 was wet and smelled of fuel so I know its getting fuel, and trims seem to say that it's the right amount short term.
Post-distributor the misfire intensity halved but is still present.

 

Appreciate the assistance regardless already seeing stuff that I hadn't thought about, I was actually looking through posts last night and noticed how involved you are in these, really respect the work you do on here!

 

1. Cam retard can be measured relatively cheaply, take a look at the article on inexpensive code readers by Les Meyer
2. You always have to verify repairs afterwards. In the modern era of cheap aftermarket parts sometimes you actually create new problems. I highly encourage you to repeat the fuel pressure leak down test
3. Fuel trims are only valuable if you combine short term and long-term trims on a given Bank being mindful of sign. What are all four values at idle and 2500 RPM
4. another valuable thing, especially when you’re diagnosing these difficult problems is to look at the pre-cat O2 sensor graphing in real time

You look like a guy who is willing to do the work and take a diagnostic approach so I’ll stick with you as long as you’re willing to do the work and we’ll get you fixed up

George

 

1. I can view the retard easily, it's set at 2 right now which is within spec but I'll try getting it to 0, I did try changing it but couldn't figure it out on the scanner I was using, will let you know if I figure out a way with what I already have access to.
2. I'll get the pressure tested tomorrow night and let you know what's going on
3. I'll grab a video of the trims at idle, at 2500, and I'll make sure to include the 02 graphs to see if they're sweeping proper
I'll also make sure to capture MAF, MAP, ECT, IAT, etc.. in the video.

I'm an auto tech by trade for 3 years now so I know a bit about a diagnostic flow, but have tons to learn, I've done everything I can think of short of pulling the driver valve cover to check it out. My way hasn't worked so I'm more than willing to listen and provide as best I can.