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.

 

Alright I have gathered some information as requested, no arcing when misted in the dark at the distributor or plug wires, retard at idle is 2, and at 2500 is 1.
Fuel pressure

1. EO: 51PSI
2. KOEO: 55PSI
3. 10MIN: 52PSI
4. 20MIN: 46PSI
5. 2HR 30MIN: 32PSI
6. 9HR: 29 PSI

Fuel Trim% at IDLE

• STB1: -4~0
• STB2: 4~0
• LTB1: 8~9
• LTB2: -6

Fuel Trim% at 2500

• STB1: -10~0
• STB2: -7~0
• LTB1: -2~4
• LTB2: -1~-4

Other Info at IDLE

• ECT: 186F
• IAT: 80F
• MAF: 6.60 g/s
• MAP: 11.2~10.9Hg
• MAP: 1.43~1.48V

Other info at 2500

• ECT: 186F
• IAT: 84F
• MAF: 19.5 g/s
• MAP: 7.3~7.6Hg
• MAP: .79~.84V

02 graphs video linked below

 

We will get back to your specific details in a minute but since you are serious about learning and have a good analytical approach I will take a minute for education to help you and future members who get here.

Misfires can be a challenge to chase down and correct, especially on engines with high mileage. It can be somewhat of a catch all diagnosis. It’s starts with what a P03xx code actually is. Yes it’s titled a general or cylinder specific misfire and it’s easy to assume that this means that there is an ignition or fuel delivery problem on say cylinder 3 but not necessarily. There is no direct measure of ignition or fuel. The PCM is monitoring the crank and cam sensors and expects to see a slight increase in rpm when every cylinder fires but if it sees behavior different from that and all other conditions are met then it declares a misfire on the assumption that the cylinder that should be firing at that point has not contributed properly to rpm. So what can produce this condition?

1. Well first as with all codes the possibility exists that the detection and reporting is the problem. Example: Is the crank sensor and the reluctor wheel functioning properly? Has the PCM learned that crank sensor?The power and wiring for same? Is the cam sensor working and aligned properly and operating smoothly without excessive timing chain or gear wear? Is the PCM functioning properly?
2. Second is there engine mechanical issues that would cause erratic rpm behavior that has nothing to do with fuel or air. Examples: valves, lifters, bearings, compression, etc. Les is a big proponent of a dynamic compression test which can identify more engine wear issues. Obviously as engines get this old there can be significant wear
3. Fuel/air delivery can certainly be a problem. Insufficient fuel pressure, injector issues, exhaust problems, O2 sensors, etc
4. Ignition is the most obvious possibility. Plugs, wires, distr, ICM, etc.

One of the reasons that I start with questions like do you feel it and what is the history of the crank and cam sensors is because we should first make sure that say in your case that there is an actual problem with say cylinder 3 that actually needs to be repaired.

Specifically for your situation.

1. Get cam retardant closer to zero to make sure it’s cylinder three.
2. You might want to pull the crank sensor and make sure there is no impact damage or reluctor wheel damage from a past unshimmed sensor.
3. You can do a drop cylinder test by pulling the ignition wire and routing the spark to ground on cylinder 3 compared to a “good” cylinder (opportunity to get bit again, lol. Ask me how I know) to see if cylinder three is really weak.
4. If you looked at the plugs then any clues there. The engine misting can be revealing
5. Can you clarify the fuel trim numbers, are those ranges?
6. The pre cat O2 sensors look reasonable
7. On those sensors, get readings on a dead cold engine and compare them to atmospheric conditions in your area
8. Your MAF reading at idle is typical

George

 

Another thing that may be helpful. Since you’re an automotive tech, you may have access to gear and techniques that most people here do not. Analyzing ignition waveforms can tell you a lot about ignition and fuel injector behavior at each of the six cylinders. creating a tutorial on that here is beyond the scope of most of the people that would read this so I would direct you to scanner Danner. He’s probably the best on the Internet for ignition wave form analysis and drop cylinder tests. The best mechanic in general on the Internet is South Main auto another great resource for a guy like you.


George