Cylinder 4 Misfire

[CapriceX2]

My 2011 Caprice started throwing intermittent P0300, P0304, and P219B codes several weeks ago. I thought I’d start a thread to document diagnosis and repair. I’ve found several other reports of misfires, but very few document what problem was found.

I’m currently assuming that the P219B code is due to the misfire, so I’m ignoring it.

My first step was to swap the coil with plug wire from cylinder 2 to cylinder 4. A short test drive reset the code and confirmed the miss was still in cylinder 4. The coil and plug wire are not at fault.

Next I swapped injectors between cylinder 2 and 4. Another short test drive again confirmed that cylinder 4 was still missing. The injector was not at fault.

While swapping injectors, I bled the rail pressure off using the rail test port and it seemed to release air with the fuel. This led me on a search of this site (found nothing) and then a broader internet search to see if this was normal or not and also how to fix it. I was only able to find one other report that seemed to be related to an aftermarket fuel pump. My tank and pump are original but that doesn’t mean there could not be some problem. I decided to set this information aside and do other diagnosis before dropping the tank. If anyone would like to check the fuel flow from your rail test port (after shutting the car off) and let me know if you get any signs of air, I’d appreciate it.

Researching misfire posts on this forum, I found a thread started by CrashTestDummy that was corrected by changing plugs and wires. I had been planning to examine the plug anyway, so I pulled it and swapped it with #2. There were no obvious visible signs of any problems and I didn’t even have to leave the driveway before I threw a code for #4 again. I think I’m going to change the plugs anyway since I can’t remember for sure if I changed them when I did the DOD delete. Pretty sure I did.

Next up I decided to check compression in all of the holes. Compression measured:
1 – 197 psi
2 – 214 psi
3 – 194 psi
4 – 197 psi
5 – 207 psi
6 – 225 psi
7 – 207 psi
8 – 245 psi (yikes!)

Compression looks ok HOWEVER… While testing #4, I found that it was not “hitting” consistently. Some revolutions would not add any pressure to the gauge and had no change to the sound of the starter. AH HA! I may be on to something.

I’m currently convinced I have some sort of valve problem. Most likely a valve hanging open. My next step will be to pull the valve cover for an inspection.

This car does have a DOD delete and a TSP 220 cam. I have previously broken a valve spring and partially dropped a valve. The head of the valve got bent and the piston was dinged. I replaced the valve and all valve springs (dual springs from BTR). The new valve did not show any signs of binding in the guide when I installed it. I’m pretty sure it was cylinder 5 that dropped the valve. If I happened to swap the heads from side to side when I did that work (quite possible), then that would line up with #4 and my current problem.

I’m going to get some lunch and head out to pull the valve cover. I’ll update when I know more.

[CapriceX2]

I got the valve cover off and could not see anything odd. Valve springs and retainers all look fine. Rockers haven't lost their bearings.
Rolled it over by hand several times and the valves are moving consistently and seem to match the others. When lifter is on the base circle, I can wiggle the rockers a bit, but there is no free play.
Contemplating my next step. Considering pulling plugs again, installing the compression tester (so I can hear if the problem is present) and having a helper crank it while watching the valves. Or just starting it with the valve cover off.

[CapriceX2]

Ah, the frustrations of intermittent problems...
I pulled all the plugs and put the compression tester back on #4. Cranked it several times, but it was hitting evenly so there was nothing to see. I put it all back together and called it a day. Fired it up when I was done and sure enough -- cylinder 4 is dead again.
I'll try again in a day or two.

[KillaWhale]

If the tickets are moving normally try taking the pushrods off for affected cylinders crank it over 15 secs and let it sit overnight before re assembly. Maybe you have a lifter preload issue

[Mooseman]

Did you confirm that coil #4 actually fires? What about injector #4? Could just be a broken wire to either. Try to get an endoscope in the spark plug hole. If the piston is wet and clean, I'd suspect no spark. If it's dry and dirty, no fuel. It will also give you a chance to look at the valves.

[CapriceX2]

I pulled the valve cover again tonight and ran it for a bit. I couldn't pick up on any odd movement from the valves. It was throwing a misfire code when i first started it up, but it smoothed out and was running fine most of the time.
I was shocked at how long it took to get oil out the pushrods to the rockers. I didn't time it - or even notice at first that the oil was missing - but I'm guessing something like 40 seconds before oil started flowing. I'm not sure what to make of that. I had turned it by hand for several revolutions, so could have pumped the oil out of them, but it shouldn't have taken that long to fill them back up.
Got a couple pictures of the piston after idling for a couple minutes.

piston2.jpeg (114.51 KiB) Viewed 1713 times

piston1.jpeg (216.25 KiB) Viewed 1713 times

piston3.jpeg (143.19 KiB) Viewed 1713 times

These photos were taken after it had been running without the misfire and code. I should have taken a look at another piston for comparison, but did not think of it at the time. That'll be one of my next steps. I may grab another scope as well to see if I can get some better pics.
I'm going to get the timing light out so that when it starts missing I can clamp the pickup on the wire and see if it triggers the light consistently.
Since it had been running without a miss, I put it back together and was going to give it a test drive. It ran fine backing out of the shop, but as soon as I started pulling forward the code set and it started missing.

[BluesBro]

Have you verified the integrity of the injector and coil harness?

The injector harness are notoriously bad.

The coil harness is not usually an issue, but I had a real bad intermittent miss on my avalanche. The connector that collects all four coil wires on the valve cover had gotten hot or something and a couple of the pins would walk around based on temp and vibration and drop coils.

[CapriceX2]

No, I have not yet verified the wires to the coils or injectors. Once I found the odd compression test results with #4 only adding compression to the gauge on SOME of the strokes and doing nothing on others, I started trying to verify a sticking valve. The wiring is not off the table, but the evidence so far is pointing me toward valves. I am planning to get a vacuum gauge installed (another check for sticking valves), an inductive timing light on the #4 wire (checking for power out of the coil), and listen to the injectors with a screwdriver as my next few steps.