Hi Scotty - one for the record books here.....
Short Summary:
-1998 Jeep Wrangler, 4.0L, manual gearbox, 210k miles (95% highway).
-Owned since new, meticulously maintained, Mobil 1 always in crankcase.
-Ran fantastic, but had persistent P0305 (cyl #5) misfire code.
-Perfomed combustion chamber cleaning (multiple times), injectors bench cleaned/tested, plugs/wires/coil checked/changed, fuel pressure checked, compression and leak down numbers were per Factory specs.
-Usually do my own work, 35-year veteran at my dealership still recommended head removal (even with stellar compression/leakdown numbers) - said he has seen instances where numbers didn't tell whole story (sticky value, etc).
-Had dealer remove head. Machine shop indicated head was in fantastic condition and spotless for the mileage. They indicated they simply lapped the valves, lightly cleaned bottom surface, and changed valve guides (with OEM guides that I provided).
-Dealer re-installed head. Now, during light throttle input around 2-3k rpm, I get light pinging (sounds like pre-ign or detonation). Idles smooth as it always did and runs right up to redline with no issues.
-Almost always, no codes are reported. In very cold temps (i.e.even leaner conditions) sometimes I now get P0300 code accompanied by some or all of the separate cylinder misfire codes (P0301,302,303,304,305).
-Fuel trims and O2 sensor readings have been recorded during a drive and plotted by me personally. All are all normal and do not vary much even when light pinging episode occurs.
-I changed the fuel pump as a precaution as the original was sitting in very old fuel at one point.
-I ran a very high octane boost to see if pinging would be eliminated or diminish (i.e. did compression increase due to machine shop error or wrong(thinner) head gasket) - no change.
-I checked/re-installed distributor per Factory manual to insure proper ignition timing.
-Compression re-checked good.
-Smoke tested intake and exhaust system - no leaks.
Before I personally remove the head and start over do you have any suggestions as to what may have happened? Burrs on valves causing hot spots?
Thank you for your time,
Anthony
Wow, that's a knotty problem. The only likely suspect I can identify is that a fragment of head gasket is sticking into your combustion chamber. If I were you, I'd get this tool Scotty recommends and take a good close look at the inside of #5 and check for that or valve burrs before I went to the trouble/expense of pulling the head.
https://www.amazon.com/dp/B07PD22FLN?ref=exp_scottykilmer_dp_vv_d
Thank you for your response. The cylinder #5 misfire no longer appears by itself. Since the head was removed and put back on, I now get P0300 and other cylinder misfire codes as well (never got the other cylinder misfire codes before the head was pulled, only #5). I have a borescope - maybe I should look into every cylinder. Thank you