Bad Miss From Bad Gas? 98XC70 220K

[StoningtonXC70]

Just got in from a somewhat scary last 30 miles of a ride. Was coming down I95 and all of a sudden, I felt a lumpy vibration develop, which I initially thought might be a tire going flat. I pulled into a rest stop that was fortunately only about a half mile ahead, and when I stopped, noted that I had a real lumpy idle, could see the engine shaking. Nothing wrong with the tires, which was a big relief. No obvious sight, sounds, or smells of anything eating itself, like a timing belt or a serpentine belt, no codes thrown at that point. Decided to try to make it home, engine down a little on power, and would get really lumpy at about 2K /60 mph in top gear, but would smooth out if I goosed it and downshifted. Eventually I got a FLASHING CEL,which I have never seen before, usually its either solid on or off, eventually it went solid on. Going through town, I had heavy lumpy vibration going into and and out of stops at stop signs. When I got home, I read the codes and got a P0302, cylinder No. 2 misfire. Could hear and feel the misfire at the exhaust pipe. Starting to think its a fouled No. 2 plug, possibly caused by a slug of bad gas, seems more than coincidence that this all started about 10 minutes after I put 14 gallons of 93 octane Hess in the tank, although I never have had a problem with gas from the plaice I got it before. I did a complete tune up about 7 months ago, new everything in the ignition department, OEM Volvo plugs, wires, cap and rotor. Up until now, the car has been running great ever since the tuneup, have put about 10K on it since then.

Will pull the plug and check it tomorrow. In the meantime, does my suspicion of a fouled plug from bad gas make sense? This ever happen to anyone else after a tank refill with bad gas?

[JRL]

Doubt it

[cattlecar]

+1 on JRL's comment.
It would be a really really odd occurrence for fuel to do anything cylinder specific.
Cracked center electrode on spark plug, bad plug wire, engine was washed and the water steamed up and allowed arcing down plug are things that come to mind as possibilities. Fuel injector failures are extremely rare but if the injector lead didn't latch or is missing the latch wire then injector lead coming off could do it too.

Flashing CEL is the ECM detecting misfire long enough and with indication from rear O2 sensor that catalytic converter could be harmed if condition persists.

[StoningtonXC70]

Sadly, it looks like it's probably cylinder No. 2 valve related. No. 2 plug was badly carbon fouled, changed it with no improvement. While I was at it, inspected and changed all the plugs with NGK Iridiums. All the others looked like what you would expect for a normal plug with about 10K. Checked for strength of spark on No. 2 vs the others with an extra plug on the lifted spark leads and no difference on 2 vs the others. Also substituted a replacement plug wire on No. 2. No improvement from any of this. Checked cylinder compression and have ZERO psi on No. 2, 170-195 psi on the rest. No. 4 is at 170 psi, the 1, 3 and 5 cylinders are at 190-195 psi. Adding a little oil to No.2 and rechecking had no effect.

Considering this problem suddenly came out of no where, whats the most likely cause, broken, stuck, or cracked valve, or something else?? Was just cruising at about 65 mph on the highway when it developed, no unusual noises at the time or since, just have a heavy miss at low RPMs/high load that smooths out somewhat at higher RPMs. Running at 3000 RPM in 3rd at about 60 mph almost feels normal. Will not keep running it though, to prevent damaging the cat converter.

I don't have the time, or experience to pull the head myself so if this going to run into big bucks, it is going ti be a parting of the ways between us and the 98XC. Probably will replace it with a lower mileage used mid size pickup, which would a better fit with our needs now.

[cattlecar]

Burnt valve can happen like that and that quick in some cases. Hardened valves for some reason fail like a cutting torch was taken to it. When the leak occurs the hot gases don't just erode a small section of the seat contact area like they did in the past.
Small chance of a tappet issue, really small. The bad part of tappet suspicion is there is no good "normal" diagnostics to rule it out. Those new flex probe video cameras with a rear looking camera probe can let you see the valves. Leakdown tester will let air out of exhaust for burnt valve or stuck open valve just the same.
Burnt valves have a track record but stuck open valves while running do not.

[StoningtonXC70]

Thanks for all the comments...I really feel bad about the way this ride of seven years and 115K miles is ending...considering the car's present otherwise overall good condition and all of the work I've put into it along the way, I wouldn't mind fixing this issue, and redo the timing belt and tensioner that now are at 70K at the same time (another Dean Wormer please give us one more chance situation). I figure that the engine internals will then be good for another 100K at least. But my wife told a neighbor today when were talking about what happened that she would set the car on fire if I even brought up the subject again! Will ask my son who has over 500K total between his 90 740 wagon and 98 V70 wagon if he wants it. Otherwise, if it can't stay in the family, I will likely put it up on the for sale forum here.