My first post here so please forgive any technical or social missteps.
I have a 99 V70 that appears to be missing on Cyl 3 (according to codes accessed at a local Auto Zone).
Symptoms: While I was on my normal commute, almost two weeks ago, my Volvo suddenly began to shake violently as I was ramping up to speed on the highway. I limped into my first call of the day then immediately to a Advance Auto to buy and put in some Z-Max. Frankly, at the time, I thought I had blown a head gasket and I was just doing all I could to allay any further damage. Since then I've done some troubleshooting and the blown head gasket may or may not be the right diagnosis.
Troubleshooting: I haven't seen any billowing white smoke and my coolant has not gone down. Also, there is no oil in the coolant nor is there water in the oil. Because of the mis-fire code I replaced all the spark plugs (car bought used so no clue how old the previous ones were) and have checked all the coils (known-good coil in cyl 3 makes no difference, coil from position 3 works well in another cyl). From a post I read here I pulled the connecter from the MAF sensor and the car still ran. I cleaned the sensor but no change in performance and the "disconnect test" presents the same problem.
Things I've done: Changed the oil (synthetic blend, new filter too - just yesterday). Changed the air filter (because I hadn't in a while). Changed spark plugs. Put in the BG44K in the fuel system (just last night) and filled up with 93 octane gas. Had the fuel injectors cleaned.
Things I still want to / need to do: Change the MAF sensor. Do a compression test.
Confessional: This is a car that could have had more attention put to it. It burned oil and I did more adding oil (typically synthetic and/or synthetic blends) than changing it over the last year and a half. I heard valves ticking from time to time, added oil, and all was better. My concern is that I have a stuck (or worse) bent valve. I'm not hearing any ticking or tapping right now, though.
Wrap up: The miss is pretty pronounced, although I can tell it is just on Cyl 3. I'm trying to figure out if the MAF sensor alone could be causing this issue. I'm a pretty technical person (used to work on Sikorsky helicopters in the USMC) but have less hands-on car time. The bottom line is that I have a strong troublshooting mind but not always the background, or tools on hand, to carry out the next steps. Some advice from you "old salts" would be greatly appreciated.
One other quick point. It seems like there's a notable hiss, like a vacuum leak of some sort, from under the hood but I've not been able to localize it yet. I think it sounds most pronounced at the front of the engine, just above the manifold pipes.