2007 Wave - All Cylinder Misfire

[MichaelP]

Hi everyone,
Our Wave has been misfiring when under load (when accelerating on the highway and when going up hills) and gave me a CEL yesterday. I pulled the following codes:
P0300 - Random/Multiple Cylinder Misfire Detected
P0301 - Cylinder 1 Misfire Detected
P0302 - Cylinder 2 Misfire Detected
P0303 - Cylinder 3 Misfire Detected
P0304 - Cylinder 4 Misfire Detected
P0314 - Single Cylinder Misfire (Cylinder not Specified)

Some history:
My wife has owned the car since 2009 and it had been very reliable with regular maintenance (oil changes) until we took it to the dealer for a wheel bearing replacement in June 2014. They mentioned that the valve cover gasket was leaking and replaced it at that time.

In January 2015, the engine started misfiring and I took it to an independent mechanic to pull the codes. They diagnosed the misfire as a leaky valve cover gasket, and I was in a hurry, so they replaced the valve cover gasket again and the spark plugs. At the time, it was suggested to budget for a timing belt replacement as the car was approaching 100,000 kms/60,000 miles.

We were planning to sell the car this year, so in April 2015, I took the car back to the independent mechanic to pre-emptively get the timing belt (including idler, tensioner pulley) and water pump replaced, with a kit that I had bought off Amazon. The engine didn't sound right when I drove it home, but it seemed to drive alright. The next day, the car was misfiring and it sounded worse, so we took it back to the mechanic, who realized that one of the cam gears was off by one tooth. I was greatly concerned that some of the valves had been damaged, but since it drove much better, they assured me it was fine. It still didn't sound right.

In late May 2015, I noticed the misfire returning and started to hear a rattle that seemed to be coming from somewhere inside the timing belt cover. I traced the misfire to some oil on a couple of the spark plugs, which meant leaky valve cover gasket. I took it back to the independent mechanic. They warrantied and replaced the valve cover gasket, this time adding some sealant to the spark plug tube seals. As for the rattling sound, they found two pieces of the brand-new tensioner had sheared off and were rattling around. They replaced the timing belt, tensioner, and idler pullies, and I returned the broken kit to Amazon for a warranty refund. The engine finally sounded right again.

On Saturday morning (June 28th), it started misfiring again on the highway, but no CELs. So suspecting the valve cover gasket, I checked the spark plugs and found oil on all four. I cleaned the outside of the valve cover, all four spark plugs, and the tips of the spark plug wires with a paper towel. I took it for a little test drive, and everything seemed good again. I didn't drive it for the rest of the weekend, but it seemed fine on the way to work on Monday (mostly downhill). On the way back home it began to misfire again, this time with the CELs I mentioned at the top of this post. I checked all the spark plugs and wires again, but there didn't seem to be any oil. I cleared the CELs, took it for a short round-the-block test drive and no misfiring.

Drove it back to work on Tuesday, and everything was fine on the way there (again downhill), but there was more misfiring on the way back home. I just checked and there was oil on the fourth spark plug.

Anyways, thanks for reading this long post. It seems like the valve cover gasket is going to be a persistent issue.