You're making a number of mistakes here. The first was to replace the battery. While that IS important on '87 and newer GM vehicles, disconnecting it erased whatever diagnostic fault codes were in the Engine Computer at that time, so that valuable information was lost.
The second mistake was code 306. Worn spark plugs were a good guess but that's all it was. Fault codes never say to replace parts or that they're bad. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. You could have replaced all six spark plugs for maintenance if nothing else, but since only one cylinder was affected, the best approach would be to switch the two spark plugs between cylinder six and, lets say cylinder two, and switch injectors between cylinder six and cylinder four. Erase the fault code, drive the car, then see if a misfire code sets again. If the cause is a worn spark plug, the misfire will be for cylinder two now, and if the injector is the cause, the misfire will be for cylinder four. If cylinder six is still misfiring, suspect a mechanical issue with that cylinder.
Third, you should have noticed the misfire and a noise if a rocker arm was broken or loose. A mechanic would have heard that right away if it was broken. If it was just out-of-adjustment due to stripped threads on the stud or nut, there wouldn't have been any unusual noise but that also will not damage the piston. What might have happened is someone over-adjusted that rocker arm and the valve was hit by the piston, but that would bend the valve and you'd have a very noticeable misfire. A compression test would have identified the low compression, and a cylinder leakage test would have identified the cause.
Be aware too that GM has a lot of trouble with their injectors on high-mileage engines. Chrysler buys their injectors from Bosch in flow-matched sets and failures are extremely rare. GM just grabs a handful of injectors out of a big bin with no regard to matching their flow rates. With normal wear and varnish buildup from high mileage, a few injectors won't flow at the same rate as the others. That results in one or two cylinders running lean, and that is detected by the oxygen sensor(s). As a result, the Engine Computer commands more fuel to all the cylinders, so fuel mileage goes down, but the misfire can remain. That misfire can be subtle enough that you don't feel it, but it's a common reason for a single-cylinder misfire code to set.
To compound this problem, it is not normal to replace one injector. If one were to fail, they're all the same age and have the same mileage, so just as with spark plugs, you'd replace all six, and in this case you would buy a flow-matched set of six rebuilt injectors.
Be aware too that GM has a real big problem with their generators. Due to their design, they develop huge voltage spikes that can damage the internal diodes and voltage regulator, and they can interfere with computer sensor signals. GM generators are often responsible for elusive engine running problems that defy diagnosis. It is common to go through four to six replacement generators in the life of the vehicle. To reduce the number of repeat failures, replace the battery at the same time unless it is less than about two years old. The battery is the main component in damping and absorbing those voltage spikes, but as they age and the lead flakes off the plates, they lose their ability to do that. Now that you've replaced the battery, if the misfire problem remains, consider having the charging system tested for maximum output current and "ripple" voltage. You need a professional load tester for those tests. If one of the six diodes has already failed, you'll only be able to get exactly one third of the generator's rated maximum current. 30 amps from the common 90 amp generator isn't sufficient to run the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over days or weeks, and the voltage spike problem will be even worse than normal.
The next time you consider buying a car, check for diagnostic fault codes first, and if the Check Engine light is on, walk away. You know the seller knew the light was on, and for sure he had it checked and knew why it was on. No one is going to sell a car with a known problem if it's an easy or inexpensive fix. To do so lowers the value. It is real common for the seller to give up on the repair because of cost or he can't figure out the cause, but you know no one is going to admit the repairs will be expensive. Too often we hear, "it just needs this little thing" to make the light go off. Well, ... Then DO that little thing, THEN sell it to me! The people at most auto parts stores will read the codes for you for free. There's web pages on this site where you can look up the code numbers to get the descriptions, and we can suggest the best course of action for each one. Remember too that of the over 2,000 potential codes, only about half of them refer to things that could adversely affect emissions, and those are the codes that turn on the Check Engine light. Too many people incorrectly think there can't be any codes in the computer if the Check Engine light isn't on.
The last mistake is your comment about "what to tell them to check next time I go". You don't tell your doctor which tests to perform. You don't tell your mechanic what to check either. If you do, you can be expected to pay for the service regardless if it leads to a diagnosis. Your job is to provide the most accurate description of the symptoms as possible, any observations or clues that might be related, and a detailed list of the history of events and services performed so far. Let the mechanic decide which tests to spend your money on. Their reputation is based in part on how efficiently they diagnose a problem and on how they approach the repair. In most states you determine a dollar limit which they can't exceed without contacting you. Based on your description of the symptoms and history, the person writing up the repair order has an idea of what is a reasonable amount to not exceed. At that point, once you hand over the keys, all you have to do is wait for the phone call, then approve any additional dollar amount that's needed to complete the repairs. To keep your car for a week suggests they've already reached your dollar limit so they set the car off to the side, or they don't know what to do next. If they don't know what to do, you need to find a different shop with mechanics who specialize in engine performance and emissions issues.
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Saturday, April 4th, 2015 AT 5:26 PM
