First let me start with the parts that won't cause misfires. The throttle position sensor tells the Engine Computer position, direction of throttle change, rate of throttle change, idle, and wide-open throttle. All fuel metering calculations based on that sensor affects all cylinders equally, and that sensor has about the least impact on those calculations.
The idle air control valve, or automatic idle speed motor, controls the amount of incoming air that is able to bypass the throttle blade. When the computer commands that "motor" to turn a fraction of a turn to let more air in, it commands the injectors to remain open longer during each pulse to let more fuel go in to go along with that air. That's how it controls idle speed, but that's all it controls. Raising or lowering idle speed won't cause a misfire.
The EGR valve can cause some cylinders to misfire but not in the same way a fouled spark plug, for example, would. This actually is a big problem on Ford trucks, but GM doesn't have a lot of trouble with this. If you suspect this, slide a metal shim under the valve temporarily as a test. That will block the flow of exhaust gas into all the cylinders so those cylinders will only get fresh air.
GM has had a lot of trouble with fuel pressure regulators, but mostly on truck engines. You'll have much more than little misfires. There will be a lot of raw fuel dumping into the intake manifold through the regulator's vacuum hose. If the engine runs, there will be a lot of black smoke from the tail pipe, and it will run so poorly you'll wish you had only simple misfires.
Only use the spark plugs specified on the emissions label under the hood. Fancy or more expensive plugs, in the misguided thinking those are of higher quality, often cause misfires and other running problems. I don't claim to know all the theory behind the story, but a number of high-level national trainers who have shops that specialize in the one-out-of-a-hundred cars that no one else has been able to solve, tell us split-fire and similar plug designs are gimmicks and cause way more problems than they claim to prevent. If your engine doesn't need them, you won't solve anything by using them.
I do read a lot about replacing fuel filters on GM vehicles, but the problems they solve usually don't include misfires. Most of the time people replace them as a guess, and the problem they're trying to solve is still there.
Chrysler is the only manufacturer that has been able to make an engine run right with just a MAP sensor. It has the biggest say in how much fuel goes into the engine, but even there it won't cause misfires. On GMs it is used to measure barometric pressure, and as a backup strategy if the mass air sensor fails. Then it works like on the Chrysler system.
Weak spark from the coil pack is a logical guess but that should also cause "lean exhaust"-type of fault codes to be set. Unburned fuel and air will go into the exhaust where the unburned oxygen will be detected as a lean condition. A weak coil will cause misfires for the same two cylinders, and the fault codes would indicate exactly which ones those are. Code 304, for example, would mean a misfire on cylinder four.
That leaves the injectors. I don't know of any professional who actually measures their resistance. A failing injector might have a resistance of 0.5 ohms lower than normal. Your meter leads can have 2.0 to 5.0 ohms of resistance, so how are you going to identify a bad injector with resistance readings?
There is something else though to consider. This is from Jim Linder who used to have an injector rebuilding service near the Indianapolis Speedway, and who used to put on very high-level classes around the country:
"Chrysler buys their injectors from Bosch in flow-matched sets. Failure of an injector is extremely uncommon and is the last thing to suspect. GM grabs a handful of injectors out of a big bin with no regard to flow-matching them. It's only a matter of time before varnish, wear, and debris cause one of them to flow considerably less fuel than the others".
His web site used to list the ten most common injector-related problems, and eight of them were for GM products. The mismatched flow rates don't cause a problem at lower mileages, but eventually you'll have some cylinders running a little lean. The Engine Computer will detect the resulting intermittent misfires and count how many occur in a given amount of time to determine when to set the fault codes, but you are likely to not feel them.
The solution for this problem is to install a full set of rebuilt injectors that have been flow-matched.
There are other potential causes for random misfires but replacing random parts is the most expensive and least effective way to find them. A mechanic much more experienced than I am in running problems can look at the waveform of the current through an injector on a scope to see exactly when the pintle pops off its seat and how long it remains open. By looking at the oxygen sensor waveforms he can determine if a misfire is due to fuel, spark, oxygen, or compression.
He can also introduce an artificial lean condition by removing a vacuum hose, and an artificial rich condition by removing and plugging the vacuum hose to the fuel pressure regulator, or by spraying in propane, to see how the Engine Computer responds.
Sunday, March 15th, 2015 AT 9:33 PM