GM has had a big history of running problems caused by injectors. They aren't necessarily defective. Rather, they do not test them for even flow rates. The minor variations in engine speed are much easier to notice at lower speeds. Usually, if this is bad enough to feel, the Engine Computer will notice it too and set a diagnostic fault code for a specific cylinder misfire. The reason you feel a misfire is because the rotational speed of the crankshaft slows down very slightly when it occurs. The computer knows which cylinder should have fired when that misfire occurred. Pulses from the crankshaft position sensor are evenly spaced. When the misfire occurs, the time increases a few microseconds between two of those pulses. That's enough for you to notice. There are aftermarket companies that specialize in rebuilding injectors. Their biggest success comes from measuring their flow rates and selling them in matched sets.
When one injector delivers less fuel than intended, the corresponding unburned oxygen is detected in the exhaust system by the oxygen sensor. There are two "upstream" sensors on V-6 engines, one on each bank, or side. The computer can't tell which of the three cylinders needs more fuel so it just increases the amount of time all three of those injectors remain turned on. The weak cylinder gets a little more fuel, but the other two cylinders get more fuel than they need. Extra fuel is wasted but does not cause a noticeable misfire. Ths is how one injector can flow slightly less fuel than intended and cause the vehicle's fuel mileage to be lower than another identical car.
The computer can also set a fault code for a random cylinder misfire. This would be typical of an air flow problem or fuel supply problem that affects all cylinders. A dirty sensing element in the Mass Air Flow sensor limits its ability to accurately measure the amount of air entering the engine. A similar problem occurs if there is any leak in the air intake tube. The computer commands the injectors to add a specific amount of fuel to mix with the amount of air it thinks is entering the engine. Little power needs to be generated at idle so the computer is already limiting fuel delivery to the lowest possible level. Any further reduction in fuel due to incorrect MAF sensor readings can result in random misfires.
Some engine designs are prone to misfires when cold due to fuel "puddling". This is where the vaporized fuel hits the back of the cold intake valve and condenses back to a liquid. Liquid gasoline does not burn; it must be in vapor form. Chrysler had a problem in the 1980s with this on some truck engines. The fix was to increase fuel pressure to the injectors to improve atomization, and to modify the angle of the injectors' spray to miss the intake valve. Those design changes are still in use today.
A similar problem can occur if an engine is heavily carboned due to condensed, unburned fuel vapors. Fuel sprayed from an injector can momentarily be absorbed into the carbon around the intake valve and passage. Fuels are much cleaner today and most brands have additives to prevent carbon buildup, so this is less of a problem than in the past. Running fuel system cleaner in the gas tank doesn't always help because it is designed to remove varnish that builds up around the tip of the injector's nozzle. Mechanics use different chemicals that are still fed, in part, through the injectors while the fuel pump is disabled, and in part through the air intake system until the engine stalls. The main part of this chemical is a soap that dissolves the carbon and loosens it. The process is called "decarbonization" and is a service that some shops offer as preventive maintenance. It's usually not necessary although a lot of people are certain their engines run smoother after this is done.
Anything that affects spark can cause a misfire too but this is usually accompanied by loss of power and fuel mileage. If spark is totally missing for one cylinder, you will smell the unburned fuel by the tail pipe, but oxygen sensors only detect the corresponding unburned oxygen. As a result, the computer increases the length of time that group of injectors are pulsed open in an attempt to provide enough fuel to mix with the unburned oxygen. No matter how much extra fuel it commands, there will always be that unburned oxygen from the misfiring cylinder. Unlike the low injector flow rate story, the extra fuel will not overcome a misfire due to a spark problem, but fuel mileage will still suffer.
Mechanical problems in the engine can cause misfires too. Burned and leaking valves used to be common when we first switched away from leaded gasoline. Better metals and other fuel additives have pretty much solved that problem. GM had some trouble with camshaft lobes wearing down in their early V-6 engines due to lack of lubricating oil running onto them. That problem occurred in the late 1970s and hasn't been a problem since then except for engines that don't see proper oil change intervals. Even lack of maintenance isn't such a problem. Engine design was the issue that prevented oil from reaching the affected parts. Worn camshaft lobes cause valves to not open to let fresh fuel and air in or to let exhaust gases out.
One more thing to look for, if you look under the hood and the engine seems to be running very smoothly, inspect the engine mounts and exhaust system for parts rubbing metal against metal. Exhaust pipe hangers always have some type of rubber isolator to prevent vibrations from transferring into the passenger compartment. If two parts of a bracket are touching you will feel a much higher frequency vibration than the "thump thump" of a misfire. Engine mounts have rubber isolators too. At the mileage you listed, it's much too soon to expect to find a deteriorated or collapsed mount, although one of them can be repositioned to center the engine. If it is holding the engine off-center, the two metal parts of one of the other mounts could be touching.
Other oddball things to consider are a pulsing steering wheel when turning due to worn seals on the rack and pinion's spool valve, worn bearings in the radiator fan motor, and even a chunk missing from the flat serpentine belt that runs the power steering and generator.
Every one of these scenarios can have their exceptions, so don't use this information to argue with your mechanic. Instead, use it as a guide to help you understand the types of things they will have to look at to find the problem. Anyone can find the easy stuff. Some of the things listed here, especially the low flowing injector, are hard-to-find things other people have found in the past.
Sunday, March 14th, 2010 AT 6:47 AM