As usual, the mechanic ripped you off. Do you also blame the doctor when he doesn't hit the correct diagnosis the first time? Doctors just bury their mistakes; ours keep coming back. Isn't it funny that people will go from doctor to doctor looking for a diagnosis. Each one does the same tests over and over, and no one gets angry. But heaven forbid the mechanic doesn't solve the problem on the first try, the whole industry is labeled "crooked". Doctors only have to learn two models and three sizes. Only the cures change over time. Mechanics are expected to be experts on every brand, every model, every system, and every circuit with thousands of variations that often change more than once per year. By the time he learns how to diagnose the new engine computer system on your van, fifty more new systems have been introduced. Chrysler is one of the best at sticking with what works well over the years. GM is by far the worst at coming up with new circuits all the time. The mechanic must constantly relearn new systems, new troubleshooting procedures, buy new specialty tools, and keep showing up to work when he knows most customers are already angry before they come through the door. And then to be called a rip-off artist? There are bad doctors, teachers, chefs, salespeople, and carpenters, but only the mechanics and tv repairmen are all considered crooks due to the actions of the few bad ones. I've known two really crooked tv repairmen and a half dozen bad mechanics. All were driven out of business by the other people in their industries. Nevertheless, the perception persists; we're all bad. At the very nice family-owned Dodge dealership I worked at, we always had grateful customers bringing us treats because they knew how hard we worked to find some of their problems.
Intermittent problems are the biggest misery in both of my careers, tv repair and car repair. People can't seem to understand that with an intermittent problem, when the van runs good, is it fixed or is it just not acting up right now? You obviously don't know because you drove it away from the shop multiple times when it was running ok. The poor mechanic doesn't know either. He has to hope he solved the problem and you don't come back screaming. Makes for a lot of sleepless nights.
I had a dome light issue once that acted up so seldomly, the customer came back nine times before I solved it. Luckily, the customer never got angry, and my boss stopped charging him for the return visits in return for his patience. In most cases with intermittent problems, you only know you didn't fix it when it acts up again. If it doesn't act up, you don't know if it's fixed or not.
Electrical problems are the worst for being intermittent. When a spark plug fails to fire for an extended period of time, the unburned fuel builds up in the exhaust system. When it finally ignites, ... Well, you know what happens. Unplugging the injector to prevent that fuel from entering the exhaust, suggests the mechanic is on the trail of the problem. The worst thing you can do now is to take it to a different shop where they will have to start diagnosing from square one.
The will be some unintended consequences to running the engine with the injector unplugged. One problem will be greatly reduced fuel economy. The engine computer knows very closely how much fuel to spray in for any set of conditions, but once warmed up, it looks at readings from the oxygen sensor to fine tune fuel metering. With an injector unplugged, the air in that cylinder will enter the exhaust system and be detected by the O2 sensor. The engine computer will think the mixture is too lean, so it will add fuel to every cylinder. The exhaust will be very rich with unburned fuel, but the O2 sensor does not detect fuel, only oxygen. There is still the unburned oxygen from cylinder number 8, so the computer continues to add more fuel to go with that oxygen. You will smell the raw fuel coming out the tail pipe, but the computer will still think there's not enough fuel. The raw fuel mixes with normal pulses of extra air and the unburned air from cylinder 8 in the catalytic converter where it is burned. This much fuel will cause the converter to get red hot. It will be a fire hazard and can lead to melting the catalyst which will plug the exhaust system.
When there's so much built up fuel in the exhaust system that it blows the muffler apart, you should have noticed a rough running engine just before that happened. I suspect something got lost in the translation. "After he checked it out again he said that it was a faulty valve, #8. He stated that the spark plug wasn't firing." These are two different things that aren't likely to develop at the same time. Typically a burned or sticking valve will cause a popping through the intake system which is not particularly harmful, or it will cause a thumping sound in the exhaust system. Based on the buildup of fuel, I'm leaning toward an electrical issue that affects how well the spark plugs fire. But there has to be more to the story. One spark plug misfiring will not cause the engine to stall like yours did, so the mechanic must have solved that problem. Not sure what you mean by a "slight tune-up", but there isn't much that can be done on these newer vehicles. It is possible the backfiring now is due to a different, related problem. If the original problem caused too much fuel to be burned in the catalytic converter, it could be melted and restricting exhaust flow. This will cause pressure buildup that forces exhaust gas to try to flow back to the engine. It will lead to popping out of the intake manifold and very low power. In severe cases, we've had a hard time driving the cars into the shop. It's a struggle just to keep the engine running. Sometimes you will also hear an unusual hissing sound as exhaust gas is forced out the seams and joints in the system. Because of the low power, you have to push harder on the gas pedal which introduces more fuel in the exhaust and makes the problem worse. There are special back pressure gauges made to test for a restricted exhaust system.
Anything that affects the spark plugs should be inspected including the distributor cap, rotor, spark plug wires, and ignition coil. The cap and rotor are normal parts of a tune-up; the wires not always; the coil, never, unless it's defective. The ignition coil, and the pickup coil in the distributor can be intermittent, but they will affect all the spark plugs, not just one. Older Dakotas often developed worn bushings in the distributor that allowed the spinning shaft to wobble. There would be a misfire when it wobbled away from the pickup coil, but that alone would never cause the engine to stall without months of warning. I never heard of that problem occurring on a full-size van, but it shouldn't be overlooked.
Most of the time the cause of these intermittent electrical problems turns out to be something real little, and therefore, hard to find. A single loose or corroded pin in an electrical connector, a small crack in a rubber vacuum hose, a broken solder connection inside a computer module, etc. Simply unplugging an electrical connector can inadvertently solve a problem for a short time. The scraping action of the pins cuts through the corrosion and it makes a good connection for a few days or months until the corrosion grows back. Simply tugging on a hose could cause it to bend, temporarily closing up the gap or making it bigger. Solder connections can be heat-sensitive. At highway speeds, the air flow can keep computers cool, while city driving can allow them to heat up. These are just three examples of all kinds of things that can cause intermittent problems. Any of these could lead the mechanic to thinking he did something to fix the problem; after all, it's running better now. Simply replacing the air filter could disturb some electrical connectors and vacuum hoses. On top of that, computers are extremely sensitive to heat and vibration, and the manufacturers are sticking more and more of them on new cars. When you look at the environment they live in, it's a wonder they don't all fail on a regular basis. Some of the new GM cars have up to 47 computers that are extremely expensive and unreliable.
I know how frustrating intermittent problems can be to find, but it sounds like your mechanic is on the right track. You might consider letting him drive it for a week to be sure it's fixed. I'm pretty sure he wants it fixed as much as you do. After you've verified it's back to running normally for a week or two, you might consider showing your gratitude with some treats for the crew. Donuts and chocolate chip cookies are always appreciated, and will earn you a little more special attention on your next visit.
Wednesday, June 24th, 2009 AT 5:09 AM