Wow! I've heard of people getting bad haircuts, but none of them considered getting a face lift to correct it.
It's time to get a second opinion. How do you know a different engine won't have the same problem? If your engine runs fine under low load, why do they think there's a mechanical problem inside it, and if there is, why don't they diagnose it and fix it? That's what they do.
First of all, most scanners will display live sensor data during a test drive. Experienced mechanics can interpret those readings and find valuable clues to the problem. Many scanners have a "record / playback" feature that allows it to record the events taking place when the problem occurs. Reviewing that data later can provide clues. When the Check Engine light turns on, there is a diagnostic fault code stored in memory that will lead to the circuit or system that needs further diagnosis. GM has one of the few systems that can turn the light on and not store a code, leading to lots of frustration among mechanics and owners, but eventually a code will be set, especially if the light turns on while you're noticing a running problem. The light must turn on when the problem detected could adversely affect tail pipe emissions. Most of the time that involves the fuel or ignition, (spark) systems, not internal mechanical problems. You might have a failing ignition coil, a shorted injector, or a similar type of problem that will still be there with a new engine.
Also be aware that GM is one of the few companies that grabs a handful of injectors out of the bin on the assembly line and throws them randomly into the engine. They have a huge problem with mismatched flow rates. That results in one cylinder running lean, and that extra unburned oxygen shows up in the exhaust where it is detected as ALL cylinders on that side of the engine are running lean. The Engine Computer tries to fix that by commanding more and more fuel from all of the cylinders on that side. No matter how much extra fuel it commands, there will still be that unburned oxygen in the exhaust. The fix is to install a matched set of rebuilt injectors that have been flow-matched. Many GM owners comment that their engine never ran so smoothly even when it was new. Chrysler, and probably most other manufacturers, buy their injectors from their suppliers already in flow-matched sets, and injector problems are unheard of for the life of the vehicle.
If you're hearing a knock from inside the engine, you have to determine if it is spark knock or a mechanical knock. Either of those will be detected by the knock sensor and cause the ignition timing to be retarded. That does not result in misfires. It only results in low power and poor fuel mileage. A mechanical problem inside the engine might be heard as a knock, but it's not going to set a misfire fault code. Misfires for a specific cylinder are detected by the fact that the crankshaft physically slows down during that brief period when it occurs. It's that slight slowdown in rotational speed that causes us to FEEL a misfire. A mechanical knock from something inside the engine won't cause the crankshaft to slow down if the fuel system is working properly and the spark is igniting the fuel and air properly.
By the way, I have a horrendous mechanical knock from inside my '88 Grand Caravan with 380,000 miles. It's been there for over 8 years and gets people out of my way in parking lots. They're afraid of being hit by flying parts, I guess. The engine runs smoothly, and the noise goes away above idle so I haven't bothered to fix it. Given that history, I find it really hard to believe the dealer has to resort to replacing your engine after just 110,000 miles, ... And for a misfire, of all things. I'm willing to listen to their reasoning, but they will have to explain why they can't repair your engine.
Sunday, December 18th, 2011 AT 9:33 PM