This is what happens when mechanics speak a different language than car owners, (just like doctors, accountants, and airline pilots), and things get mixed up in translation.
First of all, while you may indeed have a fault code for the speed sensor, they have nothing to do with engine performance. That is going to be a separate problem. Sensors are responsible for setting a code only about 50 percent of the time. Other causes include a corroded splice in a wire, a broken wire, a stretched terminal in a connector, and things like that.
Second, one mechanic's explanation of "current going in but not out" is messed up. Either he doesn't understand electrical theory and shouldn't be working on that problem, or he isn't very good at explaining what he found. Very few mechanics have good customer communication skills. Current is always the same in a series circuit. A series circuit can be visualized as a river flowing through many cities. Whatever water flow, (current), flows into the city is the same as what's flowing out. A garden hose is maybe a better example. Whatever flows in one end is the same as what's flowing out the other end. If the mechanic "diagnosed" it and came up with that explanation, why didn't he follow through and fix the cause? Diagnosis of electrical problems is where 90 percent of the time is spent. Solving the problem is just the other ten percent.
Speed sensors produce a voltage signal. That's electrical pressure, not flow. That's arguing a point irrelevant to the issue but it reveals my suspicions that this mechanic might not be your best choice for this problem.
Reflashing the computer means reinstalling or updating the software. There's nothing you're going to "look at" to tell when that's necessary. When no updates are listed, it just means they didn't see the need to make changes to correct a common emissions or running problem affecting many cars of that model. The software could have become corrupted from some other cause, just like your Windows operating system can become corrupted and need to be reinstalled. While I'm skeptical that reflashing a computer will solve an intermittent problem, I have to admit it does happen.
Check Engine lights don't reset after 50 miles. They might turn off on their own but it has nothing to do with mileage. Some models erase fault codes that don't reoccur after 50 engine starts. As for the light, there are varying degrees of fault code severity that determine how the light acts. If the code is related to something that won't affect emissions, it likely won't even turn on at all. If it's a minor problem, the light will go off while you're driving. If it's a little more severe but the intermittent problem goes away, the light will stay on until you stop and restart the engine. If the problem is even more serious, even if it goes away, the light will turn on every time you start the engine. When the light is flashing, way too much raw fuel is entering the engine and will damage the catalytic converter if the engine isn't stopped right away.
Since no other codes are being set related to the running problem, that suggests it's not related to a failing sensor. I'm actually dealing with the same issue with my minivan right now, and based solely on observations when it stumbles or when it stalls and won't restart without cycling the ignition switch a few times, I'm suspecting a tired fuel pump. Fuel pressure is not monitored on my van, and I suspect on your vehicle either. Only the RESULTS of low pressure might show up if the problem occurs long enough. I'm going to prove my suspicions by driving around with a fuel pressure gauge clipped under my wiper arm so I can see what happens when the engine stalls, (as soon as I can find the darn thing).
The problem with this type of running issue with no related codes and being intermittent is you can't have a mechanic standing right by it when it occurs, but there are a few things you can look into. Most commonly, most of the scanners the mechanics use have a record feature that records about a five-second snap shot of sensor data while you're driving. Rather than borrow you their multi-thousand dollar scanner, there are some "flight recorder"-type units made for this purpose. Almost all new car dealers have something along those lines. They leave it in the car while you drive it, then when the problem occurs, you press the record button. Since the data travels through the unit's memory, the recording actually starts a few seconds before you pressed the button. Later, that can be played back frame-by-frame while watching how each sensor responds.
If you had to guess, the mass air flow sensor is a good suspect, but also look for any leak in the tube between it and the throttle body. If any air sneaks in that the sensor doesn't know about, the Engine Computer won't command enough fuel to go with it.
A cause of low fuel pressure can be a plugged pickup screen inside the gas tank. If your vehicle has a fuel pressure regulator on the fuel rail on the engine, a plugged screen will cause stalling when the largest volume of fuel is pumped, ... Which is during coasting. The engine will typically run fine under load or when cruising at a steady speed. I never stopped to think about it, but if your vehicle uses the newer "returnless" fuel supply system with the pressure regulator in the gas tank, I think the stumbling would occur on acceleration instead of coasting.
Those are just some other things to consider. Intermittent problems are always the most frustrating to find. You might also do a search for related service bulletins although they typically are produced after reviewing numerous warranty claims for relatively new vehicles seen at the dealerships. When common problems occur on older vehicles, the manufacturer typically doesn't write bulletins for them.
Hope to read soon that this is solved.
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Wednesday, March 21st, 2012 AT 5:40 PM