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I have a 1991 Buick Park Avenue with the 3800 engine. The problem is that it stays in third gear, will not up shift or down shift. I can manually shift to second gear but not to first. Also, if I put it in neutral or park the engine idols at almost 3000 RPM. When driving at 55 MPH even though it is in third gear I can floor the accelerator and have almost no power. It gets bad gas mileage and overheats very easily. And occasionally the odometer will erotically bounce around between 0 and 6000 RPM for a few minutes. The weird part is that every now and then the check engine light goes off, it idols down, shifts perfectly, and drives like a new car. Then just as suddenly, it will start having issues again. A few other things, sometimes for about 20 seconds after starting there is a kind of rough, grinding noise from the serpentine belt area. Usually after the noise stops it will run good for a few minutes…. And sometimes after starting when cold, instead of idling too fast it will hardly stay running…. I am quite certain that it is an electrical problem, because several times when driving I have turned on my headlights and immediately the check engine light goes off and it shifts into overdrive. According to the trouble codes, the following sensors are bad. Knock sensor, coolant temperature sensor, manifold air temperature (MAT) sensor, and throttle position sensor. However, after replacing the latter two sensors, and resetting the trouble codes, it drove just as badly and still said that the same sensors were bad. One other thing, when it runs badly I loose my electronic brake assist and cruise control. I think that’s pretty much it. I have put in new spark plugs, new air filter, a new fuel filter, and a new crankshaft sensor. I also tried fuel injector cleaner without results. Have any suggestions?

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