Have you put new sparks plugs in it. If you havent I would try them first. Only run the ac delco plugs listed for the truck. I have had many people come threw the shop with misfiring and trucks running like **** and come to find out they put bosch or ngk plugs in it. Your manual says there good for 100,000 miles but there really only good for about 75,000 or so. Also run some lucas fuel treatment or b12 chemtool. both you ad straight to the gas tank. If it calls for one can for 20 gallons buy two and double the treatment it wont hurt you engine.
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Misfire could be a number of things: a) spark plugs - change them if used over 15K miles - inspect/regap. b) spark plug wires - change them if over 15K miles - inspect c) ECM (electronic control module) - test with HEI (high energy ignition) tester.
The shaking tells me that the engine is running on less than its normal compliment of cylinders - the shaking is produced by the imbalance caused in the engine by one (or more) cylinder(s) not firing at all.
The "Check Engine" light probably is coming on due the the misfire condition. Go to Pep Boys or Advance Auto Pats, where they'll tell you (free) your codes causing the "Check Engine' light to be lit - the code coming from your truck's computer will probably be one or more of: P0300 - Random/Multiple Cylinder misfire detected P0301 - Cylinder 1 misfire detected P0302 - Cylinder 2 misfire detected P0303 - Cylinder 3 misfire detected P0304 - Cylinder 4 misfire detected P0305 - Cylinder 5 misfire detected P0306 - Cylinder 6 misfire detected P0307 - Cylinder 7 misfire detected P0308 - Cylinder 8 misfire detected P0313 - Misfire detected with low fuel
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The Code(s) should be telling the specific cylinder(s) that are misfiring. Do you know waht the code is?
Target those cylinders 1st before testing the unaffected cylinders.
The following should isolate the problem(s).
1. Spark plugs - take out-reconnect back to wires and test spark plugs for getting sparks. If none: Don't replace them yet. GO TO 2. If the spark plugs are worn, dirty them replace them.
2. Remove spark plugs form end of ignition wires and test for spark from the wire. If none: Don't replace them yet. GO TO 3.
3. Remove the wires from the coil connection and test the coil for spark for each cylinder. If none: Don't replace it yet.
4. Test each spark plug and wires this way.
5. Coil Packs normally provide fire for 2-cylinders at the same time. The two are paired across from one another.
6. Test the coil for fire, amount of fire and if there is both cyclinders firing.
Misfire and the O2 Sensor got replaced??
Good Luck and get back with me if you need additional help or if this solved the problem.