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Ok i have a 1996 chevy silverado 5.7 vortec 4x4 ok.....so, when i start it, it runs fine for about 2 to 3 minutes then it starts to miss and back fire for about 15 to 20 min. and the check engine light comes on. I park, come back about 10 min later to leave and it does not want to start. After it does start it still misses and then it all stops after running it for about another 30~40 min. and the light goes off and runs fine


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contributions: 1898
Images: 37
Location: Oregon
you could scan the engine and see if you can access the misfire counter. see which cylinders are causing the miss. See if there are misfire codes.

Very common on this motor is the distributor cap burning through and shorting cylinders. When it first starts acting up, there is a misfire cold. after a while the misfire will get worse and be there warm. When it gets really bad the cap will actually short to ground and engine won't start anymore. If you check for spark with the coil wire you will see that is sparks fine but if you check to see if you have spark ay the spark plug wires there will be no spark or very weak spark.


   
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I did and it says bad cats but would the make it miss when its cold


   
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contributions: 1898
Images: 37
Location: Oregon
bad cat means different things. it could be that the rear o2 sensors are not seeing the cats cleaning out the hydrocarbons up. it could also mean that the cars are plugged up. You could check for back pressure and see if the exhaust is plugged up. If an engine misfires it sends raw fuel into the cat. that's when the check engine comes on and is flashing on and off as you drive. The flashing check engine light is telling you that you are damaging your cat if you continue to drive. Raw fuel in the cat will make the cat run too hot. It is supposed to burn off the unburned fuel from your engine. Raw fuel is too much to burn off so it will overheat and start to melt and then break up into pieces. When it melts of breaks up it plugs off the outlet for the engine. It can cause all sorts of problems. Eventually it can stop the engine from running altogether. The engine becomes an air compressor.

but first look at what is happening when it is missing. Do you have a misfire counter on your scanner? do you have any codes in the P0300-p0308 range? when the engine doesn't start have you checked for spark, fuel pressure and injector pulse to see what is not working?


   
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