I have a 99 Silverado 5.3L 4x4 with 189,000 miles. Starts great but begins idling rough after a few minutes. Has p0300 code(random misfire) and o2 sensor high voltage. I ...

muredhawks · by **muredhawks** [OP] » Jul 29 2012, 6:38pm

I have a 99 Silverado 5.3L 4x4 with 189,000 miles. Starts great but begins idling rough after a few minutes. Has p0300 code(random misfire) and o2 sensor high voltage. I have changed the plugs and wires, fuel filter and o2 sensors before and after cat. At one point it gave a p0302 code which we then changed the coil pack. The thing with it is that if you run it hard for a bit it will begin to run fine until you turn it off and restart it.

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ASTE25 · by **ASTE25** » Jul 30 2012, 7:04am

Have you checked to see if it has any vacuum leaks?

muredhawks · by **muredhawks** [OP] » Jul 30 2012, 4:56pm

That's what we're leaning towards. I'm gonna change intake gasket this weekend and go from there.

RNGRDVE · by **RNGRDVE** » Jul 30 2012, 7:26pm

They are known for upper plenum gasket leaking. Usually they will have a rough idle that goes away after warmup due to expanion sealing the leak. Easy way to check is spray brake cleaner where the plenum bolts up on cold start when it is running rough, if the engine speed changes then you found the problem.

reynoldsc58 · by **reynoldsc58** » Aug 06 2012, 11:45pm

Try the simple things like an air filter and clean the MAF sensor. I have the same truck with 182,000 miles and it did the same thing but i found out it was the O2 sensors, they were black as coal, so you beat me there. See if you unhook the MAF and start it up if there is a difference. Sense you already replaced the O2 sensors it will tell you its the MAF if it doesnt idle rough with it unhooked.

Lt295 · by **Lt295** » Sep 13 2012, 8:44am

I had a similar problem with my 2000 Silverado with the 5.3 Turned out it was a defective fuel injector.

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I have a 99 Silverado 5.3L 4x4 with 189,000 miles. Starts great but begins idling rough after a few minutes. Has p0300 code(random misfire) and o2 sensor high voltage. I ...

99 5.3L starts fine but idles rough

99 5.3L starts fine but idles rough

Re: 99 5.3L starts fine but idles rough

Re: 99 5.3L starts fine but idles rough

Re: 99 5.3L starts fine but idles rough

Re: 99 5.3L starts fine but idles rough

Re: 99 5.3L starts fine but idles rough

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