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SES code P0300

GMSea97

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GMSea97 Enthusiast

GMSea97

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The computer says random multiple cylinder misfire. It has been starting hard and not running very smooth, so I have done a very complete tune-up, and checked every thing I can think of. I think the coil pack is bad, because I have put new plugs and wires twice. I am almost sure it isn't on the fuel side of things because when she starts hard and sets the code there is plenty of fuel fumes coming out of the exhaust, but I maybe wrong. Those are my thoughts what do you all think?

95Sierra2500 Senior Enthusiast

95Sierra2500

Posted
Posted

Did you replace the cap and rotor too? Those can go bad as well. Also, check your fuel pressure. Sometimes the fuel pump can get weak and lead to symptoms like you're describing.

 

another thing to check is your cat. Those can cause misfires if they're completely plugged.

GMSea97 Enthusiast

GMSea97

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Posted

I am sure it is not the rotor or cap, because I don't have one on my truck. It has some sort of coil right where those should be. So it didn't seem like a thing that got replace as easily as a rotor and cap. How do I check the cat, I should be able to just punch it out if it is the problem right. The fuel pump got replaced almost a month ago when it went up. It was doing this a few months before the fuel pump went up.

95Sierra2500 Senior Enthusiast

95Sierra2500

Posted
Posted
I am sure it is not the rotor or cap, because I don't have one on my truck. It has some sort of coil right where those should be.  So it didn't seem like a thing that got replace as easily as a rotor and cap. How do I check the cat,  I should be able to just punch it out if it is the problem right. The fuel pump got replaced almost a month ago when it went up.  It was doing this a few months before the fuel pump went up.

 

 

 

 

Trust me, taht's a cap and rotor, and you may want to consider replacing them (they're more expensive than the old style though).

 

As for the cat, sure you can punch it out, but you'd better be finding an O2 simulator. Good luck as I'm having trouble finding one for my fiancee`'s '99 Jimmy. The cat can be tested by using a non-contact thermometer and checking the temps before and after the cat...But I can't remember what the difference should be.

 

You may also want to try running some of GM's Top Engine Cleaner through it, to clean your injectors/poppet nozzles (I'm pretty sure your '97 has that CMPFI setup). Someone else may want to chime in on that one though.

redvett Senior Enthusiast

redvett

Posted
Posted
I am sure it is not the rotor or cap, because I don't have one on my truck. It has some sort of coil right where those should be.  So it didn't seem like a thing that got replace as easily as a rotor and cap. How do I check the cat,  I should be able to just punch it out if it is the problem right. The fuel pump got replaced almost a month ago when it went up.  It was doing this a few months before the fuel pump went up.

 

 

 

 

Your sig. says 97 and they had a distributer & SFI and not a coil pack. It was not until 1999 that the LS engines came available and they had wasted spark ign.

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