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Mazda Ed
Mazda Ed, Auto Service Technician
Category: Chevy
Satisfied Customers: 2223
Experience:  ASE Master Technician, Mazda Master Technician, GM and Ford factory trained. 15 years experience.
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I have a 2004 suburban with code p0300 and rough idle, misfire

Customer Question

I have a 2004 suburban with code p0300 and rough idle, misfire counters are continuous on cyl 3 and 4 with 4 being double 3. at 2000 rpm's the msfires on cyl 3 and 4 go away. have tried swapping injectors and coils with different cylinders and the misfires do not follow
Submitted: 7 years ago.
Category: Chevy
Expert:  Mazda Ed replied 7 years ago.

You need to verify the injectors and the coils are getting the proper pulses which can be done with a noid light. If everything looks good there, then you need to check compression on all of the cylinders to see if you have an engine mechanical problem.
Customer: replied 7 years ago.
already checked injector and coil pulses with lab scope, and compression is 150 on all cylinders
Expert:  Mazda Ed replied 7 years ago.
Check your fuel pressure regulator for a leak through the vacuum hose. There should be NO fuel in that little hose at all! It's possible the intake is sucking the fuel in and #3 and #4 runners are closest to the port. Raising the rpms would allow the truck to burn that extra fuel and get rid of the misfire.
Customer: replied 7 years ago.
this year 5.3 has a regulator in the tank as it has no return line and the fuel pump module has already been replaced due to no residual fuel pressure after shut down
Expert:  Mazda Ed replied 7 years ago.
<p>I'm sorry about that, one part of my service info said it was on the fuel rail and the other said it was part of the fuel sender assembly (didn't check the second part until now.)</p><p> </p><p> </p><p>Try unplugging both front oxygen sensors and see if this makes a difference.</p><p> </p>