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Dr.Automotive, Auto Mechanic
Category: Chevy
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Experience:  Automotive Degree Dealer Experienced
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P300: 03 chevy silverado 4.8..fuel

Resolved Question:

03 chevy silverado 4.8 P0300 fault code. New fuel filter -60 psi, new cam/crank sensors, plugs & wires good, really running BAD, but idles almost perfect. Worst when hard acceleration
Submitted: 5 years ago.
Category: Chevy
Expert:  Dr.Automotive replied 5 years ago.
Hello and welcome to Justanswer. My name isXXXXX am a Automotive Mechanic here to help you with your problem.

Check the catalytic converter, on the "suspect" bank of the engine, for a restricted (plugged) exhaust system to that side of the engine. Disconnect both front O2 sensors, re-set the fuel trims and see if the problem goes away. If gone with the O2 sensors disconnected, then swap the sensors side to side and see if the problem follows the O2 sensor. Thanks

Customer: replied 5 years ago.
Does not appear to be either side. As mentioned idles good. When driving it will almost run on all cylinders under very light acceleration but breaks up upon hard acceleration. How do you reset trim ?
Expert:  Dr.Automotive replied 5 years ago.
When you have a P0300 this is multi cylinder misfire detected and based on this I suspect O2 sensor can be at fault or a restricted exhaust. But you can reset the system by disconnecting the battery cables for 5 minutes touching them together and then reconnect them. if the spark and fuel are good I would look into the fuel. It can be the MAF but normally you will get a code below are the book reasons for the code.

  • Base engine mechanical fault that affects one or more cylinders
  • Check for air leaks in the EGR system and in its related hoses and/or valve
  • Check for air leaks in the PCV system and in its related hoses and/or valve
  • EVAP system problem or the EVAP canister is fuel saturated
  • Fuel metering fault that affects more than one cylinder
  • Fuel pressure too low or too high, fuel supply contaminated
  • IC control circuit is shorted to ground (an intermittent fault)
  • Ignition system fault (a coil) that affects more than one cylinder
  • MAF sensor contamination (this can cause a very lean condition)
Dr.Automotive and 4 other Chevy Specialists are ready to help you
Customer: replied 5 years ago.
I should say I am an industrial diesel tech for 40 years and not at all unfamiliar with engines. I am somewhat unfamiliar with computer controls, especially gas jobs. I would wager engine has no base faults and the exhaust is not plugged. The good idle ( it does have a very slight miss, not a dead miss ) kind of takes out the PCV and EGR ( I believe ) . I treated the engine with Sea Foam which didn't change it as far as I could tell. A list I saw said ECU ,this was the direction I was going to go. What is IC circuit and what could be up with MAF sensor? This problem showed up while driving as a slight loss of power and quickly got worse. I had unhooked each coil and saw no change in idle other than missing on the disconnected cylinder . When you talked about the battery cables was that reseting the trim?
Expert:  Dr.Automotive replied 5 years ago.
Yes on the bat cables. I would check compression could be a ring problem. IC is ignition control. I would really look at the MAF could be faulty you need a handheld to check it out fully but I have found sometimes disconnecting it will allow it to run better than connected when no tool available to check it out if this is so the MAF is bad. Something funky is up here I would start with the above. If this all check out need to scope the crank and cam sensor can easily be at fault here. Thanks

Customer: replied 5 years ago.
As mentioned crank and cam sensors were replaced. I will try unplugging the MAF sensor. Didn't change it much. Maybe a little worse but hard to tell, it is running so poor . It does seem to run a little better right when your first run it like maybe a minute or so.
Expert:  Dr.Automotive replied 5 years ago.
Really sounds like the catalytic convertor its not hard to pull the O2 sensors and see if it helps. Than check compression and verify its good. If the compression is good than a leak down test check the timing using the leak down it could be out of time for sure!
Customer: replied 5 years ago.
I have delt with plugged converters before, does not act like any I have experienced. I will work on O2 sensors and items you suggested .
Expert:  Dr.Automotive replied 5 years ago.
Yes I have also dealt with so many convertor issues and they can seem like its not but can be. I would not overlook very easy to check it and eliminate this from the list. Your fuel is good but the other things that really come to mind is the convertor MAF and timing.


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Customer: replied 5 years ago.
Is there any possibility of it being fuel injector problems? The timing is tied into the ECU. After I work with the O2 and converters I will most likely try a salvage ecu. Thanks for trying.
Expert:  Dr.Automotive replied 5 years ago.
normally all the injectors will not cause this issue without having a hard misfire on some cylinders. This can all be tied into the ECU. You have the most common probably causes you need to verify all of this than you will have to check powers and grounds at the computer and replace if they check out. I would be very curius to see what cylinders are misfiring on a handheld because it could be one cylinder and the computer is just reporting P0300 this will also allow you to check the O2 sensors for a problem and the MAF.


Customer: replied 5 years ago.
Very definatly multiple. 15 mph on hills. Missing comes and goes . If this was an old distributor system I would be looking at the coil wire /cap etc. Really seems like an ign miss problem. I just do not have much experience with gasoline fuel injection issues. This one is teaching me!
Expert:  Dr.Automotive replied 5 years ago.
This sounds like a bad connection which can be the wiring to the injectors or the ECU so we will need to do some testing or verify the missing cylinders than diagnose those cylinders this could be a weak coil or bad injector connection to the injector timing problem or just a issue with the ECU. Thanks

Customer: replied 5 years ago.

Well, you were correct in that was a plugged converter, along with an intake manifold leak which was very small- caused the idle to be off a little.


Expert:  Dr.Automotive replied 5 years ago.
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