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Pete, GMC Mechanic
Category: GMC
Satisfied Customers: 27834
Experience:  GM training and dealer tech for 14+ years. Diagnostics
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Need help with codes P0300 and C0035. Engine has a heavy

Customer Question

Need help with codes P0300 and C0035. Engine has a heavy intermittent misfire and feels like a jerk on the highway. I replaced the plugs, wires and all ignition coils and the same 4 cylinders 1,4,5,7 are still misfiring on the scan tool. I can monitor the misfire data on the snap on solus scanner, and at idle all cylinders are great, when accelerating they are showing misfire. The vehicle has 350k miles on the engine and had a recent Jasper transmission installed. I ohmed out the front left ABS sensor, and the resistance is within specs, and its producing the voltage it needs to when I spin the wheel, so likely the wheel hub assembly needs to be replaced. Im pretty sure the jerking is the engine misfiring and not the transmission. I think this is an internal engine problem, not transmission. Please advise. Thanks
2006 GMC Savana 3500 Base 4.8 L GAS 1GDGG31V361900296
Submitted: 11 months ago.
Category: GMC
Expert:  Pete replied 11 months ago.

Hi my name is ***** ***** I will be happy to assist with this today.In some cases I may need more info to assist better.I apologize for any delay.

Is this cylinders 1,3,5 and 7 misfiring instead of #4 cylinder by chance?

Let me know so we can continue,thanks Pete

Customer: replied 11 months ago.
sorry. 1,3,5,7 are misfiring on the misfire data with code P0300 and the EVAP leak code too, but I kind of disregarded that code for now.
Expert:  Pete replied 11 months ago.

O.k.,your model has 2 catalytic converters as shown in the diagram below.One right below the manifold pipe connection on each side of the engine.I suspect the drivers side is restricted.This commonly causes misfires with all cylinders under load on that side of the engine.If you have not had an exhaust back pressure test done this needs done.You can usually rent a back pressure tester from a local parts store.Remove the oxygen sensor before the catalytic converter on the drivers side,thread in the tester.Start the engine and rev it up.The back pressure should not exceed 1.5 psi at any rpm range.If it does then the catalytic converter needs replaced.

If you have already tested this and found it to be o.k. and not the issue here let me know but I do suspect this is the cause as this is most common for misfires on one bank like this here.

If more help is needed, use the reply tab to continue our conversation. If no further assistance is needed, kindly rate my service,your 5 star rating is appreciated.You can rate at any time and we can continue to work on your question as this will not close out your question.Keep in mind in some cases it can be difficult to fully diagnose or help repair your vehicle without seeing it and I work on delivering the best possible answer from what I see to work with.

Thanks Pete

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