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Gary, Chevy Technician
Category: Chevy
Satisfied Customers: 13794
Experience:  15 years GM master tech
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Code p0300b random engine missfire

Customer Question

code p0300b random engine missfire
Submitted: 4 years ago.
Category: Chevy
Expert:  Gary replied 4 years ago.
Is it misfiring just # XXXXX at idle?
Customer: replied 4 years ago.
Expert:  Gary replied 4 years ago.
ok do you have a vacuum gage?if so can you plug it into a manifold vacuum line and idle it and tell me what it is reading?and also if the needle is steady or not
Customer: replied 4 years ago.
in between 5 & 10 the needle is not steady
Expert:  Gary replied 4 years ago.
whats it bouncing too?between 5 and 10? also if you rev it up to @ 2000 will the # XXXXX skip go away?and how will the vacuum needle respond if the skip goes away or gets allot better?
Customer: replied 4 years ago.



Expert:  Gary replied 4 years ago.
what is the needle bouncing to at idle?for what to what and was it bouncing fast?
Customer: replied 4 years ago.
5 to 10, yes it is bouncing fast
Expert:  Gary replied 4 years ago.
ok what that is showing is the intake valve more than likely for # XXXXX cylinder is not closing properly causing you to see that gage bounce you are actually seeing compression entering threw the intake valve not closing all the will either be a burnt valve or there is allot of carbon on the backside of the intake valve holding it open.either way that intake valve is not closing and sealing
Customer: replied 4 years ago.
we checked for leak down on cylinder one and there was zero percentage--then checked #2 and #3 for reference.. where do we go from here
Expert:  Gary replied 4 years ago.
there is an intake valve not seating all the can be sticking valve guides slightly holding the valve open when the engine is running or carbon on the back of the valve.if that needle is jumping means compression is getting into the intake threw a or intake valves.there is no other way compression can get into the intake.
Customer: replied 4 years ago.
how do you determine that with zero leak down?
Expert:  Gary replied 4 years ago.
the problem being is the leak down will push the valve back up and more than likely seal it.when you had the intake off did you look and see what the back side of the valve looked like?to narrow it down # XXXXX is misfiring and the vacuum gage shows compression getting into the intake the only way it can is threw the intake would have to be a problem with the # XXXXX intake what you can do is remove the valve cover and fill the cylinder with air.remove the valve spring for # XXXXX valve hold the tip of the valve and let the air out of the cylinder and see if that valve feels like its binding in the guide.DO NOT drop that valve down just feel it all the way up and then down @ 1 inch see if it feels real smooth or binding.if it feels really good remove the intake and look for excessive carbon on back side of valve.the last thing i have seen causing this and it is rare is the valve seat is loose in the head and when the engines running the seat is moving when the valve opens.also try with the intake off opening and closing the valve by hand while looking in threw the intake opening get a good look at that valve

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