could be multiple things plugs wires mas intake maniflod 02sensor i had an 03 avalanche 5.3 l vortec it had the same issue it turned out to be intake manifold gasket. do a leak test and check compressinon of each cylinder

 

It could be a bad coil or a vacuume leak. Both are fairly common problems. Since it is a random misfire I would lean towards a vecuum leak. The intake gaskets are pretty common to leak.

 

+1 on the intake gasket. i had the same problem. changed all my plugs and wires just for service and didnt help. i then tested all my coils and they were good. tested the injectors, all good. so since the intake gasket is a very common problem with these motors, that's what i will be changing next.

 

Thanks! Ill start looking into coils and the intake....whats the best way to check the intake gasket? Spraying the base with brake cleaner I'm assuming?

 

Ya, I used carb cleaner

 

yup. or a starting fluid will do the trick

 

I had the same problem along with two other codes, and it led to an O2 sensor. You've got it right by checking your intake gasket by spraying it down as I did that test as well. My intake is good....... For now.

 

Just had the same thing happen a few days ago - getting a P0300 and P0332 (Misfires / Knock Sensor II Low Input)...so I'm going to get to replace that mother and see if it takes care of the P0300 - this may be the same issue you are looking at?

 

So I had this problem on thursday and spent the whole weekend figuring it out! Check both banks of you spark plugs and see if one bank is running richer or leaner than the other, ie blacker or whiter (my bank 1 was running VERY rich causing the P0300 code). Also you can swap your two upstream O2 sensors with eachother, and see if you start getting a bank 2 random misfire code if you are hesitant to blow $100 on diagnostics

 

Plugs were all in really good shape. Used the GM Tech II Scan tool to look at fuel trim and O2 sensor readings. All seem to be within spec. The misfire count is on ALL cylinders but 1 , 4 and 7 seem to have the highest misfire counts. cylinder 2 for example has a miss count of 16,000 where 7 has a miss count of 54,000 . Fuel pressure is also perfect. 60Lbs. Ill try and check my intake tomorrow, hopefully with some results.

 

make sure 1 of your plugs aren't cross threaded. 1 of mine were during my misfire diagnostics

 

Interesting. Did this start happening all the sudden, or has something been going on for a while and been getting progressively worse?

 

been an ongoing issue but began as a once in 3,000 mile thing....and now its an every mile thing.. :/ my engine light is on constantly seeing as i take the highway to work every day.

 

I have this code (P0300), as well as P0171 and P0174. I've replaced the fuel filter and fuel pressure regulator and also cleaned the mass airflow sensor with the recommended cleaner. Also, I've tried to spray all around the intake manifold with carb cleaner and had no response to that. Since it's throwing both bank 1 and 2 I didn't think it would be one of the O2 sensors or any coils, wires or plugs... Any suggestions on where to go from here? Thanks.

 

I had the same problem along with two other codes, and it led to an O2 sensor. You've got it right by checking your intake gasket by spraying it down as I did that test as well. My intake is good....... For now.

What were the other codes? P0171 and P0174 by chance? And which O2 sensor did it lead to?

 

I had the same problem along with two other codes, and it led to an O2 sensor. You've got it right by checking your intake gasket by spraying it down as I did that test as well. My intake is good....... For now.

What were the other codes? P0171 and P0174 by chance? And which O2 sensor did it lead to?

I believe those were the codes. If I remember correctly, it was the sensor on the passenger side before the cat.

 

I am having a similar problem with P0171 and P0174 codes. I bought some cleaner to spray around the intake and vacuum fittings. Anyone have a diagram of the vacuum system?

 

I know it's been years but did you figure out what was causing the problem? I'm having same problem with the same 3 codes

 

@GMCain Yes, it's been 11 years and the OP has not been on this site for 7+ years. If you have something with which you need help, start a new thread including the year/make/model/engine/drivetrain/trim of your Silverado/Sierra.