Need your input! Misfire. P0300 but Only #7 misfiring?

JN85Chevy · August 27, 2013

This is my first post so I am not sure if this is where I need to be posting this but here goes anyways. Bought 1999 Silverado 5.3 used last year everything worked great, just some minor issues that I fixed on my own (brakes, rotors, brake lines). I am no mechanic, just a guy with a garage and a few tools I would like to add.

Currently it has 225,xxx miles on it and is my daily driver. Here are the problems I have had in the past few weeks. Idles rough, worse when engine is cold but otherwise always there. Misfires slightly only when under a load but ses does flash, only flashes after the engine is on and in drive at same exact time every morning and only once in a while in the afternoon. SES light was on so I had it checked pulled up misfire on cylinder 7 and p0300 code as well as the code for the EVAP and passenger side o2 sensor. This is what I have done so far, Replaced Fuel pump, fuel filter, fuel pressure regulator, all spark plugs, all plug wires, #7 cylinder coil pack, local mechanic replaced a part on top of the engine towards the front, it has a connector running to it, not sure what it is or called. Ran lucas injector cleaner thru it, and seaform also, no change. Cleaned the MAF sensor, retightened all my vacuum hoses and done a self vacuum leak test while engine was running, no change in idle and the misfire is still there. Also my temp gauge is running above normal but not overheating either when I have my a/c on or a/c off and drive for more than 45 min straight, unless it is in the morning when the air is cooler outside then the temp stays normal. Its NOT leaking antifreeze or oil. replaced thermostat twice, not sure why. What do I do next? Sorry for the long post but im sure I also forgot to mention something. If I think of it I will repost.

Edited August 28, 2013 by txab

TJay74 · August 27, 2013

Running rough back in the past could be attributed to the coolant temp sensor, the ECM relies on it so that it knows when to switch over from open loop to closed loop running. You are going to throw money at this, at some point you may need to heed and take it to a certified shop. I am like you, I try it myself if it is point blank a found known issue, running issues though can be hard to diagnois.

Edited August 27, 2013 by TJay74

dgrinnell · August 27, 2013

Check your intake manifold gasket, I had an 2001 that did the exact same thing. Thats just somthing to look at.

JN85Chevy · August 27, 2013

First off thanks for your quick replies. I am trying to hold off on taking it to a Chevy shop until I absolutely have to. I will try tonight look at the intake manifold gasket. I get out of work at 5 pm so it will be at least 6pm tonight until I get to it. More idea posts welcome, thanks.

TJay74 · August 27, 2013

I totally forgot as well, could be false knock. There was a issue with water leaking under the intake manifold and it would go under the knock sensor gasket and corrode the knock sensor. There was a TSB for it somewhere, I remember replacing a set for a friend on her 2002 Tahoe with the 5.3l. Took about 2 hours to pull the intake, clean everything up and put in the new knock sensors (there are 2). I then used a thin film of silicone sealant to seal the rubber boots down so water would not penetrate them again.

JN85Chevy · August 28, 2013

Ok so I was not able to change the intake manifold gasket last night but this morning my SES light did not flash at the time it usually does and only after I was driving for about 10 minutes did it misfire but only for a few seconds other than that, only a minor misfire in my 30 minute drive to work. The idle was still off, @400-500 rpms and still shaking truck. The weather is @ 84 no rain but high humidity, not sure if that would have affected it. Also would like to add that I had replaced idle sensor and had all the injectors balanced checked and all seemed fine but their computer still showed that it was cylinder #7 misfiring? I don't think my misfire is gone yet but it has been misfiring intermittently on a consistent base other than this morning, any ideas? Air Idle control Valve?

Edited August 28, 2013 by txab

JN85Chevy · August 28, 2013

Bought 1999 Silverado 5.3 used last year everything worked great, just some minor issues that I fixed on my own (brakes, rotors, brake lines). I am no mechanic, just a guy with a garage and a few tools I would like to add.

Currently it has 225,xxx miles on it and is my daily driver. Here are the problems I have had in the past few weeks. Idles rough, worse when engine is cold but otherwise always there. Misfires slightly only when under a load but ses does flash, only flashes after the engine is on and in drive at same exact time every morning and only once in a while in the afternoon. SES light was on so I had it checked pulled up misfire on cylinder 7 and p0300 code as well as the code for the EVAP and passenger side o2 sensor. This is what I have done so far, Replaced Fuel pump, fuel filter, fuel pressure regulator, all spark plugs, all plug wires, #7 cylinder coil pack, local mechanic replaced a part on top of the engine towards the front, it has a connector running to it, not sure what it is or called. Ran lucas injector cleaner thru it, and seaform also, no change. Cleaned the MAF sensor, retightened all my vacuum hoses and done a self vacuum leak test while engine was running, no change in idle and the misfire is still there. Also my temp gauge is running above normal but not overheating either when I have my a/c on or a/c off and drive for more than 45 min straight, unless it is in the morning when the air is cooler outside then the temp stays normal. Its NOT leaking antifreeze or oil. replaced thermostat twice, not sure why.

this morning my SES light did not flash at the time it usually does and only after I was driving for about 10 minutes did it misfire but only for a few seconds other than that, only a minor misfire in my 30 minute drive to work. The idle was still off, @400-500 rpms and still shaking truck. The weather is @ 84 no rain but high humidity, not sure if that would have affected it. Also would like to add that I had replaced idle sensor and had all the injectors balanced checked and all seemed fine but their computer still showed that it was cylinder #7 misfiring? I don't think my misfire is gone yet but it has been misfiring intermittently on a consistent base other than this morning, any ideas? Air Idle control Valve?

Edited August 28, 2013 by JN85Chevy

Doug_Scott · August 29, 2013

Have you checked compression? When you get a rough running engine, and the computer says misfire on a cylinder, you check for spark, fuel, and compression on that cylinder. Don't head straight to throwing parts at it.

nickvrebel · August 29, 2013

Check your compression as stated above.

How does the sparkplug look now?

next time you are thinking about changing a coil pack it's cheaper to do one more step diagnosing.... just swap the coil pack with another a couple cylinders away and see if the problem follows the coil pack, if it doesn't then the coil pack isn't the problem.