Misfire Without The Miss

bmgmike · October 18, 2009

I know this has been discussed alot but here is my story. I have a 2003 Silverado 2500HD ECLB 6.0L, 135,000 miles. A few months ago my SES started blinking. Took it to a friend at a dealer and it was the dreaded P0300 code, misfires on #1, 8 and 5 but I did not feel the misfires the light just started blinking. I started with the plugs and #1 was cross threaded and not all the way in and the tip on #5 was burned away and the dealer said #8 was probably because it followed #1 in the firing order. I fixed the two plugs and figured that was that but it was not to be. The light still blinks and every time it is P0300 code. The weird thing is it is just a light, I feel no miss, it runs a little rough at idle but that is it. The truck has tons of power, I was running it at 80mph and punched it and the computer topped it out at 100mph in a matter of seconds. And the only time the SES light blinks is at highway cruising speed. I have looked at the MAF, looked for vaccum hoses broke, bad wires on O2 sensors. I actually had two other plugs with the tips burned off and they weren't even listed as misfiring. Any ideas would be greatly appreciated, I just have to know what is causing this and stop the blinking.

bmgmike · December 5, 2009

I know this has been discussed alot but here is my story. I have a 2003 Silverado 2500HD ECLB 6.0L, 135,000 miles. A few months ago my SES started blinking. Took it to a friend at a dealer and it was the dreaded P0300 code, misfires on #1, 8 and 5 but I did not feel the misfires the light just started blinking. I started with the plugs and #1 was cross threaded and not all the way in and the tip on #5 was burned away and the dealer said #8 was probably because it followed #1 in the firing order. I fixed the two plugs and figured that was that but it was not to be. The light still blinks and every time it is P0300 code. The weird thing is it is just a light, I feel no miss, it runs a little rough at idle but that is it. The truck has tons of power, I was running it at 80mph and punched it and the computer topped it out at 100mph in a matter of seconds. And the only time the SES light blinks is at highway cruising speed. I have looked at the MAF, looked for vaccum hoses broke, bad wires on O2 sensors. I actually had two other plugs with the tips burned off and they weren't even listed as misfiring. Any ideas would be greatly appreciated, I just have to know what is causing this and stop the blinking.

Update, the SES only flashes from 70mph to 80mph, dealer diagnosed as intake leak. Changed gaskets, no change, any ideas.

dmaxpower · December 7, 2009

I know this has been discussed alot but here is my story. I have a 2003 Silverado 2500HD ECLB 6.0L, 135,000 miles. A few months ago my SES started blinking. Took it to a friend at a dealer and it was the dreaded P0300 code, misfires on #1, 8 and 5 but I did not feel the misfires the light just started blinking. I started with the plugs and #1 was cross threaded and not all the way in and the tip on #5 was burned away and the dealer said #8 was probably because it followed #1 in the firing order. I fixed the two plugs and figured that was that but it was not to be. The light still blinks and every time it is P0300 code. The weird thing is it is just a light, I feel no miss, it runs a little rough at idle but that is it. The truck has tons of power, I was running it at 80mph and punched it and the computer topped it out at 100mph in a matter of seconds. And the only time the SES light blinks is at highway cruising speed. I have looked at the MAF, looked for vaccum hoses broke, bad wires on O2 sensors. I actually had two other plugs with the tips burned off and they weren't even listed as misfiring. Any ideas would be greatly appreciated, I just have to know what is causing this and stop the blinking.

Update, the SES only flashes from 70mph to 80mph, dealer diagnosed as intake leak. Changed gaskets, no change, any ideas.

I've got the same problem with a 4.8L. Some things I've read could be the cause are plugged cat coverters or bad cam sesor. I don't know where I'm going to start.

minibike132 · December 7, 2009

Has anyone made sure the crank variation has been relearned?

bmgmike · December 11, 2009

Has anyone made sure the crank variation has been relearned?

I have heard of this being an issue but I figured the dealer knew what they were talking about. I did hear of clogged cats causing this but it has way too much power to be clogged. I did the cam sensor when I had the intake off because it is easier to get to when the intake is off. Is there a scantool I can buy reasonably that will do the crank relearn? Thanks Mike

Nytemare · December 11, 2009

Has anyone made sure the crank variation has been relearned?

I have heard of this being an issue but I figured the dealer knew what they were talking about. I did hear of clogged cats causing this but it has way too much power to be clogged. I did the cam sensor when I had the intake off because it is easier to get to when the intake is off. Is there a scantool I can buy reasonably that will do the crank relearn? Thanks Mike

Sorry man...its a function of the Tech II or similar Diagnostic tool...they start at about 2000 bones for a cheap one.

JRLZ71 · December 12, 2009

Well this morning my SES light came on in my 2008 GMC Sierra and I had no idea why. It came on a few months back and that was because my gas cap some how unscrewed. But anyway I looked in my manual to see what the problem was. It says that any modification made to the the engine, transmission, exhaust, intake, fuel system, or even the replacement of tires can affect your vehicle's emission controls thus causing your SES light to come on. If your light flashes it means a misfire condition has been detected. A misfire increases vehicle emissions and could damage the emission control system on your vehicle. if the light stays on it means that an emission control system malfunction has been detected on your vehicle.

My light came on because I got a new exhaust yesterday. I called around and asked people what they thought I should do and one of my friends said that I should take my positive battery cable off and let it sit for a few minutes. I tried this and luckily it worked because I really did not feel like driving to the dealership today. I also had the same problem on my 2006 Chevy Colorado and it was the accelerator sensor that was causing the problem. hope this helps

Nytemare · December 12, 2009

Well this morning my SES light came on in my 2008 GMC Sierra and I had no idea why. It came on a few months back and that was because my gas cap some how unscrewed. But anyway I looked in my manual to see what the problem was. It says that any modification made to the the engine, transmission, exhaust, intake, fuel system, or even the replacement of tires can affect your vehicle's emission controls thus causing your SES light to come on. If your light flashes it means a misfire condition has been detected. A misfire increases vehicle emissions and could damage the emission control system on your vehicle. if the light stays on it means that an emission control system malfunction has been detected on your vehicle.

My light came on because I got a new exhaust yesterday. I called around and asked people what they thought I should do and one of my friends said that I should take my positive battery cable off and let it sit for a few minutes. I tried this and luckily it worked because I really did not feel like driving to the dealership today. I also had the same problem on my 2006 Chevy Colorado and it was the accelerator sensor that was causing the problem. hope this helps

You know....this is why they make code readers...so you don't have to guess and/or toss parts at the vehicle hoping to fix things.

You read the code...and look it up, the list tells you something like this;

For misfires...you will get

P0300 Random/Multiple Cylinder Misfire Detected

If detected on a specific cylinder you will get one or more of the following, depending on the number of cylinders yo have;

P0301 Cylinder 1 Misfire Detected

P0302 Cylinder 2 Misfire Detected

P0303 Cylinder 3 Misfire Detected

P0304 Cylinder 4 Misfire Detected

P0305 Cylinder 5 Misfire Detected

P0306 Cylinder 6 Misfire Detected

P0307 Cylinder 7 Misfire Detected

P0308 Cylinder 8 Misfire Detected

P0309 Cylinder 9 Misfire Detected

P0310 Cylinder 10 Misfire Detected

P0311 Cylinder 11 Misfire Detected

P0312 Cylinder 12 Misfire Detected

Or a multitude of other codes explaining whats going on.

scoota1969 · December 30, 2009

I have the same problem with my 2003 GMC sierra 1500hd quadsteer missfires mostly 1,3,5,7 have done everything plugs,wires,fuel filter,gas pressure test,compresion test,MAF sensor,canned the k&n filter,checked everywhere for vacume leaks,only thing left is plugged cats or cam sensor i guess all the scanner says is random misfire runs rough idles low misfires spark knocks its just a mess really HELP!!!!!

Nytemare · December 30, 2009

I have the same problem with my 2003 GMC sierra 1500hd quadsteer missfires mostly 1,3,5,7 have done everything plugs,wires,fuel filter,gas pressure test,compresion test,MAF sensor,canned the k&n filter,checked everywhere for vacume leaks,only thing left is plugged cats or cam sensor i guess all the scanner says is random misfire runs rough idles low misfires spark knocks its just a mess really HELP!!!!!

That's a whole bank....I would check for head gasket issues, or loose plugs to the coil harness on that side of the engine.

scoota1969 · December 30, 2009

I have the same problem with my 2003 GMC sierra 1500hd quadsteer missfires mostly 1,3,5,7 have done everything plugs,wires,fuel filter,gas pressure test,compresion test,MAF sensor,canned the k&n filter,checked everywhere for vacume leaks,only thing left is plugged cats or cam sensor i guess all the scanner says is random misfire runs rough idles low misfires spark knocks its just a mess really HELP!!!!!

That's a whole bank....I would check for head gasket issues, or loose plugs to the coil harness on that side of the engine.

ive been all over this thing with two other tech's for a week now we've really check every plug and wire and vac line havent found any leaks or anytihng that would lead us to a head issue any other ideas like i said havent dont that cats or cam sensor yet?

Nytemare · December 30, 2009

I have the same problem with my 2003 GMC sierra 1500hd quadsteer missfires mostly 1,3,5,7 have done everything plugs,wires,fuel filter,gas pressure test,compresion test,MAF sensor,canned the k&n filter,checked everywhere for vacume leaks,only thing left is plugged cats or cam sensor i guess all the scanner says is random misfire runs rough idles low misfires spark knocks its just a mess really HELP!!!!!

That's a whole bank....I would check for head gasket issues, or loose plugs to the coil harness on that side of the engine.

ive been all over this thing with two other tech's for a week now we've really check every plug and wire and vac line havent found any leaks or anytihng that would lead us to a head issue any other ideas like i said havent dont that cats or cam sensor yet?

Highly doubt anything is wrong with the cats...that shouldn't cause a misfire...even if they were plugged.

the most common thing I have seen when an engine misfires on a whole bank is head gasket. you could also try swapping ECM's with someone...I have seen them go bad and it very well could be a cam pos sensor. If you could hook it up to an o-scope and see the cam pos sensor triggering that would rule that out....or you could just swap it...I believe its 25 bucks.