Po300

[Greg Mck]

I have been having a misfire since day one with this truck. I like the truck and when it's not misfiring it runs great.
I have replaced-- cap and rotor twice, ignition module, ignition coil, spark plugs, distributor, crank position sensor, crank position sensor connector,and plug wires. Also cleaned and reconnected plugs on the ECM. Still has an intermittent misfire. One thing i didn't do is reset the cam position retard. My question is could this be causing my problem? I don't know what else to do other than setting the retard. It runs and drives but it will start back firing and popping out of the blue. Any ideas?

 

[GarrettGmc]

Hey did you ever figure this out? Or anyone else had this code? At red light today my 99 burb started cutting out, luckily I was able to pull off then pulled the po300 code. Engine is only 3 years old. I'll be digging into it in few mins and if I cnt fix it by Monday ill have to trade the diesel for a family hauler since the burb is the only thing that holds everyone.

 

[GarrettGmc]

Ok so got it home and went to get it off the trailer and it dies within a minute of starting, I cnt even get it off the trailer. Starts up then bogs down, tried to give it lil gas and cut out an died. Anyone got any help, I've heard the po300 can be hard.

 

[Mangonesailor]

To the both of you (Garret, I saw your issue on the "What'd you do to your GMT400 today" thread)

I created THIS THREAD a few years ago when I had my own P0300 issue. Check it out as it may give ya'll some ideas on what to look for.

Greg McK, Why have you not replaced your spark plug wires yet? Have you tried watching them in the dark (with the hood light unplugged) to see if you have voltage leaks? Have you pulled the dizzy? What's your fuel pressure?


Garret, what motor do you have?

 

[GarrettGmc]

I've got the 5.7.
Well I did fuel pressure check then checked the cap an rotor, damn things were rusting and with only 30k on em. Replaced those and plug wires, one was starting to get burned. Then pulled the plugs, first 4 were fine, off a little bit on the gapping but the last 4 were rusting, not screwed in all the way and one was clogged up, see pic. Replaced all that and cleared the code took it for drive and at abt 60mph it feels like a cylinder is missing but theres no codes and my scanner isn't pulling anything up. Gunna run some injector cleaner. Had the motor rebuilt by a reputable shop 30k ago and I haven't done anything to it besides oil change so I'm not to happy with them, also missing exhaust manifold to head bolts

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[Mangonesailor]

Christ on a *******...

That's some screwed up looking plugs!!!

Looks like you're on the right track, but, you're going to either keep driving it the way it is to "make a code pop up" (That's not good practice by the way, a CEL will go away if the code is not active... errr... causing an issue at that point in time. Clearing the code resets some computers and it will attempt to go through all of the DTCs before showing codes).

What kind of scanner are you using?

 

[DerekTheGreat]

I had a plug that looked like that with my old motor and it turned out that cylinder burnt up a valve. Stick a piece of paper about 1" away from the tail pipe, if the exhaust tries to suck it up into it - burnt valve. A compression check is a good thing too.

 

[GarrettGmc]

I had a plug that looked like that with my old motor and it turned out that cylinder burnt up a valve. Stick a piece of paper about 1" away from the tail pipe, if the exhaust tries to suck it up into it - burnt valve. A compression check is a good thing too.

Never heard of the paperbtest but makes sense. I'll prolly pick up a compression tester just to make sure but it shouldn't be the valves already. Motor was completely rebuilt, only thing original are the block and crank but I'm not to happy with that shop right now so who knows what they really were doing to it.

 

[DerekTheGreat]

Ya never know, if they did a crappy job with the valve seats & guides... Just whip out a piece a paper and give that a go. It instantly turned me into a saaad panda..

 

[GarrettGmc]

Ya sucks cuz everyone says they're great, they sponsor all the the hot rod shows in town they have everything in house to rebuild engines. Mayb I just wasn't spending enough money for them to really care
Anyways I tried that paper idea of yours and it didn't suck it at all so I'm thinking that's good.