GM Truck Club Forum banner
1 - 20 of 29 Posts

·
Registered
Joined
·
15 Posts
Discussion Starter · #1 ·
My brother bought a 06 Silverado with the 4.8 new. He didnt drive it alot and just started it every now and then.About a year ago when he started it it was missing bad and didnt hardly have enough power to move and had the random multiple missfire code. It only has 38k miles. Few weeks ago I bought it. I fooled with it a little bit but never figured anything out about it. What i did figure out was that it seems like it is firing and getting gas but like its way out of time. All the plugs were completly black and I could unplug the coils one at a time and there was only 3 or 4 that changed anything and I had put new plugs in it and the coils were firing. I didnt know what else to do so i took it to a gm service garage where a buddy of mine works. He said the mechanic said it had low compression on 2 cylinders a little over a 100 pounds and thats what he thought was wrong with it. I just dont think that could be it. If it was dont look like all the plugs would be that black and if it did miss it shouldnt be as bad as it is. Ive read where people says with missfire didnt have as much power on hills and other stuff but this one dont have enough power to hardly even move. Just seems to me like its way out of time.Doesnt make any noises or nothing else thats not normal. Have read about ground wires and other things but just dont know what would have happened with it just setting. Anybody have any ideas that might help? Thanks
 

·
Registered
2001 Suburban 5.3L, 302.2k miles
Joined
·
1,829 Posts
At only 38k miles, I doubt it's out of time.
I have a 2001 suburban with the 5.3L that sat for a few months and when I decided to drive it again, it started misfiring and had a P0305 I think. Not a random misfire P0300 like yours but it turned out to be a stuck injector.
You need to do an injector flow test. Some scanners can do it together with a fuel pressure tester. But I was able to shock the stuck closed injector with just a fuel pressure tester and energizing the injector with a 9V battery. It was easy because the injectors on the 5.3L are aceesivle from the outside

I think your 4.8L has the injectors inside the intake if I'm not mistaken but it can be done.
 
  • Like
Reactions: RayVoy

·
Registered
Joined
·
15 Posts
Discussion Starter · #8 ·
At only 38k miles, I doubt it's out of time.
I have a 2001 suburban with the 5.3L that sat for a few months and when I decided to drive it again, it started misfiring and had a P0305 I think. Not a random misfire P0300 like yours but it turned out to be a stuck injector.
You need to do an injector flow test. Some scanners can do it together with a fuel pressure tester. But I was able to shock the stuck closed injector with just a fuel pressure tester and energizing the injector with a 9V battery. It was easy because the injectors on the 5.3L are aceesivle from the outside

I think your 4.8L has the injectors inside the intake if I'm not mistaken but it can be done.
 

·
Registered
Joined
·
15 Posts
Discussion Starter · #9 ·
As bad as it runs it would have to be a few of them. One might go bad but dont look like that many would. Seems like if it wasnt getting gas the plugs wouldnt be so black either. I also squirted gas in the throttle body like ive done to check for a bad fuel pump and it didnt change at all.
 

·
Registered
Joined
·
17,446 Posts
Real black plugs are caused by too rich a mixture, too much fuel, or not enough air.
Check your air box and the supply tube, maybe it has become home to critters
 

·
Registered
Joined
·
6,665 Posts
you did not post all the engine codes .............. also the fuel is old . suck it out .. then put in new fuel and add fuel injector cleaner 2 bottles ...
air cleaner/wiring/ground straps corroded ....
 

·
Registered
Joined
·
15 Posts
Discussion Starter · #12 ·
you did not post all the engine codes .............. also the fuel is old . suck it out .. then put in new fuel and add fuel injector cleaner 2 bottles ...
air cleaner/wiring/ground straps corroded ....
you did not post all the engine codes .............. also the fuel is old . suck it out .. then put in new fuel and add fuel injector cleaner 2 bottles ...
air cleaner/wiring/ground straps corroded ....
The only code on it was was the 0300 i think it is but it was the multiple misfire. How many grounds are on it or do you know. I cleaned the ones under the drivers side on the cab mount bracket.
 

·
Registered
Joined
·
6,665 Posts
The only code on it was was the 0300 i think it is but it was the multiple misfire. How many grounds are on it or do you know. I cleaned the ones under the drivers side on the cab mount bracket.
clean the injectors ,,,,,,,,,,,, gas pressure is 53-60 PSI ....then drive the vehicle and the injectors are dropping the cylinders in the fuel ..P0300 miss fires ..put marvel mystery oil in the gas tank 4 OZ lube / clean the injectors .
look up the engine gnd strap to the firewall .. then next to the gas tank on the frame ..
 

·
Registered
Joined
·
15 Posts
Discussion Starter · #16 ·
clean the injectors ,,,,,,,,,,,, gas pressure is 53-60 PSI ....then drive the vehicle and the injectors are dropping the cylinders in the fuel ..P0300 miss fires ..put marvel mystery oil in the gas tank 4 OZ lube / clean the injectors .
look up the engine gnd strap to the firewall .. then next to the gas tank on the frame ..
The fuel pressure is 55 to 60.Do you mean the injectors might be putting too much fuel instead of it being not enough air to burn the normal fuel amount. I cant drive it either. It will barley even move. Misses bad idling and wont hardly rev at all. I mentioned timing before and the engine probably aint out of time but what keeps things in time so the plugs fire at the right time? Also I mentioned compression in my original post and theres nothing wrong with it I checked it..
 

·
Registered
Joined
·
6,665 Posts
the engine is poor idling ..the P0300 engine code is one code ...take out an injector that looks up a dirty inside the port ...
many vehicles stay in the gasoline 10% alcohol over 1 year then the tank and the fuel supply has a mucky mess.
some have to look up the injectors and the dirty ports then the gas tank has to clean the gas tank..
use the gas gauge then have the drain valve .. then put a large glass bottle to see if the gasoline is clean or a cloudy color ..
 

·
Registered
Joined
·
15 Posts
Discussion Starter · #19 ·
the engine is poor idling ..the P0300 engine code is one code ...take out an injector that looks up a dirty inside the port ...
many vehicles stay in the gasoline 10% alcohol over 1 year then the tank and the fuel supply has a mucky mess.
some have to look up the injectors and the dirty ports then the gas tank has to clean the gas tank..
use the gas gauge then have the drain valve .. then put a large glass bottle to see if the gasoline is clean or a cloudy color ..
I have another question now if anybody might be able to help. Alot of the answers on here talk like the injectors may be stopped up. Im going to check them further and if thats the case ill either clean them or buy new ones. My question is though if they were stopped up it wouldnt be getting enough gas to run but mine seems the other way like its getting too much gas. The plugs are completely black and black smoke out the exhaust like its flooding. Could injectors cause this to? Thanks for any help.
 

·
Registered
2001 Suburban 5.3L, 302.2k miles
Joined
·
1,829 Posts
There's a way to check if injectors are stuck open aside from being stuck closed.

With the fuel pressure hooked up to the rail, key on, engine off, fuel pump will prime rails to 60 psi. Observe if the pressure drops quickly or it holds.
Does fuel pressure hold or drops?
 
  • Like
Reactions: j cat
1 - 20 of 29 Posts
Top