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2005 2500HD 6.0 - Troubleshooting P0300 Codes

chev78fan

By chev78fan
in Ask A GM Technician

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chev78fan Enthusiast

chev78fan

Posted
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Truck has approximately 125,000 miles on it (odometer doesn't work). CEL is on and flashes above 2500 rpm. Truck also falls on its face above 2500 rpm. I am the original owner and truck is stock.

 

Changed plugs and wires. Checked the coils with a spark tester and they all have good spark.

 

After a 2 mile drive i checked the exhaust with an IR gun.... 410F and 350F at the inlet of the cats / 390F and 350F at the outlet of the cats. (i don't think they are clogged). Good flow out of tailpipe.

 

Fuel pressure after sitting all night (below zero) was at 0psig / key on goes to 50psig / start it goes to 62psig / shut off goes to 56psig and slowly drops... down to 28psig after a half hour.

 

I have Lucas fuel injector cleaner in the tank.

 

idle is smooth.... when i pull a plug wire off to check for spark there is no noticeable difference in the idle.

 

Any thoughts or suggestions?

Jsdirt Senior Enthusiast

Jsdirt

Posted
Posted

You've got a bad, BAD misfire. Could be any number of things from intake gaskets, injector issues, to mechanical failures.. Got to diagnose that one ... it WILL melt your cats & plug them solid, if it hasn't already ... those temps are VERY low for after cat temps. SHould be close to 4 digit temps there ....

chev78fan Enthusiast

chev78fan

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  • Author
Posted

Thanks for the response.

 

I just did a vacuum test using the port that typically attaches to the PCV.

 

In park - at idle 15" Hg / at 3,000rpm 18" Hg. Needle is steady in both cases. With a plugged cat wouldn't there be less vacuum with RPM due to the back pressure?

 

Next idea is to pull the front O2 sensors and test drive to see if it still runs flat...

Doug_Scott Senior Enthusiast

Doug_Scott

Posted
Posted

Is the fuel filter outside the tank? Has it been changed? I have a friend that came to me one day saying his transmission was on its way out. He took me for a drive, and as soon as he got on the highway and tried to accelerate and merge into traffic, the truck just would not rev. If he tried to accelerate with less gas pedal, he could get to the speed limit, and then from then on it was good, unless he tried to accelerate. My first thought was plugged exhaust, simply because it behaved just like a plugged exhaust. Hard acceleration, engine would just refuse to rev. Let off on the gas, and it would pull away.

 

He was spending the night, and the more I thought about it the more it just did not really match what a plugged exhaust did. Took a look under the truck and saw a fuel filter that looked it may be original. Buddy did not ever change it, and he had owned and driven the truck for about 120k miles. He changed the filter and the truck was like new again. It also fixed the noisy fuel pump he had. If your truck has the filter outside the tank, and has never been changed, change it first. Checking fuel pressure will show this as being an issue if you are driving the truck with the gauge attached. Cannot really duplicate engine load and fuel demands with truck not moving.

chev78fan Enthusiast

chev78fan

Posted
  • Author
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Thanks for the replies. Filter is in the tank. The truck also has the returnless fuel system with the regulator as part of the pump assembly.

 

I'm going to try and drive it with the fuel pressure gauge attached - I think the gauge hose is long enough. If pressure is good I'm going to pull the front O2 sensors (unplug the exhaust) and drive it.

chev78fan Enthusiast

chev78fan

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I drove the truck with the fuel pressure gauge installed and the pressure is dead steady at 62psig.

 

i'm not looking forward to trying to remove the O2 sensors.

Doug_Scott Senior Enthusiast

Doug_Scott

Posted
Posted

I drove the truck with the fuel pressure gauge installed and the pressure is dead steady at 62psig.

 

i'm not looking forward to trying to remove the O2 sensors.

 

Really have to wonder why GM put the filter in the tank. I have heard they did that so you would be forced to change the pump when changing the filter or you would have to change the filter when changing the pump. Depends on if you are an optimist or pessimist I guess.

 

Did you try running the truck at WOT through the first 3 gears and watch fuel pressure? Can't you just unplug the O2 sensors instead of removing the sensors? Not sure that the open circuit in the O2 sensor circuit cause though.

chev78fan Enthusiast

chev78fan

Posted
  • Author
Posted

Yes I ran it thru the gears and the fuel pressure is a steady 62psig.

 

I want to remove the O2 sensors to relieve any back pressure associated with clogged cats.

 

I tried removing the O2 sensors and the offset O2 sensor wrench I bought twisted.

Jsdirt Senior Enthusiast

Jsdirt

Posted
Posted

Get that sensor red hot at the base - it will just about come out with your fingers, if you could touch it. I would have a new one on hand first, however.

gmtech4 Senior Enthusiast

gmtech4

Posted
Posted

Thanks for the replies. Filter is in the tank. The truck also has the returnless fuel system with the regulator as part of the pump assembly.

I'm going to try and drive it with the fuel pressure gauge attached - I think the gauge hose is long enough. If pressure is good I'm going to pull the front O2 sensors (unplug the exhaust) and drive it.

Do not remove the O2 sensors, remove both pipes before the cats and DRIVE THE TRUCK. As loud as it is , just do it , that ALWAYS reveals plugged cats . And it's cool and very loud !!!!

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