03 chevy silverado misfires

[stinkbelly]

My son's 2003 chevy 2500HD 6.0 gas is getting the p0300 code. This is multiple misfire code. It is idling rough. The service engine light flashes when idling, but turns off when driving. I don't think it is plugs or wires because I wouldn't think multiple would break at the same time. I thought maybe bad gas. He filled up in brownwood, drove 2 hours and went to bed the next morning it started acting up. 120,000 miles of it matters.

Thoughts?

[Muskles]

I'm going through pretty much the same with my son's 01. I hate those codes, because it can be multiple things. I'd start with some good gas treatment and check for vacuum leaks first. You can get some brake parts cleaner, or carb cleaner and spray around the top of the engine, where vacuums are pulled, including the throttle body and intake. If the engine idle changes, there's a vacuum leak where you sprayed. There's another tool, other than a code reader (I think it's a programmer), that will help narrow it down more accurately. They're pretty pricey, so you might find a mechanic, or someone who has one you can borrow. Spark testers are pretty cheap at Harbor Freight, so you can check each plug and coil that way. It could also be a number of different sensors. Sorry I'm not more helpful, but I feel your pain.

[diamond10x]

I've had two 6.0's, a 99 and an 03. I got that stupid 0300 code in my 03. Took a while to get figured out like said above. I replaced plugs wires and coils, cleaned MAF sensor and throttle body, checked for vacuum leaks, looked over the intake tube for any hairline cracks, replaced manifold gaskets. Finally took it in to a mechanic buddy of the family so he could do an ECM relearn which finally got rid of the code.

[Kingfisher789]

Does it make any ticking noises hot or cold?

[easeup]

here is some reading to prime you on the subject, and maybe help you decide the next step

https://www.carparts.com/blog/p0300-code-random-or-multiple-cylinder-misfire-detected/

[shwacker1911]

My 01 2500 6.0 ended up being knock sensors and a coil pack

[muzzlebrake]

There was also a run of these 6.0 engines in the 2500 trucks that would have a knock sensor problem and would throw a P0300 code. These sensors are located under the intake manifold, one at the front and one at the rear and screw into the valley pan. Seems that water gets in there and stands in the depression and eventually rusts or corrodes the wire connector. Sometime this code would throw the truck into limp mode. It would run rough and barely have enough power to get you home.
Replacing these sensors is a PITA as the intake has to be pulled to access the sensors and 2 pin connectors.
Knock sensor code is Po325
Coil paks are also another culprit but easy to replace and diagnose.
A loose spark plug is the embarrassing one especially if you are the one that replaced your plugs.

[RifleBowPistol]

If you have not put plugs in it, by now, put some plug in it. Preferably NGK, Denso or Delco, no Champion or Bosch. Just common replacement plugs, not any triple platinum, or some other gimmicky BS.

If the plugs are of a good brand and part number, and less than 50,000 miles on them and not fouled, I would look at other problems, such as things that will cause it to go lean. The fact you said it died, I would check fuel pressure, make sure it's good at all times, meaning not just sitting there idling in neutral. To test a fuel system, basically fuel pump and filter, you will need to drive it, and put a heavy load on the engine, get the vehicle nice and hot. Drive it for a while, during the hotter times of the day, in town traffic, will produce the most heat.

If the pump is weak, heat will kill it. So taking it out and driving it for 30 minutes or so, during a time of the day, when it's pretty hot, then put a heavy load in the engine, intermittently, is the way you test out a fuel pump.

Clogged fuel filter should show up right off, as soon as you put a heavy load in the engine. Meaning in gear, lots of throttle, in a higher gear, higher speeds.

You could have a very bad vacuum leak or air leak that could cause the engine to run lean and possibly die. A air leak or vacuum lean that causes the engine to die would have to be a pretty big leak. Carb cleaner is how you find those leaks. Engine running, spray some carb cleaner around the top of the engine and the intake air hose, from the MAF to where the intake and the heads are bolted together. Then spray along any vacuum hose.

[PYBUCK]

Put some injector cleaner in the gas tank and you should be good.

[Kingfisher789]

I'm going through pretty much the same with my son's 01. I hate those codes, because it can be multiple things. I'd start with some good gas treatment and check for vacuum leaks first. You can get some brake parts cleaner, or carb cleaner and spray around the top of the engine, where vacuums are pulled, including the throttle body and intake. If the engine idle changes, there's a vacuum leak where you sprayed. There's another tool, other than a code reader (I think it's a programmer), that will help narrow it down more accurately. They're pretty pricey, so you might find a mechanic, or someone who has one you can borrow. Spark testers are pretty cheap at Harbor Freight, so you can check each plug and coil that way. It could also be a number of different sensors. Sorry I'm not more helpful, but I feel your pain.

There aren’t many places for that engine to look. Intake and brake booster

[stinkbelly]

No odd sounds. It did not die.

Obe weird thing it started when his ac died. The ac was only a blower motor resistor. So it shouldn't be related.

[Kingfisher789]

Put some injector cleaner in the gas tank and you should be good.

How about a bottle of Techron and a tank of hi detergent fuel? There is a reason HEB Costco etc are cheaper……

[Kingfisher789]

No odd sounds. It did not die.

Obe weird thing it started when his ac died. The ac was only a blower motor resistor. So it shouldn't be related.

I’d take it to a GMC dealer and have them diagnose it. Best $200 you can throw at it. Tell them you want a certified engine performance tech to diagnose your concern.

[stinkbelly]

It will go to the dealer on monday if I can't figure it out this weekend